Question: Q. 4. (a) Why are infra-red waves often called heat waves? Explain.
(b) What do you understand by the statement, “Electromagnetic waves transport momentum”?
A&E [CBSE D/OD 2018]
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Solution:
Ans. (a) Reason for calling IR rays as heat rays 1 (b) Explanation for transport of momentum 1
(a) Infrared rays are readily absorbed by the (water) molecules in most of the substances and hence increase their thermal motion.
(If the student just writes that “infrared rays produce heating effects”, award $1 / 2$ mark only) 1
(b) Electromagnetic waves can set (and sustain) charges in motion. Hence, they are said to transport momentum. (Also accept the following : Electromagnetic waves are known to exert ‘radiation pressure’. This pressure is due to the force associated with rate of change of momentum. Hence, EM waves transport momentum).
[CBSE Marking Scheme 2018]
