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CBSE Class 12 · 2018 · EM WavesShort Answer

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(a) Why are infrared waves often called heat waves? Explain.
(b) What do you understand by the statement, "Electromagnetic waves transport momentum"?

Solution

(a) Infrared waves have frequencies just below visible light and are strongly absorbed by the molecules of most substances. The absorbed energy increases the molecular (thermal) vibration, raising the body's temperature --- so they produce a heating effect and are called heat waves.
(b) An electromagnetic wave carries not only energy but also linear momentum. If total energy U is delivered to a surface, the momentum transferred is

So when the wave is absorbed (or reflected) it imparts momentum to the surface, i.e. it exerts a small radiation pressure. This is the meaning of 'transporting momentum'.

CBSE Class 12 Board · 2018 · Subjective

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