Reflection, Refraction, and Dispersion
When light travels from one medium to another, three phenomena can occur; reflection, refraction, and dispersion. Reflection takes place when the light does not pass into the material, but is instead "bounced" off the surface. Refraction occurs when the light passes into the material. It is bent toward or away from the normal according to the index of refraction of the material. The normal is a line perpendicular to the surface of the medium. Dispersion occurs when different wavelengths of light are refracted different amounts, separating the light into its constituent colors. The wave property of light prevents all light from passing directly though a medium. As light passes through different mediums, part of it is reflected and part is transmitted through.
When light travels from one material to another, it is refracted, obeying Snell's Law. This law states that the sine of the angle of incidence for a ray multiplied by the refractive index for the medium, equals the product of the refractive index and the sine of the incidence angle for the next medium. Every translucent material has an index of refraction, n. This is an index that tells how the speed of light is effected by the material. Since n=c/v, (where c equals the speed of light in a vaccuum and v equals the speed of light through the medium) the higher the index, the lower the velocity of light inside the material. If the light is traveling from a lower index to a higher index, such as going from air (n=1) to glass (n=1.33), the light is bent towards the normal. If traveling from a higher index to a lower one, light is bent away from the normal. The effects of refraction are apparent when viewing fish from a boat. This is because light is being refracted as it passes from air to water. Water has a higher index of refraction so light is bent towards the normal. However, when we view an object, we assume the light travels in a straight path. As a result, we will think the fish is actually closer to the surface of the water. If you aimed a spear at the fish, you'd miss! Your best bet when spear fishing is to jab straight down (perpendicular to the water's surface) since light traveling along the normal is not refracted (the sine of zero degrees is one).
When light is reflected, the incident angle is equal to the reflection angle, as measured from the normal. When the light is exiting from a medium, total internal reflection can occur if the angle of incidence is greater than the critical angle. The critical angle is the angle that causes the light to travel along the surface of the material when refracted. Total internal reflection is the basic phenomenon behind fiber optic light; once it enters the fiber, is totally internally reflected, keeping it inside the fiber until it exits the other end.
Dispersion is the effect associated with the separation of light into colors by a prism. As the light enters the glass, it is refracted. Each wavelength of light is refracted a different amount, with the blue wavelengths being refracted the most. We know that the amount of refraction is dependent on the index, n, which is itself dependent on the velocity of the light in the medium. The velocity is wavelength dependent, v being equal to the frequency, f, multiplied by the wavelength, lambda. When light is dispersed by a prism, red light has a higher velocity in the glass and is not refracted as much as the slower, blue light. Any refractive process using non-monochromatic light has an element of dispersion. This leads to effects such as chromatic aberration in lenses. Each wavelength of light exiting from a lens that exhibits this kind of aberration has a different focal length; therefore, light is not totally recombined into white light and a slight rainbow effect can be seen in the image.
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