Module 5
1. Module 5
1.23. Page 6
Module 5—Wave Theory of Light
Lesson Summary
At the start of this lesson you were asked the following essential questions:
- What is the cause of refraction?
- What is an index of refraction, and how does Snell’s Law use it to describe refraction mathematically?
- What is total internal reflection? How does it relate to the critical angle?
- How does a prism produce a rainbow, and how is the prism similar to a natural rainbow?
Refraction is a change in the direction of a light caused by a change in its speed as it passes at an angle from one medium to the next.
The index of refraction is a relative measure of how fast a light wave travels in a specific medium. The higher the index of refraction, the slower light moves through the medium. When light travels from a low-index medium into a high-index medium, the ray bends toward the normal as the wave slows down. When light travels from a high-index medium into a low-index medium, the ray bends away from the normal as the wave speeds up.
Mathematically, refraction is described by the general form of Snell’s Law, which is based on the transverse wave nature of the oscillating electric and magnetic fields making up light:
When light travels from a high-index medium into a low-index medium, if the angle of refraction reaches 90°, the light ray is reflected internally and does not escape the high-index medium. This occurs once the angle of incidence is larger than the critical angle, which is the boundary between refraction and total internal reflection. If the angle of incidence is less than the critical angle, light is refracted as it exits the higher-index medium. If the angle of incidence is greater than the critical angle, the light is reflected internally and does not escape the higher-index medium.
The refraction of white light as it enters and exits a prism or a water droplet produces a spectrum of colours, which is known as dispersion. Refraction demonstrates that white light is composed of a variety of colours, each associated with a specific wavelength. This occurs because the index of refraction is related to the speed of the wave and its unique wavelength, according to Snell’s Law and the universal wave equation, which is an explanation that supports the wave model of light.
Lesson Glossary
critical angle: for any two mediums, the size of the incident angle that causes the angle of refraction to be 90°
refraction: a change in the direction of a wave due to a change in its speed
refractive index: a ratio comparing the speed of light in a vacuum to its speed in a given medium
Snell’s Law: for any angle of incidence greater than zero, the ratio of 
total internal reflection: the reflection of a wave that is travelling from a high-index medium into a low-index medium at an angle equal to or greater than the critical angle