What Causes the Dispersion of White Light?

Visible light is made of a mixture of frequencies of light. What we see as white light includes all the colors of the rainbow, from the high-frequency violet to the low-frequency red. When white light is passed through a triangular glass prism, it is separated into a spectrum of colors: red, orange, yellow, green, blue, indigo and violet. This process of separating white light into colors is known as dispersion.



Visible light is a small portion of the electromagnetic spectrum, which ranges from low-frequency radio waves to ultra-high frequency gamma rays. Higher frequency light has a shorter wavelength, and lower frequency light has a longer wavelength. Visible light ranges in wavelength from 400 to 700 nanometers. Electromagnetic radiation with longer wavelengths is known as infrared, which we recognize as heat. Electromagnetic radiation with shorter wavelengths is ultraviolet, which is the type of radiation that causes sunburns. The light will behave differently as it passes through a transparent material, depending on its wavelength.


Optical density is a term that describes how light behaves when it passes through the transparent material. If something has a high optical density, then it has a greater ability to slow light down as it passes through. As light strikes something transparent, it is absorbed by the material’s atoms, then released. This causes the light to pass from atom to atom through the material. However, the process of absorption and emission can be slightly faster or slower, depending on the light’s wavelength. Light with shorter wavelengths is slowed down more than light with longer wavelengths.


When light passes at an angle from one material to another, it will bend, or refract. This happens because the two materials have different optical densities. For example, when light passes from air to glass, the glass has a greater optical density than air. This causes the light to bend closer to the normal, which is an imaginary line perpendicular to the surface. When the light reaches the other side of the glass and exits into the air, moving from a greater optical density to a lesser one, the light bends away from the normal.


Light with shorter wavelengths bends more as the wavelengths pass from one material to another. So when white light enters one side of a prism, the violet light components are bent the most, then the indigo ones, then the blue ones, followed by green, yellow, orange and red. Were the prism a flat piece of glass, the light would emerge as a white light on the other end because it would exit at the same angle as it entered, and the light would simply bend back to its original angle. However, because a prism is triangular, the angle at which it exits is different from the one it entered. When the light exits the prism, the violet light is bent even further, as are the other colors in succession. The result is the dispersion of the white light into its full spectrum.


A prism results in an ordered dispersion of light into its component colors. However, light can also be dispersed in less-ordered ways. Uneven reflective surfaces can disperse light into its component colors by reflecting it at various angles. This is most commonly seen on the iridescent exoskeletons of some insects or on the burned side of a CD or DVD. Diffraction, which is the bending of light waves around an edge, can also disperse light into its component colors. This happens when the bent or reflected light waves interfere with each other, which is different than how a prism actually separates the pure light.

Courtesy of sciencing.com, By Kevin Carr.