Physics Essay 1.5 updated Fall 2011

Refraction

What happens to light when it strikes a surface, besides being reflected?

When an incident ray strikes a surface, it can also pass through the surface and into the material. As it passes out of the first material, through the surface, and into the second material it usually bends its path at the surface. This bending is called refraction and the two materials are referred to as medium-one and medium-two.

As the light beam passes into medium-2 (the glass block), it bends or changes paths. This is called refraction. Notice how all of the refraction occurs right at the surface.

As with reflection, we can draw a normal that is at a 90o angle to the surface and use this line to measure an incident angle and a refracted angle. In the picture below the incident ray bends toward the normal line as it refracts into the prism. This means that as the ray enters the glass, instead of continuing along a straight line, the ray bends to a path that is closer to the normal as it moves from the air into the prism.

refracted ray

incident ray

incident

angle

refracted

angle

39o

52o

normal

In the photograph below, notice how the ray refracts as it moves out of the prism and into the air. This time it refracts away from the normal. In other words, as the incident ray passes through the surface, it does not travel in a straight line, but refracts (or bends) to a path that is farther away from the normal. Click here to see a demo where the incident angle changes for light entering a prism.

Click here (EWS) to view a movie of a laser beam refracting into and out of water with an changing angle of incidence.

In the movie, how does the angle of refraction change as the angle of incidence becomes larger? When the angle of incidence is zero (when the incident ray is parallel to the normal), what is the refracted angle?

What happens to the angle of refraction, as the angle of incidence becomes larger?

refracted ray

incident ray

incident angle

reflected angle

35o

normal

Looking at a second half of the video demo, what happens to the ray that refracts back out into the air? As the ray of light moves through the water and strikes the surface, and then moves out into the air, how does it refract? The rays I am referring to are similar to the rays in the picture below.

In the movie, how does the angle of refraction change for this second refracted ray as the angle of incident changes?

In the EWS demo movie, what happens to the refracted ray as the angle of incidence becomes very large for the situation where light is trying to leave the water? Does a ray disappear at this point? Which ray disappears?

As the incident ray tries to leave the water and as the angle of incidence becomes large, no refraction occurs and all light is reflected back into the water. This process is called total internal reflection. It only occurs when a light ray in a more dense substance (i.e. plastic or glass) strikes a surface or boundary with a less dense substance (i.e. air). The angle of incidence at which the refracted ray disappears is called the critical angle. Make sure to watch the demo videos carefully so you can understand the process of total internal reflection and the critical angle.

Total Internal Reflection occurs when the incident angle is large enough that all light is reflected back into the prism and none is refracted out of the prism. The minimum angle at which this happens is called the critical angle.

Air

Glass