Refraction is the bending of light when it crosses from one transparent material into another, such as from air into water or glass. It matters because refraction explains lenses, eyeglasses, cameras, microscopes, rainbows, and the way objects look shifted underwater. Snell's Law gives a precise mathematical rule for predicting the new direction of a light ray at a boundary.
The key idea is that light changes speed when it enters a material with a different index of refraction.
When light enters a medium with a higher index of refraction, it slows down and bends toward the normal line. When it enters a medium with a lower index of refraction, it speeds up and bends away from the normal line. Snell's Law is written as n1 sin(theta1) = n2 sin(theta2), where angles are measured from the normal, not from the surface.
For example, light going from air into glass bends toward the normal because glass has a larger index of refraction than air.
Understanding Physics: Snell's Law and Refraction
A useful way to understand the change in direction is to picture a wavefront, which is a line joining points on a light wave that are at the same stage of vibration. If a wavefront reaches a boundary at an angle, one side enters the new material first. That side changes speed first, while the other side is still moving at its old speed.
The unequal motion makes the wavefront rotate. A ray is drawn at right angles to the wavefront, so the ray changes direction as the wavefront rotates. This is why refraction is not a force pulling light sideways at the boundary.
The frequency of light does not change when it crosses the boundary. Frequency is set by the source, such as a lamp or the Sun. Since speed changes but frequency stays fixed, wavelength must change.
In a slower material, the crests are closer together. This matters when studying color because different wavelengths usually travel at slightly different speeds in glass or water. Blue light generally changes direction more than red light in glass.
A prism can therefore spread white light into a band of colors. This effect is called dispersion.
Careful angle measurement is the main skill in refraction problems. Draw the boundary as a straight line. At the point where the ray meets it, draw a normal line at a right angle to the boundary.
Both incoming and outgoing angles are measured between each ray and this normal. A ray that travels exactly along the normal does not change direction, even though its speed and wavelength change.
Students often measure from the surface by mistake. That produces the wrong result even if every calculation step is correct.
Refraction explains why a coin at the bottom of a cup can become visible when water is added. Rays from the coin bend as they leave the water and reach the eye. The brain assumes light traveled in straight lines, so it traces those rays backward to a position closer to the surface.
The coin appears shallower than it really is. The same effect makes a swimming pool look less deep. This apparent depth can be dangerous because the visual position is not the real position.
When light travels from glass or water toward air, there is a limit to the possible outgoing angle. At one particular incoming angle, the refracted ray travels along the surface. This incoming angle is called the critical angle.
At larger incoming angles, no ray exits into the air. Instead, the light reflects completely inside the material. This is total internal reflection.
Optical fibers use repeated total internal reflection to carry signals through thin glass strands. In lab work, use a narrow ray, mark several points along each ray, and draw lines with a ruler. Small drawing errors can create large angle errors, especially when the ray is close to the surface.
Key Facts
- Snell's Law: n1 sin(theta1) = n2 sin(theta2).
- Index of refraction: n = c / v, where c is the speed of light in vacuum and v is the speed of light in the medium.
- Light slows down in a medium with larger n and speeds up in a medium with smaller n.
- From low n to high n, the ray bends toward the normal, so theta2 is smaller than theta1.
- From high n to low n, the ray bends away from the normal, so theta2 is larger than theta1.
- Typical values: nair ≈ 1.00, nwater ≈ 1.33, nglass ≈ 1.50.
Vocabulary
- Refraction
- Refraction is the change in direction of a wave as it enters a new medium where its speed is different.
- Index of refraction
- The index of refraction is a number that tells how much light slows down in a material compared with its speed in vacuum.
- Normal line
- The normal line is an imaginary line drawn perpendicular to the surface at the point where the light ray hits.
- Incident ray
- The incident ray is the incoming light ray that strikes the boundary between two media.
- Refracted ray
- The refracted ray is the light ray after it has crossed the boundary and changed direction in the new medium.
Common Mistakes to Avoid
- Measuring the angle from the surface instead of the normal is wrong because Snell's Law uses angles measured from the perpendicular normal line.
- Assuming light always bends toward the normal is wrong because it bends toward the normal only when it enters a medium with a higher index of refraction.
- Forgetting that frequency stays the same is wrong because refraction changes light speed and wavelength, but the frequency is fixed by the source.
- Using n = v / c is wrong because index of refraction is defined as n = c / v, so larger n means slower light.
Practice Questions
- 1 A ray of light travels from air into glass with n1 = 1.00 and n2 = 1.50. If the angle of incidence is 30.0 degrees, find the angle of refraction.
- 2 Light travels in water with n = 1.33. Using c = 3.00 x 10^8 m/s, calculate the speed of light in water.
- 3 A light ray travels from glass into air. Explain whether it bends toward or away from the normal and why.