This article explains the effects of refraction and provides two rules to predict how light changes direction when it passes from one material into another.

From the previous article recall:

The Refractive index (or Index of Refraction) of a material is indicated by the letter “n.” The index impacts almost every aspect of light’s behavior, including how fast light travels through a material and how much light bends when it moves from one material into another.

Refraction is the reason that objects underwater are not always where they appear to be. A good example can be seen in this photograph of an aquarium.

Refraction causes the double image of the coral reef that can be seen by comparing the images seen through the left-hand pane of glass and the pane closer to the photographer.

From the viewpoint of the person taking the photo, all the seaweed and coral visible through the left-hand pane of glass near the children can also be seen by looking straight ahead.

The photographer sees a double image because light passing through the left-hand pane of glass bends as it exits the aquarium.

The drawing shows what is happening. As the light from the coral passes through the left-hand window, it bends toward the photographer, creating a double image. This change in direction occurs because water has a refractive index of about 1.3 while air has a refractive index of about 1.0.

There is a mathematical formula, called “Snell’s Law”, that predicts the bend angle, but it is not necessary to use mathematics to understand what light is doing—photons don’t use trigonometry to figure out where to go! The rules of refraction can be summarized in two simple rules-of-thumb:

  • Light refracts in the direction of the higher index material.
  • The amount of refraction depends on (1) the change in refractive index and (2) the angle of the ray.

So, in the case of the aquarium, the light coming straight toward the photographer does not bend because it is perpendicular to the glass. The light that comes through the left-hand pane, strikes the glass at an angle. As the light tries to leave the aquarium through the left-hand window, it bends back toward the aquarium.

An upcoming blog will show how refraction is used to create lenses. The next blog, though, deals with the curious situation of what happens when light bends too far into a higher-index material.

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