The Law of Refraction
Snell's Law (also known as the Snell-Descartes law) is the formula used to describe the relationship between the angles of incidence and refraction when light passes through a boundary between two different isotropic media, such as water, glass, or air.
As light passes from a medium with a lower refractive index (like air) to one with a higher index (like glass), it slows down and bends toward the normal (a line perpendicular to the surface). When it moves from glass to air, it speeds up and bends away from the normal.
Applications of Snell's Law
- Eyeglasses & Contact Lenses: Use precisely shaped glass to bend light so that it focuses correctly on your retina.
- Fiber Optics: Relies on light bending so much that it stays trapped inside a glass cable.
- Atmospheric Physics: Explains why the sun appears to set several minutes later than it actually does (the atmosphere bends the light around the curve of the Earth).
The Formula
Where:
=
Refractive indices of media 1 and 2=
Angle of incidence=
Angle of refractionExample Calculation
A beam of light in air ($n_1 = 1.0$) hits a pool of water ($n_2 = 1.33$) at an angle of $30^\circ$ from the normal.
- Calculate n1 · sin(θ1): $1.0 \times \sin(30^\circ) = 0.5$.
- Divide by n2: $0.5 / 1.33 \approx 0.376$.
- Take Inverse Sine: $\arcsin(0.376) \approx 22.1^\circ$.
The light bends into the water at an angle of $22.1^\circ$.