Question

Two black objects of the same diameter, a sphere and a disc are placed in front of a uniform beam of light. The plane of the disc is perpendicular to the light rays. The force acting upon them by the light is:
(A) zero
(B) bigger on the disc
(C) bigger on the sphere
(D) the same on both

Answer 1

Hint:The force acting upon the plane of the disc by the light when the light ray is perpendicular to the plane of the disc. The force exerted by the beam of light depends on the power of the light and the speed of the light. By using the formula, the force acting by the light is determined.

Formula used:
The force exerted by the beam of light on the plane surface is given by,
$F = 2\dfrac{P}{C}$
Where, $F$ is the force exerted by the beam of the light on the plane surface, $P$ is the power of the light and $C$ is the speed of the light at that instant of the light touches the plane surface.

Complete answer:
Given that, two black objects of the same diameter, a sphere and a disc are placed in front of a uniform beam of light. The plane of the disc is perpendicular to the light rays. Now,
The force exerted by the beam of light on the plane surface is given by,
$F = 2\dfrac{P}{C}$
From the above equation, the force exerted by the beam of the light on the surface does not depend on the shape and colour of the object. So, the same amount of the force is exerted by the beam of the light on the plane and sphere.

Hence, the option D is the correct answer.

Note:The force exerted by the beam of the light depends on the power of the light and the speed of the light. And the force exerted by the beam of the light will not depends on the shape of the object and the colour of the object which is placed in front of the beam of the light.