Particles and Waves Reflected by a Mirror

One point of view envisions light as wave-like in nature, producing energy that traverses through space in a manner similar to the ripples spreading across the surface of a still pond after being disturbed by a dropped rock. The opposing view holds that light is composed of a steady stream of particles, much like tiny droplets of water sprayed from a garden hose nozzle. This interactive tutorial explores how particles and waves behave when reflected from a smooth surface.

The tutorial initializes with particles of monochromatic red light (photons) impacting the surface of a mirror with an incident angle of approximately 60 degrees. After bouncing away from the surface, the particles travel back into space at a reflection angle that equals the incident angle (measured from a plane perpendicular to the mirror). The Particle/Wave slider, located beneath the mirror, can be utilized to morph the beam of particles into a planar wavefront. Prior to becoming a wave, the particles align themselves in waves.

An excellent comparison of the wave and particle theories involves the differences that occur when light is reflected from a smooth, specular surface, such as a mirror. Wave theory speculates that a light source emits light waves that spread in all directions. Upon impacting a mirror, the waves are reflected according to the arrival angles, but with each wave turned back to front to produce a reversed image (Figure 1). The shape of arriving waves is strongly dependent upon how far the light source is from the mirror. Light originating from a close source still maintains a spherical, highly curved wavefront, while light emitted from a distance source will spread more and impact the mirror with wavefronts that are almost planar.

The case for a particle nature for light is far stronger with regards to the reflection phenomenon than it is for refraction. Light emitted by a source, whether near or far, arrives at the mirror surface as a stream of particles, which bounce away or are reflected from the smooth surface. Because the particles are very tiny, a huge number are involved in a propagating light beam, where they travel side by side very close together. Upon impacting the mirror, the particles bounce from different points, so their order in the light beam is reversed upon reflection to produce a reversed image, as demonstrated in Figure 1. Both the particle and wave theories adequately explain reflection from a smooth surface. However, the particle theory also suggests that if the surface is very rough, the particles bounce away at a variety of angles, scattering the light. This theory fits very closely to experimental observation.