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# Snell's Law for AP Physics B

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By McGraw-Hill Professional
Updated on Feb 12, 2011

Practice problems for these concepts can be found at:

Optics Practice Problems for AP Physics B

In addition to changing its speed and its wavelength, light can also change its direction when it travels from one medium to another. The way in which light changes its direction is described by Snell's law.

To understand Snell's law, it's easiest to see it in action. Figure 24.1 should help.

In Figure 24.1, a ray of light is going from air into water. The dotted line perpendicular to the surface is called the "normal."1 This line is not real; rather it is a reference line for use in Snell's law. In optics, ALL ANGLES ARE MEASURED FROM THE NORMAL, NOT FROM A SURFACE!

As the light ray enters the water, it is being bent toward the normal. The angles θ1 and θ2 are marked on the figure, and the index of refraction of each material, n1 and n2, is also noted. If we knew that θ1 equals 55°, for example, we could solve for θ2 using Snell's law.

(1.00)(sin 55°) = (1.33)(sin θ2)

Whenever light goes from a medium with a low index of refraction to one with a high index of refraction—as in our drawing—the ray is bent toward the normal. Whenever light goes in the opposite direction—say, from water into air—the ray is bent away from the normal.

If you have a laser pointer, try shining it into some slightly milky water … you'll see the beam bend into the water. But you'll also see a little bit of the light reflect off the surface, at an angle equal to the initial angle. (Careful the reflected light doesn't get into your eye!) In fact, at a surface, if light is refracted into the second material, some light must be reflected.

Sometimes, though, when light goes from a medium with a high index of refraction to one with a low index of refraction, we could get total internal reflection. For total internal reflection to occur, the light ray must be directed at or beyond the critical angle.

Again, pictures help, so let's take a look at Figure 24.2.

In Figure 24.2, a ray of light shines up through a glass block. The critical angle for light going from glass to air is 42°; however, the angle of the incident ray is greater than the critical angle. Therefore, the light cannot be transmitted into the air. Instead, all of it reflects inside the glass. Total internal reflection occurs anytime light cannot leave a material.

Practice problems for these concepts can be found at:

Optics Practice Problems for AP Physics B

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