Theories of Light

Newton's theory - light consists of particles called corpuscles; this theory only explained reflection.

Wave theory of light (Maxwell's theory) - light behaves like a wave; this explained all the properties of light such as reflection, refraction, diffraction, and interference; it did not explain the photoelectric effect or radiation produced by an incandescent light.

Quantum theory (Einstein's theory) - light has a dual nature; when light is transmitted through space or matter, it behaves like a wave; when light is emitted or absorbed, it behaves like a particle called a photon.

Maxwell's theory of light as an electromagnetic wave:

-a changing electric field will produce a magnetic field
-a changing magnetic field will produce an electric field

A magnetic field is produced in empty space by a changing electric field. Maxwell hypothesized that if a changing magnetic field produces an electric field, the electric field must also be changing. Maxwell found that the net result of these interacting fields was the production of a wave of magnetic and electric fields traveling through space at a speed of 3 x 108 m/s. Thus, light is an electromagnetic wave.



A photocell is used in the experiment. When placed in the dark, the galvanometer reads zero. When light shines on a metal plate in the photocell, the galvanometer detects a current. When a variable voltage is used and the terminals reversed, a point is reached where no current is detected. This voltage is measures the maximum kinetic energy of the ejected electrons (or photoelectrons). The current flow does not depend upon the intensity of the light used, but upon the frequency of the light used.

Maxwell's wave theory predicts that as the intensity of light is increased, the current flow should increase. The frequency should not affect the maximum kinetic energy of the photoelectrons. According to this theory, the electric field of the electromagnetic wave exerts a force on the electrons in the metal and some are ejected from the surface.

Einstein's photon theory predicts that only the frequency of the light used affects the maximum kinetic energy of the photoelectrons. As the intensity of light is increased, no change is seen in the maximum kinetic energy of the photoelectrons. No photoelectrons are ejected until a minimum value of energy is reached, no matter how great the intensity. After this point, the maximum kinetic energy of the photoelectrons increases linearly as the frequency of light used increases.