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

When two black objects, like a sphere and a disc, are exposed to a uniform beam of light, they interact with the light in different ways due to their shapes. The disc, positioned perpendicular to the light rays, will absorb and reflect light differently than the sphere.

Understanding Light Pressure

Light exerts pressure on objects when it strikes them. This pressure is related to the area that the light hits and how much light is absorbed or reflected.

Comparison of the Sphere and Disc

• Disc: The disc has a larger surface area facing the light, which means it can absorb more light and thus experience a greater force.
• Sphere: While the sphere also absorbs light, its curved surface means that less area is directly facing the light at any given moment compared to the disc.

Given these factors, the force acting on the disc will be greater than that on the sphere because of its orientation and surface area exposed to the light.

Final Answer

The correct choice is (B) bigger on the disc.