Notes

The Newtonian Concept of Light
For centuries, scientists tried to determine just what light was and why it acted the way it did. At the time of Sir Isaac Newton (the 1700s), most scientists agreed that light must be particle-like, since light appeared to travel in a straight line and could pass through a vacuum. Because sound, which was known to be waves, could not travel through a vacuum, it seemed apparent that light must consist of particles. These particles, called corpuscles by Newton, could travel through a vacuum like little bullets and did travel in a straight line.
Using this assumption, Newton was able to prove the basic laws of reflection and refraction of light. Although scientists continued to believe in this model for a long time, subsequent experiments indicated that a wave model would be more appropriate to explain some light phenomena.
In the figure below, the shadow formations on the screen indicate that light having either a point source or an extended source travels in a straight line. In (A), the point light source behind the object casts a black shadow. With an extended light source, there will be both partially lighted and unlighted spaces beyond the object (B). Here, the shadow cast consists of a black central portion called the umbra, surrounded by a lighter, less distinct shadow called the penumbra. The penumbra gradually blends into the illuminated regions of the screen.
This illustration shows light traveling in a straight line. (A) shows a point light source and (B) shows an extended light source.
The Concept of Light as a Wave
As long as rather large objects are used, the results of experiments aren’t affected by the wave properties of light. In the figure above, the large object used to interrupt the light seems to cast a shadow that’s an exact replica of the object, but larger. If a similar demonstration was attempted using sound, such a clear-cut shadow would never be formed because of the diffraction of the sound waves around the edge of the object.
Careful observations of the behavior of light by scientists such as Fresnel, Fraunhofer, and Arago showed that interference and diffraction effects similar to those observed in sound also affect light. It’s easy to understand why these effects weren’t observed earlier once you realize that the sizes of sound waves and light waves differ so much in magnitude. A 1,000-hertz sound wave, for example, has a wavelength of approximately 0.34 meter, while green light has a much smaller wavelength of 0.00000055 meters. The interference and diffraction effects of light are proportional to the wavelength causing them. Even more time elapsed before anyone could explain how light waves can pass through a vacuum.
Sound, which also travels in waves, doesn’t travel through a vacuum.
Studies of electromagnetic waves by James Clerk Maxwell in 1865 showed that all electromagnetic waves were self-propagating and could pass through a vacuum. Self-propagation (reproducing itself) means that light doesn’t need a medium for propagation, unlike sound waves. Maxwell’s work removed a major objection to thinking of light as a wave motion.
Is Light a Wave or a Particle?
Electromagnetic waves comprise many different waves, including radio and television, ultraviolet, and visible light. These different types of waves all have the same velocity, all are transverse waves, and all are governed by the same mathematical relationships. A diagram of a transverse wave is shown in the figure.
The wave shown here is a transverse wave because the displacement of particles is in the direction of the y-axis, although the wave is traveling in the direction of the x-axis.
Maxwell’s work seemed to prove once and for all that light was a wave and not a particle, but theories published by Albert Einstein in 1905 raised the whole question again. Einstein described the photoelectric effect of light. A metal bombarded by light was observed to give off electrons, the negatively charged particles that form part of an atom. Einstein’s explanation attributed a particle-like effect to the light; that is, it behaved like an occurrence in mechanics where a particle with momentum and energy is observed to transfer some of its momentum and energy to another particle when they collide. However, Einstein noted, the electron wouldn’t be emitted from the metal unless the frequency of the light striking the metal had some minimum value. In effect, this frequency requirement means that light must be a wave, but the exchange of momentum and energy would seem to indicate that it’s also a particle.
Wave Motion of a Particle
Some scientists accepted the idea of a particle moving in a wave motion. In 1924, scientist Louis de Broglie advanced a theory of matter waves that allowed for a wave motion of particles. He developed equations that correlated the momentum of a particle with its wavelength. It remained for Davisson and Germer, working in the Bell Telephone Laboratories in 1927, to show that electrons exhibited a wave motion with a wavelength like that predicted by de Broglie’s theory. The effect noted by Davisson and Germer was the diffraction of electrons when they were used to bombard a sample of nickel. The actual observation of wave effects associated with solid particles, such as electrons, made it easier to accept the fact that light can be both particle-like and wave-like at the same time.
This dual nature of light is a distinct advantage. When working with optical systems (such as those using mirrors and lenses in image formation), you can, for all practical purposes, neglect the wave effects and assume that light is a particle-like phenomenon and travels in a straight line. When you’re working with systems that are small and have components with openings or dimensions close to the wavelength of light, you can treat light as a wave motion. Where both the wave nature and the particle nature are important (such as in the photoelectric effect), don’t be hesitant to use both the wave nature and the particle nature of light.