Refraction

Learning objectives

Section Key Terms

The Law of Refraction

You may have noticed some odd optical phenomena when looking into a fish tank. For example, you may see the same fish appear to be in two different places (Figure_16_02_Fish). This is because light coming to you from the fish changes direction when it leaves the tank and, in this case, light rays traveling along two different paths both reach our eyes. The changing of a light ray’s direction (loosely called bending) when it passes a boundary between materials of different composition, or between layers in single material where there are changes in temperature and density, is called refraction. Refraction is responsible for a tremendous range of optical phenomena, from the action of lenses to voice transmission through optical fibers.

Why does light change direction when passing from one material (medium) to another? It is because light changes speed when going from one material to another. This behavior is typical of all waves and is especially easy to apply to light because light waves have very small wavelengths, and so they can be treated as rays. Before we study the law of refraction, it is useful to discuss the speed of light and how it varies between different media.

The speed of light is now known to great precision. In fact, the speed of light in a vacuum, c, is so important, and is so precisely known, that it is accepted as one of the basic physical quantities, and has the fixed value

where the approximate value of 3.00 $\times$ 10⁸ m/s is used whenever three-digit precision is sufficient. The speed of light through matter is less than it is in a vacuum, because light interacts with atoms in a material. The speed of light depends strongly on the type of material, given that its interaction with different atoms, crystal lattices, and other substructures varies. We define the index of refraction, n, of a material to be

where v is the observed speed of light in the material. Because the speed of light is always less than c in matter and equals c only in a vacuum, the index of refraction (plural: indices of refraction) is always greater than or equal to one.

Table_16_02_Refractive_Indices lists the indices of refraction in various common materials.

Figure_16_02_Lawnmower provides an analogy for and a description of how a ray of light changes direction when it passes from one medium to another. As in the previous section, the angles are measured relative to a perpendicular to the surface at the point where the light ray crosses it. The change in direction of the light ray depends on how the speed of light changes. The change in the speed of light is related to the indices of refraction of the media involved. In the situations shown in Figure_16_02_Lawnmower, medium 2 has a greater index of refraction than medium 1. This difference in index of refraction means that the speed of light is less in medium 2 than in medium 1. Note that, in Figure_16_02_Lawnmower(a), the path of the ray moves closer to the perpendicular when the ray slows down. Conversely, in Figure_16_02_Lawnmower(b), the path of the ray moves away from the perpendicular when the ray speeds up. The path is exactly reversible. In both cases, you can imagine what happens by thinking about pushing a lawn mower from a footpath onto grass, and vice versa. Going from the footpath to grass, the right front wheel is slowed and pulled to the side as shown. This is the same change in direction for light when it goes from a fast medium to a slow one. When going from the grass to the footpath, the left front wheel moves faster than the others, and the mower changes direction as shown. This, too, is the same change in direction as light going from slow to fast.

The amount that a light ray changes direction depends both on the incident angle and the amount that the speed changes. For a ray at a given incident angle, a large change in speed causes a large change in direction, and thus a large change in the angle of refraction. The exact mathematical relationship is the law of refraction, or Snell’s law, which is stated in equation form as

In terms of speeds, Snell’s law becomes

Here, n₁ and n₂ are the indices of refraction for media 1 and 2, respectively, and θ₁ and θ₂ are the angles between the rays and the perpendicular in the respective media 1 and 2, as shown in Figure_16_02_Lawnmower. The incoming ray is called the incident ray and the outgoing ray is called the refracted ray. The associated angles are called the angle of incidence and the angle of refraction. Later, we apply Snell’s law to some practical situations.

Dispersion is defined as the spreading of white light into the wavelengths of which it is composed. This happens because the index of refraction varies slightly with wavelength. Figure_16_02_Prism shows how a prism disperses white light into the colors of the rainbow.

Rainbows are produced by a combination of refraction and reflection. You may have noticed that you see a rainbow only when you turn your back to the Sun. Light enters a drop of water and is reflected from the back of the drop, as shown in Figure_16_02_Rainbow. The light is refracted both as it enters and as it leaves the drop. Because the index of refraction of water varies with wavelength, the light is dispersed and a rainbow is observed.

A good-quality mirror reflects more than 90 percent of the light that falls on it; the mirror absorbs the rest. But, it would be useful to have a mirror that reflects all the light that falls on it. Interestingly, we can produce total reflection using an aspect of refraction. Consider what happens when a ray of light strikes the surface between two materials, such as is shown in Figure_16_02_Total_internal(a). Part of the light crosses the boundary and is refracted; the rest is reflected. If, as shown in the figure, the index of refraction for the second medium is less than the first, the ray bends away from the perpendicular. Because n₁ > n₂, the angle of refraction is greater than the angle of incidence—that is, ${\theta }_{\text{2}}$ > ${\theta }_{\text{1}}$ . Now, imagine what happens as the incident angle is increased. This causes ${\theta }_{\text{2}}$ to increase as well. The largest the angle of refraction, ${\theta }_{\text{2}}$, can be is 90°, as shown in Figure_16_02_Total_internal(b). The critical angle, ${\theta }_{\text{c}}$, for a combination of two materials is defined to be the incident angle, ${\theta }_{\text{1}}$, which produces an angle of refraction of 90°. That is, ${\theta }_{\text{c}}$ is the incident angle for which ${\theta }_{\text{2}}$ = 90°. If the incident angle, ${\theta }_{\text{1}}$, is greater than the critical angle, as shown in Figure_16_02_Total_internal(c), then all the light is reflected back into medium 1, a condition called total internal reflection.

Recall that Snell’s law states the relationship between angles and indices of refraction. It is given by

When the incident angle equals the critical angle ( ${\theta }_{\text{1}}$ = ${\theta }_{\text{c}}$ ), the angle of refraction is 90° ( ${\theta }_{\text{2}}$ = 90°). Noting that sin 90° = 1, Snell’s law in this case becomes

The critical angle, ${\theta }_{\text{c}}$, for a given combination of materials is thus

for n₁ > n₂.

Total internal reflection occurs for any incident angle greater than the critical angle, ${\theta }_{\text{c}}$, and it can only occur when the second medium has an index of refraction less than the first. Note that the previous equation is written for a light ray that travels in medium 1 and reflects from medium 2, as shown in Figure_16_02_Total_internal.

There are several important applications of total internal reflection. Total internal reflection, coupled with a large index of refraction, explains why diamonds sparkle more than other materials. The critical angle for a diamond-to-air surface is only 24.4°; so, when light enters a diamond, it has trouble getting back out (Figure_16_02_Diamond). Although light freely enters the diamond at different angles, it can exit only if it makes an angle less than 24.4° with the normal to a given surface. Facets on diamonds are specifically intended to make this unlikely, so that the light can exit only in certain places. Diamonds with very few impurities are very clear, so the light makes many internal reflections and is concentrated at the few places it can exit—hence the sparkle.

A light ray that strikes an object that consists of two mutually perpendicular reflecting surfaces is reflected back exactly parallel to the direction from which it came. This parallel reflection is true whenever the reflecting surfaces are perpendicular, and it is independent of the angle of incidence. Such an object is called a corner reflector because the light bounces from its inside corner. Many inexpensive reflector buttons on bicycles, cars, and warning signs have corner reflectors designed to return light in the direction from which it originates. Corner reflectors are perfectly efficient when the conditions for total internal reflection are satisfied. With common materials, it is easy to obtain a critical angle that is less than 45°. One use of these perfect mirrors is in binoculars, as shown in Figure_16_02_Binoculars. Another application is for periscopes used in submarines.

Fiber optics are one common application of total internal reflection. In communications, fiber optics are used to transmit telephone, internet, and cable TV signals, and they use the transmission of light down fibers of plastic or glass. Because the fibers are thin, light entering one is likely to strike the inside surface at an angle greater than the critical angle and, thus, be totally reflected (Figure_16_02_Fiber). The index of refraction outside the fiber must be smaller than inside, a condition that is satisfied easily by coating the outside of the fiber with a material that has an appropriate refractive index. In fact, most fibers have a varying refractive index to allow more light to be guided along the fiber through total internal reflection. Rays are reflected around corners as shown in the figure, making the fibers into tiny light pipes.

Calculations with the Law of Refraction

The calculation problems that follow require application of the following equations:

and

These are the equations for refractive index, the mathematical statement of the law of refraction (Snell’s law), and the equation for the critical angle.

Practice Problems

Check Your Understanding

Section Summary

• The index of refraction for a material is given by the speed of light in a vacuum divided by the speed of light in that material.
• Snell’s law states the relationship between indices of refraction, the incident angle, and the angle of refraction.
• The critical angle, ${\theta }_{\text{c}}$, determines whether total internal refraction can take place, and can be calculated according to ${\theta }_c=si{n}^{-1}\left(\frac{n_2}{n_1}\right)$ .

Key Equations

Concept Items

Critical Thinking

Problems

Test Prep Multiple Choice

Test Prep Short Answer

Test Prep Extended Response

Glossary

angle of refraction

the angle between the normal and the refracted ray

critical angle

an incident angle that produces an angle of refraction of 90°

dispersion

separation of white light into its component wavelengths

incident ray

the incoming ray toward a medium boundary or a reflective surface

index of refraction

the speed of light in a vacuum divided by the speed of light in a given material

refracted ray

the light ray after it has been refracted

Snell’s law

the law of refraction expressed mathematically as $n_1\sin {\theta }_1=n_2\sin {\theta }_2$

total internal reflection

reflection of light traveling through a medium with a large refractive index at a boundary of a medium with a low refractive index under conditions such that refraction cannot occur