Glossary
Critical Angle
The minimum angle of incidence at which light traveling from a medium with a higher index of refraction to one with a lower index of refraction will undergo total internal reflection.
Example:
For light going from water to air, if the angle of incidence is greater than the critical angle, the light will not exit the water.
Index of Refraction (n)
A dimensionless value that describes how much the speed of light is reduced in a medium compared to its speed in a vacuum. It is calculated as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v).
Example:
Water has an index of refraction of approximately 1.33, meaning light travels about 1.33 times slower in water than in a vacuum.
Normal
An imaginary line drawn perpendicular to the surface at the point where a light ray strikes the boundary between two media. Angles of incidence and refraction are always measured with respect to this line.
Example:
When drawing a ray diagram for refraction, always draw the normal first to correctly measure the angles.
Optical Density
A measure of how much a material slows down light. Materials with higher optical density cause light to slow down more.
Example:
Light travels slower in glass, which has a higher optical density, than in air.
Refraction
The bending of light as it passes from one transparent medium to another due to a change in its speed.
Example:
When you look at a straw in a glass of water, it appears to be bent or broken at the water's surface due to refraction.
Snell's Law
A formula used to describe the relationship between the angles of incidence and refraction, and the indices of refraction of two media. It states n₁ sin θ₁ = n₂ sin θ₂.
Example:
Using Snell's Law, a scientist can predict the exact angle at which a laser beam will bend when it enters a diamond.
Total Internal Reflection
A phenomenon where light traveling from a denser medium to a less dense medium is completely reflected back into the denser medium, rather than refracting out. This occurs when the angle of incidence exceeds the critical angle.
Example:
Fiber optic cables transmit data using total internal reflection to keep light signals contained within the glass strands.