Light Reflection and Refraction

Light is a complex phenomena. It exhibits both wave*-like and particle-like properties. Its exact nature is not fully understood and this complexity makes it difficult for one model to describe all of light’s properties. As a result, different models describe different aspects of light’s behavior. The electromagnetic wave theory explains light’s ability to travel through a vacuum. The light ray model, on the other hand, can describe interactions between light and matter.


*note - In this illustration the angle of the transmitted ray is called the angle of transmission and labeled as θt. Many sources (including other pages on this site) refer to this angle as the angle of refraction*.

The light ray model makes the assumption that light travels in a straight line through transparent media such as air or water. The model also assumes that light rays behave in a predictable manner when they encounter surfaces such as an interface between different media (air and water for example) or the surface of an opaque object. This makes it possible to predict the path a light ray will follow as it move from its point of origin to wherever it eventually changes into another form of energy such as heat.

Everyday examples of light encountering surfaces include movement of light from air into the water in a swimming pool, through the glass of a window pane or onto an opaque surface such as a rock or the back of your hand.

When a light ray encounters a surface, one or more of the following three things occur, the light ray:

  1. Reflects off the surface and travels off in a different direction.
  2. Passes from one medium into the other and continue on a new, straight line path.
  3. Is absorbed.

Often, more than one of these occur. The behavior that dominates depends on the type of interface and the angle at which the light ray falls on the surface.

The following terms are used to describe the behavior of light rays: The light ray that hits a surface is the incident ray*. The angle it hits the surface is the angle of incidence. This angle is defined as the angle the incident ray makes with a line normal* (perpendicular) to the surface. The path a light ray will follow after hitting a surface can be predicted based on the incident angle and information about the surface.

Light rays that reflect follow the law of reflection. The law of reflection states that the angle of reflection* is equal to the angle of incidence.

Light rays that pass through an interface are transmitted rays. These rays bend. This bending is called refraction. The direction and magnitude of refraction depends on the relative densities of the two media and the angle of incidence.

Refracted light rays behave in the following ways:

  • Light traveling from a more dense to a less dense medium refract away from the normal. This behavior is exhibited by light moving from water to air.
  • Light traveling from a less dense to a more dense medium refract towards the normal. This behavior is exhibited by light moving from air into water.
  • The greater the difference in density* between the two media, the greater the refraction.
  • The greater the incident angle, the greater the refraction.
  • For light traveling from a more dense to a less dense medium, there is a critical angle* beyond which the light will not pass through the interface. A light ray with an angle of incidence equal to or greater than the critical angle will reflect, following the law of reflection.

The interactive illustration at the top of this page explores patterns of reflection and refraction of light rays that hit a variety of interfaces at different angles of incidence. Below is a video overview of the concepts covered. Test your understanding of the concepts covered with the reflection and refraction problem set.

Reflection / Refraction Overview

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