What is Black Body?

What is black body ? and some important questions based on this topic which are important for class 11th examination provided in this blog –

To download the complete notes and questions based on this topic click on the link given here-

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Black Body – Class 11 Physics (CBSE)

1. What is a Black Body?

A black body is an ideal body that absorbs all the radiations incident on its surface, irrespective of their wavelength, frequency and direction.

It neither reflects nor transmits any radiation falling on it.

A black body is also called a perfect absorber of radiation.

Definition for CBSE examination:

A black body is an ideal body that absorbs completely all the radiations incident upon it, without reflecting or transmitting any part of them.

2. Important Characteristics of a Black Body

1. It absorbs 100% of the radiation falling on it.
2. It is a perfect absorber and a perfect emitter of radiation.
3. It does not reflect or transmit any incident radiation.
4. The radiation emitted by a black body depends only on its temperature.
5. At a given temperature, it emits the maximum possible thermal radiation.

3. Practical Example of a Black Body

A hollow enclosure with a small hole is considered a good practical approximation of a black body.

Working:

• When radiation enters through the small hole, it undergoes multiple reflections inside the cavity.
• At each reflection, some energy is absorbed by the inner walls.
• After several reflections, almost all the radiation gets absorbed.
• Therefore, the small hole behaves approximately like a perfect black body.

Important: A small hole in a hollow cavity is considered a practical example of a black body.

4. Black Body Radiation

The radiation emitted by a black body due to its temperature is called black body radiation.

When a black body is heated, it emits electromagnetic radiation.

For example:

• At a lower temperature, it emits mainly infrared radiation.
• At a higher temperature, it may appear red.
• At a still higher temperature, it may appear yellow or white.

Thus, the colour and intensity of radiation emitted by a black body depend upon its temperature.

5. Important Laws Related to Black Body Radiation

These laws are useful for understanding thermal radiation and are important for higher-level study.

(a) Stefan-Boltzmann Law

The total energy radiated per unit area per second by a black body is directly proportional to the fourth power of its absolute temperature.

E=σT4

Where:

• E = Energy radiated per unit area per second.
• σ = Stefan’s constant.
• T = Absolute temperature in Kelvin.

(b) Wien’s Displacement Law

The wavelength corresponding to maximum intensity of radiation is inversely proportional to the absolute temperature.

λm​T=b

Where:

λm​ = Wavelength of maximum intensity.

• T = Absolute temperature.
• b = Wien’s displacement constant.

Important CBSE-Based Questions on Black Body

The following questions are useful for Class 11 Physics revision and conceptual understanding.

A. Very Short Answer Questions (1 Mark)

1. What is a black body?
2. Why is a black body called a perfect absorber?
3. Does a black body reflect any incident radiation?
4. What is black body radiation?
5. Give one practical example of a black body.
6. What is the nature of radiation emitted by a black body?
7. Does the radiation emitted by a black body depend on its material or temperature?

B. Short Answer Questions (2 Marks)

1. Define a black body. Write any two characteristics of a black body.
2. Why is a small hole in a hollow cavity considered a good approximation of a black body?
3. What happens to the radiation emitted by a black body when its temperature increases?
4. Why is a black body considered both a perfect absorber and a perfect emitter?
5. Differentiate between a perfect reflector and a perfect absorber.

C. Long Answer Questions (3–5 Marks)

1. Explain the concept of a black body with the help of a suitable diagram.
2. Explain the construction and working of a cavity radiator as a practical approximation of a black body.
3. What is black body radiation? Explain how the nature of emitted radiation changes with temperature.
4. State and explain Stefan-Boltzmann law.
5. State Wien’s displacement law and explain the relationship between temperature and wavelength of maximum intensity.

D. Important Numerical Questions

Q1. The temperature of a black body is doubled. How many times will its energy radiated per unit area per second increase?

Solution:

According to Stefan-Boltzmann law,

E= σT ⁴

If temperature becomes 2T,

E= σ(2T) ⁴

E= 16σT ⁴

E′=16E​

Answer: The energy radiated increases 16 times.

Q2. The wavelength of maximum intensity of a black body is 500 nm500nm. Calculate its temperature using Wien’s displacement law.

Given:

λm​=500×10^-9 m

b=2.898×10−3mK

Using,

λm​T=b

T=2.898×10−3500×10−9T=500×10^-9  2.898×10^-3 ​

T=5796K​

E. Important MCQs (CBSE Pattern)

Test your understanding

5 MCQs

Q1. A black body is a body which:

A. Reflects all radiation

B. Absorbs all incident radiation

C. Transmits all radiation

D. Emits no radiation

Q2. A small hole in a hollow cavity behaves approximately as:

A. Perfect reflector

B. Perfect absorber

C. Perfect transmitter

D. Perfect insulator

Q3. The energy radiated per unit area per second by a black body is proportional to:

A. T

B. T²

C. T³

D. T⁴

Q4. Wien’s displacement law is:

A. λm/T = constant

B. λmT = constant

C. λmT² = constant

D. λm = constant

Q5. The colour of radiation emitted by a heated body changes with:

A. Its shape only

B. Its temperature

C. Its volume only

D. Its mass only

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To download the complete notes in PDF form click here-

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