Question 27: EM waves

Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) :
The electromagnetic wave exerts pressure on the surface on which they are allowed to fall.
Reason (R) :
There is no mass associated with the electromagnetic waves.
In the light of the above statements, choose the correct answer from the options given below :

Accepted Answer

Electromagnetic waves contain both energy (E) and momentum (p). And when an EM wave strikes a surface, it transfers its momentum to that surface.
According to Newton's Second Law, the rate of change of momentum is force, and force per unit area is pressure (radiation pressure).
Therefore, assertion (A) is true.
Electromagnetic waves consist of photons. Photons have zero rest mass. Though photons have energy and momentum, they do not have mass.
Therefore, reason (R) is true.
The reason EM waves exert pressure is because they carry momentum ( $\mathrm{p}=\frac{\mathrm{E}}{\mathrm{c}}$ ), not because they don't have mass. If waves had no momentum, they wouldn't exert pressure regardless of their mass status. Thus, while reason is a true fact about EM waves, it does not explain why they exert pressure.
So, both $(\mathrm{A})$ and $(\mathrm{R})$ are true, but $(\mathrm{R})$ is not the correct explanation of $(\mathrm{A})$.
Thus, the correct option is (B).

Answer

Both (A) and (R) are true and (R) is the correct explanation of (A)

Answer

Both (A) and (R) are true but (R) is not the correct explanation of (A)

Answer

(A) is true but (R) is false

Answer

(A) is false but (R) is true

EM waves

Choose the correct answer: