Physics Classes

Category: 12th Assignments

  • Very important numericals physics( term-2)

    Important numericals physics term-2 (CBSE)

    1. What is apparent position of the object below a rectangular block of glass 6cm thick , if a layer of water 4cm thick is on the top of the glass ? Refractive index of glass and water with respect to air is 3/2 and 4/3 respectively.
    2. A screen is placed 100cm from an object  . The image of the object on the screen is formed by a convex lens at two different locations , separated by distance 20cm . calculate the focal length of the lens used.
    3. You are given three lenses  L1 , L2,L3 each of focal length 10cm . an object is kept at 15cm in front of L1  ,  as shown in fig. the final image is formed at the focus of L3 find the separation between the lenses.
    • 4. A convex lens made up of glass of refractive index 1.5 is  dipped  , in turn ,in (a) a medium of refractive index 1.65 (b)   ) a medium of refractive index 1.33.Will it behave like converging or diverging lens in two cases, explain how focal length changes in two cases .
    • 5.Three rays  red  , green and blue are incident on a right angled prism ABC at face AB the refractive indices of the material  of the prism for red, green ,and blue are 1.39,1.44, and 1.47 respectively . Out of the three which colour of ray emerge out of face AC. Trace the path of the three rays after passing through AB.
    • 6. State the conditions under which total internal reflection occurs . one face of a prism with a refracting angle of 30 , is coated with silver . A ray incident on  another face at an angle of 45 is refracted and reflected from the silver coated face and retrace its path. Find the refractive index of the material of the prism.
    • 7. Find the position of the image formed by the lens combination given in figure   
    • 8. A diverging lens of focal length f cut into two identical parts ,each forming a plano- convex lens what is the focal length of each part. How the focal length of the lens changes if it bisected axially?
    • 9. The image of a needle placed 45cm from a lens is formed on a screen placed 90 cm on the other side of lens. Find displacement of image if object is moved 5cm away from lens.
    • 10. A small bulb is placed at  the bottom of a tank containing water to a depth of 80 cm . what is the area of the surface of water through which light of the bulb can emerge out ?Refractive index of water is 1.33 consider the bulb to be a point source.
    •  11. Yellow light of wavelength λ= 6000 A illuminate a single slit of width 1 x 10-4 m calculate (i) the distance between two dark lines on either side of the central maximum when the diffraction pattern is on screen kept 1.5 m away from the slits , (ii) the angular spread of the first diffraction minima .

       Ans- 1.8 cm , 6x 10-3 rad . 12. Two wavelength of sodium light 590 nm are used in turn , to study the diffraction taking place at the single slit of aperture  2x 10-4 m . The distance between the slits and the screen is 1.5 m . Calculate the separation between the position of first maxima of the diffraction pattern obtained in the two cases .

      Ans-6.75×10-5m . 13. (a) In a single slit diffraction , a slit of width d is illuminated by red light of wavelength 650 nm . For what value of  d will (i) the first minimum fall at an angle of diffraction of 300 , and   (ii) the first maximum fall at an angle of diffraction of 300 ? 

      (b) Why does the intensity of secondary maximum becomes less as compared to the central maximum .

         Ans- 1.3 x10-6 m , 1.95×10 ‑6‑ m 14. A monochromatic light of wavelength 500 nm is incident normally on a single slit of width 0.2 mm to produce a diffraction pattern . Find the angular width of the central maximum obtained on the screen . Also estimate the number of fringes obtained in YDSE with fringe width 0.5 mm , which can be accommodate within the region of total angular spread of the central maximum due to single slit .            Ans- ϴ=0 to 2.5×10-3 ; n=D/2 ;D+1 15.A beam of light consisting of two wavelength 800nm and 600 nm is used to obtain the interference pattern in YDSE on a screen placed 1.4 m away . If the two slits are separated by 0.28mm , calculate the least distance from the central bright maximum where the bright fringes of the two wavelengths coincide . Ans- n=3 ; 1.2 cm . 16. In YDSE using monochromatic light of wavelength λ, the intensity of light at a point on the screen where path difference is λ is K units . Find out the intensity of light of light at a point where path- difference is λ/3 . 17. The ratio of the intensity at minima to the maxima in YDSE is 9:25 , find the ratio of the width of the two slits , also find the ratio of amplitude .   Ans- 16:1 . 18. Two wavelength of sodium light of 590 nm and 596 nm are used in turn to study the diffraction taking place at a single slit of aperture 2×10-6 m . The distance between the slit and the screen is 1.5 m . Calculate separation between the position of first maxima and of the diffraction pattern obtained in the two cases .

     Ans- 6.75 nm .

    19. The electron in a given Bohr orbit has a total energy of -1.5Ev. calculate its(i) kinetic energy(ii)potential energy (iii) wavelength of the radiation emitted , when this electron makes a transition to the ground state.

    20. The energy of an electron in the ground state of hydrogen atom is -13.6 Ev. (i) what does the negative sign signify ? (ii)how much energy is required to take an electron in this atom from the ground state to the first exited state?

    21. The ground state energy of hydrogen atom is  -13.6 eV.  ( I ) What is the potential energy of an electron in the third exited state? (ii) what will be the maximum and minimum wavelength of the emitted  radiation of Lyman series of hydrogen spectrum.

    22. The energy level diagram  of an element is given below . identify using necessary calculations , which transition corresponds to the emission of the spectral line of wavelength 102.7 nm . which transition corresponds to the emission of radiation of maximum wavelength?  

    .

    23.The first line of Lyman series in hydrogen spectrum has a wavelength of 1220 Å. Calculate the wavelength of the second member of the series.

    24. A nucleus at rest splits up into two parts with velocity ratio 2 : 1 calculate the ratio of their radii of the ratio the two nuclear fragments.

    25.The ground state energy of electron in hydrogen atom is – 13.6 eV. Find the kinetic energy and potential energy of the electron in (i) ground state (ii) first excited state.

  • Physics Sample paper class12th(term-2)

    Sample paper -1

                                                            Class 12th

                                                        Physics (Theory)   Term-2

                                    

    M.M: 35                                                                                                             TIME-2Hrs

     Here Physics classes by Nayan jha providing CBSE Term-2 Physics sample paper .

    There are 12 questions in all and all questions are compulsory.

     This question paper consists of three section A,B and C.

    SECTION A has three questions of two marks each , 

    SECFTION- B there are eight questions carrying three marks each and

    SECTION- C is case study based questions of five marks.

    There is no overall choice . However , an internal choice has been provided.

    Uses of calculator is not allowed.

    SECTION-A

    1. Find the ratio of energies of photons produced due to transition of electron of hydrogen atom from its (i) Second permitted energy level to the first level and, (ii) the highest permitted energy level .

    Or,

    The ground state energy of hydrogen atom is -13.6eV. What is the kinetic and potential energy of electron in this orbit.

    2. Draw the circuit diagram showing the biasing of an LED . State the factor which controls (i) Wavelength of light (ii) intensity of light emitted by the diode.

    3. Write the photoelectric equations of Einstein . Write two basic features of photon picture of electromagnetic radiation on which Einstein’s photoelectric equation is based.

    SECTION-B

    4. Draw V-I characteristics of p-n junction diode (i) When diode is reversed bias and (ii) when diode is forward biased . Explain why the current under reverse bias is almost independent of the applied voltage up to the critical voltage?

    5.State the necessary conditions for producing total internal reflection of light. Draw the diagram to show how specially designed prism make use of T.I.R to obtained inverted image of the object by deviating rays (i) through 900 and (ii) Through 1800.

    Or,

    Three rays of light red , green and blue is fall normally on one of the equal side of an isosceles right angled prism . The refractive index of these rays are 1.39,1.44,1.47 respectively . Find which of these rays get internally reflected from diagonal side . Trace the path of rays . Justify your answer with necessary conditions.

    6. Using Bohr postulates for hydrogen atom , show that the total energy of the electron in the stationary states can be expressed as the sum of kinetic energy(K) and potential energy (U)  , where K=-2U. hence , deduce the expression for the total energy in the nth energy level of hydrogen atom .

    Or,

     Draw a labelled diagram of alpha particle scattering experiment. Give Rutherford observations and  discuss the significance of this experiment .

    7. How the size of nuclei is experimentally determined ? write the relation between the radius and mass number of the nucleus . show that the density of the nucleus is independent of its mass number. Two nuclei of mass number in the ratio 27:125. What is the ratio of their nuclear radius and , ratio of their densities.

    Or,

    Draw the binding energy per nucleon curve . using the curve state clearly how the release in energy in the process of nuclear fission and nuclear fusion can be explained.

    8. An electron of mass m  and charge q is accelerated from rest through a potential difference V  , obtain the expression for the de-Broglie  wavelength  associated with it . if electron and proton are moving with the same K.E. , which one of them will have a large De-Broglie wavelength associated with it ? give reason .

    9. I n a single slit diffraction experiment , deduce the conditions for the central maximum and secondary maxima/minima observed in diffraction pattern . Also explain why the secondary maxima go on becoming weaker in intensity as the order increases. Haw does the width of the slit affected the size of the central bright fringe?

    10. Using Young’s double slit experiment obtain the conditions for gating bright and dark fringes  also derive the expression for the fringe width.  Also write the effect on fringe width when(I)Distance between slits increases/decreases (II) Distance between slits and the screen increases/decreases(III) Monochromatic light replaced  by white light,(IV) One of slit is closed and (V) entire experimental apparatus is immersed in a liquid.

    11.  (i) Which segment of electromagnetic waves has highest frequency? How are these waves produced? Give one use of these waves.

    (ii) Which EM waves lie near the high frequency end of visible part of EM spectrum? Give its one use. In what way this component of light has harmful effects on humans?

                Section-C

    12.

     Power of a lens-

    Power (P) of a lens is given as the reciprocal of focal length (P=1/f)where f should be in meter and P is in Diopter . for convex power is positive and concave power is –ve. When two or more lenses are kept in contact then power of the combined lens is given as P= P1 + P2+P3 …….

    (I) A convex and a concave lens is separated by distance d are then put in contact then the focal length of the combination  (a) becomes 0 (b) remain the same (iii) decreases (iv)increases.

    (ii)The two lenses of power +1.5D and +1.0D are placed in contact then the effective power of the combination will be (a)2.5D (b)1.5 D (c) 0.5D (d)3.25D .

    (iii) If the power of the lens is 5D then what is the focal length of the lens?(a)10cm (b) 20cm (c)15cm (d) 5cm

    (iv)Two thin lens of focal length +10cm and -5cm are kept in contact , the power of the combination is ? (a)-10D (b)-20D (c)10D (d)15D .

    (v)A convex lens of focal length 25cm is placed coaxially in contact with a concave lens of focal length 20cm the system will be ; (a) converging in nature (ii) diverging in nature (c) can be converging or diverging (d) None of the above.

    Samle-paper2-22-12thDownload

    samplpaper-3.pdf

  • Case study based questions on Photoelectric effect

    Photoelectric effect

    Photoelectric effect is the phenomenon of emission of electrons from a metal surface , when radiation of suitable frequency fall on them . The emitted electrons are called photoelectrons and the current so produced is called photoelectric current.

    i. With the increase of intensity of incident radiation on photoelectrons emitted by a photo tube , the number of photoelectrons emitted pre unit time is (a) increases (b) decreases (c) remain the same  (d) none of these .

    ii. It is Observed that photoelectrons emission stops at a certain time t after the light source is switched on . The stopping potential can be represented as (a) 2(K.E.)max/e (b) (K.E.)max/e  (c) (K.E.)max/3e (c) (K.E.)max/2e .

    iii. A point source of light of power 3.2×10-3 W mono energetic photons of energy 5.0eV and work function 3.0eV . The efficiency of photoelectrons emission is 1 for every 106 incident photons . Assume that the photoelectrons are instantaneously swept away after emission . The max kinetic energy of photon is (a) 4eV (b) 5eV (c) 2eV (d) Zero .

    iv. Which of the following is the application of photoelectric effect ?   (a)L.E.D  (b)Diode (c) Photocell     (d) Transistor .

    v. If the frequency of incident light falling on the photosensitive metal is doubled , the kinetic energy of the emitted photoelectrons is (a) Unchanged (b) halved (c) doubled (d) more than twice its initial value.

    To solve the test based on the chapter Dual nature of matter . Atoms and nuclei, and Semiconductor devices click here-

    Test Dual nature, Atoms Nuclei and semiconductor

  • Hots questions based on semiconductor

    Hots questions based on Semi conductors.

    These questions are important for class 12th board Examination.

    Q. 1. What are different types of compound semiconductors? Give two examples of each.

    Ans- The compound semiconductors are of following types

    • Inorganic semiconductors: Cds, GaAs, CdSe
    • Organic semiconductors: Anthracene, pthalocynines
    • Organic polymers: Polyparrole, polyaniline, polythiophene.

    Q. 2. Which characteristic of a semiconductors make them useful in fabrication of electronic devices?

    Ans- The number of charge carriers and their direction of flow can be controlled within the solid semiconductor.

    Q. 3. Give three advantages of semiconductor devices over conventional vacuum tube devices.

    Ans- 1. The charge carriers can be manipulated within the semiconductor unlike the vacuum tubes where the electrons were manipulated in vacuum.

    2. The size of semiconductors devices is very small because they do not require vacuum for manipulation of the charge carriers.

    3. The semiconductor devices consume lesser energy because unlike vacuum tube, they do not require heating for electron emission.

    Q. 4. What are the characteristics to be taken care of while doping a semiconductor? Justify your answer.

    Ans-(i) The number of dopant atoms should be small and restricted to just a few parts per million (ppm) of the pure semiconductor atoms.

    (ii) The size of the dopant atom used should be nearly equal to that of the semiconductor atom.

    The above characteristics are essential so that the doping process does not cause any major distortion in the original pure semiconductor lattice. The dopant atom just substitute some of the semiconductor atoms in their lattice.

    Q. 5. Give example of the dopants used to make

           (i) n-type semiconductor                       (ii) p-type semiconductor.

    Ans- n-type semiconductor are formed by using pentavalent impurity e.g. Arsenic (As);  Antimony (Sb), Phosphorous (P) p-type semiconductors are formed by adding trivalent impurity to pure semiconductor e.g., Indium (In), Boron (B), Aluminium (Al).

    Q. 6. What is the energy required to make electrons free for conduction in pure (i) germanium (ii) silicon? How does the energy value change in a doped semiconductor?

    Ans- In a pure semiconductor, the energy required to make electrons available for conduction is large. It is about 0.72 eV for germanium and about 1.1 eV for silicon. In doped germanium, the energy required becomes nearly 0.01 eV and in doped silicon it is about 0.05 eV.

    Q. 7. What is the net charge on a p-type extrinsic semiconductor? Justify.

    Ans- Zero, the crystal maintains its overall neutrality because the charge on additional charge carriers is just equal and opposite to that of the ionised cores in the lattice.

    Q.8. What should be the value of the reverse breakdown voltage of the diode used in a half-wave rectifier and why?

    Ans- The reverse breakdown voltage of the pn-junction diode used in half-wave rectifier should be higher than the peak voltage of the a.c. voltage at the secondary of the transformer. The condition is necessary to protect the pn-junction diode from reverse breakdown.

    Q.9. How is a sample of n-type semiconductor electrically neutral though it has an excess of negative charge carriers?

    Ans- n-type semiconductors are fabricated by adding group 15 impurity atom which are neutral and having one extra proton for every conduction electron. So the sample in neutral.

    Q.10. A student has to study the forward and reverse characteristics of a pn-junction. What kind of circuit arrangement should he use?

    Draw the typical shape of input and output characteristic curves likely to be obtained.

    Mark the knee voltage and reverse breakdown voltage on the curve. How can the static and the dynamic resistance be determined from the curves?

    In which of the regions does the pn-junction remain when used as (i) full-wave rectifier (ii) stabiliser?

    Q.11. What do you mean by host atoms in semiconductors?

    Ans- In impure semiconductors, the semiconductors atom (e.g. Ge or Si) are called host atoms.

    Q.12. What are optoelectronic devices? Give any two example of such devices.

    Ans- The semiconductor junction diodes in which the charge carriers are generated by photons are called photoelectronic devices. The examples of such devices are

    • Photodiodes
    • Light emitting diodes
    • Photovoltaic or solar cells.

    Q.13. Two semiconductor pieces X and Y of same length and equal area or cross-section are provided. One of the materials is pure semiconductor and the other is doped.

    (i) How will you identify the impure semiconductor from the two give pieces X and Y?

    (ii) For a given rise in temperature what will be the relative charge in the behaviour of the two materials?

    Ans- (i) Measure the electrical resistance offered by X and Y. The piece offering lower resistance will be impure semiconductor.

    (ii) Both X and Y will have negative temperature coefficient of resistance. However, the resistance of pure semiconductor will decrease sharply with rise in temperature as compared to the impure semiconductor.

    Q.14. A photodiode is operated is reverse bias region only although the forward current is known to be more than the current in reverse bias. Give reason.

    Ans- A photodiode is sensitive to light and is used to measure/detect intensity of light incident on it. In a pn-junction, photodiode; consider n-type region. The electrons in this region are majority carriers and the holes are minority charge carriers

    ؞                              ne >> nh.

    When the pn-junction is illuminated the bonds between same atoms breaks up due to photo-excitation leading to increase in ne and nh say by ∆ne and ∆nh

    We have                       ∆ne = ∆nh

    ؞                                    ∆nh/nh >>  ∆ne/ne

    i.e. The fractional increase in minority charge carriers is very large as compared to that in minority charge carriers.

    The current through pn-junction in reverse bias is due to drift of the minority charge carriers. The number density of minority carriers being sensitive to light intensity; the reverse current show a rapid change with intensity of incident light.

    So a photodiode is always operated in reverse bias.

    Q.15. What is the minimum frequency that can be detected by a photodiode?

    Ans- The minimum frequency (vm) that can be detected by a photodiode depends on the energy gap Eg of the semiconductor and is given by

                                H vm = Eg

        ؞                         vm = Eg/h.

    Q.16.What are the factors taken into consideration while selecting material suitable for fabrication of solar cell?

    Ans-The following factors are taken into consideration while fabricating solar cell:

    • Band gap is 1.0 eV to 1.8 eV
    • High optical absorption
    • Reasonable electrical conductivity
    • Availability of raw material
    • Cost.

  • Case study question on Compound microscope

    Compound microscope-

    A compound microscope is an optical instrument used for observing high magnified image of tiny objects. Magnifying power of compound microscope is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended by the object , when both the final image and object at the least distance of distinct vision from the eye . It can be given that:m=mexm0, where me is the magnifying power of eyepiece and m0 is the magnifying power of objective lens. Suppose a lens of focal length 2.0cm and another of 6.25 cm separated by distance 15 cm .

    (i) The object distance of eye piece , so that final image is formed at the least distance of distinct vision will be (a) 3.45cm  (b)5cm (c)1.29cm (d) 2.59cm.

    (ii)How far from the objective should an object be placed in order to obtain the condition given in (i). (a)4.5cm (b)2.5cm (c) 1.5cm (d) 3.0cm .

    (iii) What  is the magnifying power of the microscope in case of list distinct of vision? (a)20 (b)30 (c)40 (d)10 .

    (iv) The intermediate image formed by the objective of a compound microscope is (a) real, inverted and magnified (b) real erect and magnified (c) virtual , erect and magnified (d) virtual, inverted and magnified .

    (v) The magnifying power of compound microscope is increases with .(a) f0 increases and fe decreased (b)fe increased and f0 decreased (c) both increased (d) both decreased .

  • Case study question on astronomical telescope

    Astronomical telescope-

    An astronomical telescope is an optical instrument which is used for observing distinct images of heavenly bodies libe stars, planets etc. It consists of two lenses . In normal adjustment of telescope , the final image is formed at infinity . Magnifying power of astronomical telescope in normal adjustment is defined as the ratio of the angle subtended at the eye by the angle subtended at the eye by the final image to the angle subtended at the eye , by the object directly , when the final image and the object both lie at the infinite distance from the eye . It is given by m = f0/fe. To increase magnifying power of an astronomical telescope in normal adjustment , focal length of objective lens should be small.

    (i)An astronomical telescope of magnifying power 7 consists of two thin lenses 40cm apart , in normal adjustment . The focal length of the lenses are (a) 5cm,35cm   (b) 7cm , 35cm (c)17cm , 35cm (d)5cm, 30cm.

    (ii)  An astronomical telescope has a magnifying power of 10. In normal adjustment , distance between the objective and eye piece is 22cm . the focal length of the objective lens is ,(a)25cm (b)10cm(c)15cm (d)20cm.

    (iii) In astronomical telescope compare to eye piece , objective lens has (a)negative focal length (b)zero focal length (c)small focal length (d)large focal length .

    (iv)To see stars use (a)simple microscope (b)compound microscope (c)endoscope (d)astronomical telescope.

    (v)For large magnifying power of astronomical telescope (a) f0<<fe (b)f0=fe (c)f0>>fe (d)none of these  .

  • Case study questions on ray optics

    Case study based questions-

    1. Total internal reflection –

    Total internal reflection is the phenomenon of reflection of light into denser medium at the interface of denser medium with rarer medium . For this phenomenon to occur necessary condition is that light must move from denser to rarer and angle of incidence in denser medium must be greater than critical angle for the pair of media in contact. Critical angle depends on nature of medium and wavelength of light . We can show that µ=1/sinC (where C is the critical angle).

    (i) critical angle for glass air interface , where µ for glass is 3/2 , is (a)41.80 (b) 600 (c)300 (d)150 .

    (ii)Critical angle for water air interface is 48.60 , then what is the refractive index of water/(a)1 (b)3/2 (c)4/3 (d)3/4

    (iii) critical angle for air water interface for violet colour is 490 its value for red colour will be (a)490(b)500 (c)480 (d)can’t say.

    (iv) Which of the following is not due to total internal reflection?(a) working of optical fibre (b)difference between real depth and apparent depth of a pound.(c)mirage on hot summer day (d) brilliance of diamond.

    (v) critical angle of glass is θ1 and that of the water is θ2 critical angle for water and glass surface would be (µg > µw) (a) less than θ2 (b) between θ1 and θ2 (c)greater then θ2 (d)less than θ1 .

    2. Power of a lens-

    Power (P) of a lens is given as the reciprocal of focal length (P=1/f)where f should be in meter and P is in Diopter . for convex power is positive and concave power is –ve. When two or more lenses are kept in contact then power of the combined lens is given as P= P1 + P2+P3 …….

    (I) A convex and a concave lens is separated by distance d are then put in contact then the focal length of the combination  (a) becomes 0 (b) remain the same (iii) decreases (iv)increases.

    (ii)The two lenses of power +1.5D and +1.0D are placed in contact then the effective power of the combination will be (a)2.5D (b)1.5 D (c) 0.5D (d)3.25D .

    (iii) If the power of the lens is 5D then what is the focal length of the lens?(a)10cm (b) 20cm (c)15cm (d) 5cm

    (iv)Two thin lens of focal length +10cm and -5cm are kept in contact , the power of the combination is ? (a)-10D (b)-20D (c)10D (d)15D .

    (v)A convex lens of focal length 25cm is placed coaxially in contact with a concave lens of focal length 20cm the system will be ; (a) converging in nature (ii) diverging in nature (c) can be converging or diverging (d) None of the above.

    3.

    4. Refraction through a prism

    A prism is a portion of a transparent medium bounded by two planes face inclined to each other at a suitable angle . A ray of light suffers two refraction on passing through a prism and hence deviates through a certain angle from its original path . The angle of deviation of a prism is ɗ = (µ-1 )A , through which a ray deviates on passing through a thin prism  of small refracting angle A . If µ is the refractive index of the material of the prism then the prism formula is µ = sin(A+ɗm/2)/sin(A/2).

    (i) For which colour , angle of deviation is minimum ?(a) Red (b)yellow (c) violet (d) blue.

    (ii) When white light moves through vacuum (a) all colours have same speed (b)different colours have different speed (c) violet has more speed then red (d)red has more speed then violet.

    (iii)The deviation through a prism is maximum when angle of incidence is (a)450 (b) 700 (c)900(d)600 .

    (iv)What is the deviation produced by a prism of angle 60 ? (µ=1.644) (a)3.840 (b)4.595(c)7.259(d)1.252.

    (v) A ray of light falling at an angle of 500 is refracted through a prism and suffers minimum deviation . If the angle of prism is 600, then the angle of minimum deviation is (a) 450 (b) 750 (c)500 (d)400 .

    5.

    6.

  • Short answer type questions of ray optics

    Short answer type questions- (2 marks),

    This assignment provides Very important short answer type questions of ray optics which is very important for the class 12th board examinations.

    1.What are the necessary conditions for the phenomenon of total internal reflection to occur . Also write the relation between refractive index and critical angle for a given pair of optical media.

    2. State the principle on which the working of an optical fiber is based. What are the necessary conditions of  phenomenon to occur?

    3. Define critical angle . Obtain a relation between refractive index and critical angle for a pair of media . Does critical angle depends on the colour of light ? Explain.

    4. Draw ray diagrams to show how specially designed prism make use of total internal reflection to obtain the inverted image of the object by deviating rays (i) through 900 (ii) through 1800.

    5. Find the relation between real depth and apparent depth with the refractive index , when light moves from denser medium to rarer medium.

    6. A fish in the water tank sees the outside world as if fish at the vertex of cone such that the circular base of the cone coincide with the surface of water . Given the depth of water were fish is located being h and critical angle for water-air interface being ic , find out by drawing the ray diagram the relationship between the radius of the cone and height h.

    7. Draw the ray diagram showing the path of a ray of light entering through a triangular glass prism . Deduce the expression for the refractive index of glass prism in terms of the angle of minimum deviation and angle of the prism.

    8. A convex lens of focal length f1 is kept in contact with another lens of focal length f2 . Find an expression for the focal length of the combination.

    Watch the video of related topic combination of lenses click here-

    9. Why does the sun appear reddish at sunset or sunrise? For which colour the refractive index of prism material is maximum and minimum?

    10. Draw the ray diagram showing formation of image by a magnifying glass when the image is formed at infinity also find the angular magnification of image.

    11. Define the magnifying power of compound microscope when the final image is formed at infinity. Why does objective and the eyepiece of a compound microscope has short focal lengths ? explain.

    12. Draw  a schematic diagram of reflecting telescope (Cassegrain). Write its two advantages .or, Modern telescope prefer using suitable mirrors over using suitable lens , give two reasons for this preferences.

    13. Use a lens formula and show that (i) Object placed at the focal length of the convex lens for image at infinity.(ii) Object is placed between O(optical center) and F (focus) the image will be virtual.

    14. Draw a plot showing the variation of power of a lens with the wavelength of the incident light.

    15 A double convex lens made from a material of refractive index µ1 is immersed in liquid of refractive index µ2 where µ2>µ1 . What change if any, would occur in the nature of the lens?

  • Magnus effect and dynamic lift

    Magnus effect and dynamic lift-

    The upward force experience by a body when it moves through a fluid is called dynamic lift . Such a lift is felt by airfoil , aircraft win etc. Couse behind it can be explained by Bernoulli’s theorem . It also explain why a spinning ball is deviated from its parabolic path .This deviation is usually observed in many games like cricket, football golf, tennis etc.

    As a ball moves through air without spinning , it divides the streamline  above and below it symmetrically . Thus there is no pressure difference on the two sides of the ball.

    But when a ball spins as it moves through air , it drags along a layer of air with itself. When the ball moves forward , the air ahead of the ball rushes backward to fill the space left vacant by the ball. Thus the streamlines of air around this ball is shown in the fig. given below.

    The velocity of air above the ball opposite to the direction of spin and it is in the direction below the ball. Thus the relative velocity of air  with respect to the ball is larger at the upper part than that is in the region below  the ball . The streamline thus gets crowded above and rarified below .This cause the pressure above the to be smaller and that below it to be greater . This pressure difference causes an upward force on the ball. Hence the ball experiences a dynamic lift and is deviated from its path . This effect arising due to spinning of the ball is called ‘Magnus effect’.

    Airfoil and lift on aircraft wing:-  An airfoil is streamlined shaped , Solid body that is capable of generating a dynamic lift as it moves through a fluid .  streamline shape is special shape given to bodies moving in fluids, So as to produce minimum drag .

    The cross-section of air wings is also shaped like that  .Its upper portion is more curved then the lower surface .Its front edge is broader and the rear edge is narrow . When the airfoil moves through air , streamlines of air moves backward . The air above the foil has to travel a longer distance then that  below in the same time . Thus the flow speed on top becomes greater then that below it . The higher speed on reduces air pressure there. This pressure difference causes to lift the foil. Such dynamic lift also acts on the wings of an air craft and balances the weight of the plane.     

  • Total internal reflection

    In this topic we will discuss , what do you mean by the term Total internal reflection, critical angle . And we will practice some questions based on these topics.

    before to know this topic students must know Real depth and apparent depth. To know this topic click here-

    Total internal reflection and critical angle.

    Before to know about the total internal reflection we must know critical angle

    Critical angle –  It is the value of angle of incidence for which angle of refraction becomes 900, when light enters from denser to rarer medium .

    As we know ,

    1µ2 = sin i /sin r

    Since light is moving from denser medium to rarer medium

    So,  1µ2 = sin r /sin I;

    i.e. µ = sin 900 / sin c  [ here r= 900 then I is written by C (critical angle).

    So, sin C = 1/µ

    Then critical angle C = sin -1(1/µ).

    Total internal reflection-   (TIR) When the light in the denser medium strikes an interface separating the denser medium and a rarer medium at an angle of incidence greater then critical angle total light is reflected without any refraction through the rarer medium . This phenomenon is called “total internal reflection”.

    As shown in fig.

    I2< C ( critical angle) in this case refraction of light occurs .

    I1> C  in this condition T.I.R. occurs.

    Essential conditions for T.I.R is

    (i) Light must move from denser to rarer and (ii) angle of incidence should be greater then critical angle.

    Questions based on refraction of light ( apparent depth and real depth, total internal reflection and  

    Q1. A ray of light is incident on a glass plate as an angle of 600. What would be the refractive index of glass , if reflected and refracted rays are perpendicular to each other? (a)√3 . (b)3/2 (c) √3/2 (d) ½ . ans-a

    Q2. A glass of thickness 12mm is placed on a table . The lower surface of the slab has a black spot .At what depth from the upper surface will the sport appear when viewed from above ? (µg=1.50) . (a) 2mm (b) 4mm (c) 6mm (d) 8mm. ans-d

    Q3. Light enters at an angle of incidence in a transparent rod of refractive index of the material of the rod the light once entered into it will not leave it through its lateral surface whatsoever be the value of angle of incidence? (a) µ>√2 (b) µ = 1 (c) µ=1.1 (d) µ=1.3. ans-a

    Q4. A transparent cube contains a small air bubble . Its apparent distance is 2cm when seen through one face and 5cm when seen through other face . If the refractive index of the of the material of the cube is 1.5 the real length of the edge of the cube must be (a) 7cm (b) 7.5 cm (c) 10.5 cm (d) 14/3 cm. ans-c

    Q5. For the given incident ray as shown in figure , The condition of T.I.R of the ray will be satisfied , if the refractive index of the block will be  ,

    (a) (√3+1)/2      (b)(√2+1)/2     (c)√3/√2     (d)√7/√6. ans-c

    Q6. A ray of light travels from optical denser medium to rarer medium. The critical angle for the two media is C . The maximum possible deviation of the ray will be (a)2C (b) π/2 – C (c) π-C  (d) π-2C . ans-b

    Q7. The apparent depth of water in the cylindrical water tank of diameter 2R cm is reducing at the rate of x cm/min. when water is being drained out at a constant rate . The amount of water drained in cc/minute is : (µ1 refractive index of air and µ2 refractive index of water) (a) xπR2µ1/µ2  (b) xπR2µ2/µ1          (c)  2πRµ1/µ2  (d) xπR2. ans-b

    Q8. Refractive index of glass with respect to medium is 4/3  .If the difference between the velocities of light in medium and glass is 6.25 x 107m/sec , then velocity of light in medium is (a)2.5 x 108m/s  (b) 0.125 x108m/s (c) 1.5 x 107m/s (d) 3 x 107m/s . ans-a