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.
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.