Definition Of Thermal Radiation
All the objects, which are above the absolute zero gives out (liberates) some energy. This energy travels mainly in the form of electro-magnetic waves (infra-red waves or light waves) and these waves can travel through empty space. If any object absorbs (receives) these waves, they heat up those objects. This process is knows as thermal radiation.
Example 1: On earth sun is the main source of energy.
Earth receives the energy from the sun due to thermal radiation.
More About Thermal Radiation:
- All objects give out some thermal radiation.
- As the temperature of the object is higher, more energy is radiated from the object.
- Black body absorbs more radiation.
- Black body also emits more radiation.
- Silvery body absorbs least radiation.
- Silvery body also emits least radiation.
- Thermal radiation is mainly because of infrared waves, but very hot objects also give out light waves.
- Infra-red waves are invisible.
- Radiation is the method of transformation of heat without using any medium.
Worked Example
Why people are advised to wear white clothes in summer?
A. White clothes absorb the radiation quickly.
B. White clothes do not absorb the radiation quickly.
C. White clothes are very soft.
D. White clothes emit radiation quickly.
Correct answer is B.
Solution
Step 1: Black cloth absorbs more radiation, so "A" is not correct choice.
Step 2: White objects will not absorb the radiation. Hence we will not absorb more radiation by wearing white clothes. "B" is the correct choice.
Step 3: All the white clothes are not soft, so "C" is not the correct choice.
Step 4: Black body only emits radiation quickly. White bodies will not emit the radiation quickly. So "D" is not correct choice.