Which observation best illustrates that light behaves like particles?

• A. Light travels in space at a speed of about 3.0 x 108 m/s.
• B. Light bounces off a white cement sidewalk.
• C. Light bends as it strikes a window.
• D. Light increases in speed as it moves through matter.

Answer: (B)

Light can exhibit particle-like behavior when it interacts with matter through discrete collisions that transfer momentum. Seeing light bounce off a surface is a clear way this happens: photons strike the surface and reflect, like tiny billiard balls exchanging momentum with the surface. This direct, momentum-carrying interaction is a hallmark of the particle view of light.

Other observations either describe properties shared by waves or describe incorrect behavior in matter. Speed in a vacuum is a wave/particle property that doesn’t single out particle behavior, refraction involves bending due to speed change (explained by both models), and light actually slows in most materials, not speeds up. So the act of bouncing off a surface best illustrates the particle aspect of light.