Table of Contents
What is Snell's Law?
Snell's Law (also called the Law of Refraction) describes how light bends when it passes from one medium to another with a different refractive index. When light enters a denser medium (higher refractive index), it bends toward the normal; when entering a less dense medium, it bends away from the normal.
This fundamental law of optics was discovered by Willebrord Snellius in 1621 and independently by Rene Descartes. It is essential for understanding lenses, prisms, fiber optics, and virtually all optical systems.
The Formula
Refractive Indices
| Medium | Refractive Index (n) |
|---|---|
| Vacuum | 1.000 |
| Air | 1.0003 |
| Water | 1.333 |
| Glass (crown) | 1.52 |
| Glass (flint) | 1.62 |
| Diamond | 2.417 |
Total Internal Reflection
When light travels from a denser to a less dense medium (n1 > n2), there exists a critical angle beyond which all light is reflected back into the denser medium instead of being refracted. This phenomenon, called total internal reflection, is the basis of fiber optic communication, where light bounces along the fiber core without escaping.
Frequently Asked Questions
What causes refraction?
Refraction occurs because light travels at different speeds in different media. In a denser medium, light slows down. The refractive index n = c/v where c is the speed of light in vacuum and v is the speed in the medium. The change in speed at the interface causes the wavefront to bend.
What is the critical angle for glass-air?
For crown glass (n=1.52) to air (n=1.0), the critical angle is arcsin(1.0/1.52) = 41.1 degrees. Any light hitting the glass-air interface at an angle greater than 41.1 degrees from the normal will undergo total internal reflection.
What is the Brewster angle?
The Brewster angle is the angle of incidence at which reflected light is completely polarized. It equals arctan(n2/n1). At this angle, the reflected and refracted rays are perpendicular. Polarized sunglasses exploit this effect to reduce glare.