Snell's law
Snell's law relates the angles of incidence and refraction to the refractive indices of the two materials: a ray bends more the greater the difference between them.
Practise this conceptSnell's law pins down exactly how much a ray bends when it crosses into a new material: the refractive index of the first material times the sine of the angle of incidence equals the refractive index of the second material times the sine of the angle of refraction. Feed in any three of those four quantities, and the law hands you the fourth.
Refractive index is a fixed property of a material for a given colour of light — it doesn't change depending on the angle at which light happens to strike it. Shine light on a piece of glass at a steep angle or a shallow one, and the glass's refractive index in the equation stays exactly the same; the angle of refraction is what adjusts to keep the law satisfied.
A ray travelling exactly along the normal — a 0° angle of incidence — passes straight through undeviated, refracting by 0° regardless of what materials are involved, since the sine of a 0° angle is itself zero on both sides of the equation.
Key ideas
Requires: Reflection and refraction
Unlocks: Dispersion and chromatic effects, Lenses and focal length, Total internal reflection
Formulas
| Symbol | Name | Unit |
|---|---|---|
| refractive index | 1 (dimensionless) | |
| refractive index | 1 (dimensionless) | |
| angle | deg (degree) | |
| angle | deg (degree) |
Common misconceptions
- A ray of light passing straight along the normal refracts by the largest possible angle.
- The refractive index of a material depends on the angle of incidence.
- Snell's law only applies to light, not to other types of waves.
Resources
Part of these UT courses
This concept is mapped as covering part of the following course(s) in the University of Tartu curriculum.
- The physical world viewLOFY.01.002
- OpticsLOFY.01.008
- Physics practical III — opticsLOFY.01.124
- Materials physics practical III — opticsLOFY.02.007