##### Very important questions of ray optics-

In this topic we will get some, Very important questions of ray optics. Students preparing for board examination must solve all the questions given below.

Very important questions of ray optics

1. Derive the mathematical relation between refractive indices μ_{1} and μ_{2} of two radii and radius of curvature R for refraction at a convex spherical surfaces. Consider the object to be a point source on the principle axis in rarer medium of refractive index μ_{1} and a real image formed in the denser medium .

Light from a point source in air falls on a convex spherical glass surface of refractive index 1.5 and radius of curvature 20 cm . The distance of light source from the glass surface is 100cm . At what position the image will be formed?

2. A point object is placed in front of a double convex lens with its spherical faces of radii of curvature R_{1} and R_{2} . Show the path of rays due to refraction at first and subsequently at the second surface to obtain the formation of the real image of the object. Hence obtain the lens maker’s formula for a thin lens.

(i) A double convex lens having both faces of the same radius of curvature has refractive index 1.5. Find out the radius of curvature of the lens required to get the focal length of 20 cm . (ii) How the focal length of the above lens changes when it is placed in liquid of refractive index (a) 1.3 (b) 1.5 and (c)2.5 .

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3. Draw the ray diagram to show the refraction of a ray of monochromatic light passing through a glass prism. Deduce the expression for the refractive index of glass in terms of angle of prism and angle of minimum deviation.

A ray of light incident on face AB of an equilateral glass prism ABC , shows minimum deviation of 30^{0} calculate the speed of light through the prism. Also find the angle of incidence at face AB so that the emergent ray along the face AC .

4.What do you mean by the term total internal reflection also write the conditions to get the total internal reflection, define the term critical angle for a pair of media , how the critical angle is related to the refractive index of the medium. A point source is kept at the bottom of the pond 2m deep ( μ= 4/3 )draw the ray diagram and calculate the area of water surface through which light emerges out of the pond.

5. Draw a ray diagram showing the image formation of compound microscope, hence obtain the expression for magnifying power when (I) image is formed at infinity (II) image is formed at least distance of distinct vision. (a)Why must both the objective and eyepiece of a compound microscope have short wavelength? (b) while viewing through a compound microscope, why should our eyes be placed not on the eye piece but at short distance away from it for best viewing?

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6. Draw a labelled ray diagram of astronomical telescope, for(I) normal adjustment, (II) at least distance of distinct of vision. Also derive the expression for magnifying power .You are given the three lance L_{1} , L_{2 }, L_{3} having power 3D , 6D and 10D respectively with aperture 8cm,1cm and 1cm respectively, which two lances will you use as an eyepiece and objective to construct (a) compound microscope and (b) Astronomical telescope. Give reasons.

7.Explain reflecting type telescope draw the ray diagram for (I) Newtonian and (II)Cassegrainian type telescope. Write advantages of reflecting type telescope over refracting type telescope.

8.Derive lens formula in a convex lens using suitable ray diagram 1/v -1/u = 1/f . (a) A convex les is used to obtain the image of an object on the screen 10 cm from the lens . if the magnification is 19, find the focal length of the lens. (b) An illuminated object and a screen are placed 90 cm apart. Determine the focal length and nature of the lens required to produce a clear image on the screen , twice the size of the object.

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