The Law of Refraction Explained

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To find out more about the diagram above . . . . read on!

The Law of Refraction Explained

Look carefully at the diagram at the top of the page. Now check out the following questions (and answers)!

  1. What is another term used for refractive index?
  2. What is the other term used for the law of refraction?
  3. What is the sine of an angle?
  4. What can be calculated if we know the refractive index of two adjacent media?
  5. How do the refractive indices of different media affect the direction in which light travels?

About the Diagram

Introducing the diagram! Read back and forward between the image at the top of the page and the explanation below!

Have you already checked out An Introduction to Reflection, Refraction and Dispersion?

It is the opening page of our Reflection, Refraction and Dispersion Series and contains masses of useful information. This is the table of contents:

Overview of this page

  • This page explains how to use the law of refraction (also called Snell’s law).
  • Related terms, including reflection, refraction and chromatic dispersion are covered on earlier pages of this series.
  • An introduction to term refractive index and how the refractive index of a transparent medium is used also appears in the series.

An overview of refraction

  • Refraction refers to the way that light (electromagnetic radiation) changes speed and direction as it travels from one transparent medium into another.
  • Refraction takes place as light travels across the boundary between different transparent media and is a result of their different optical properties.
  • Refraction is the result of the differences in the optical density of transparent media. Gases have a very low optical density whilst diamonds have a high optical density.
  • When light is refracted its path bends and so changes direction.
  • The effect of refraction on the path of a ray of light is measured by the difference between the angle of incidence and the angle of reflection.
  • As light travels across the interface between different media it changes speed.
  • Depending on the media through which light is refracted, its speed can either increase or decrease.

An overview of refraction and wavelength

  • Every wavelength of light is affected to a different degree when it encounters a medium and undergoes refraction.
  • Every wavelength of light changes both speed and direction by a different amount when it encounters a new medium and undergoes refraction.
  • The change in angle for any wavelength of light undergoing refraction within a specific transparent medium can be predicted if the refractive index of the medium is known.
  • The refractive index for a medium is calculated by finding the difference between the speed of light in a vacuum and its speed as it travels through the medium.
Colour wavelength (nm) Refractive index
Red 640 1.50917
Yellow 589 1.51124
Green 509 1.51534
Blue 486 1.51690
Violet 434 1.52136

The refractive index for crown glass is often given as being 1.52. This table shows how that figure alters with wavelength

The diagram

This diagram deals with the Law of Refraction (Snell’s law) and explains how to use the equation to predict what will happen to the direction in which light travels when it crosses the boundary between two transparent media. The law defines the relationship between the angle of incidence and angle of refraction of a ray of light with reference to the refractive indices of both media. It can be stated as follows:

When electromagnetic radiation (light) of a specific wavelength crosses the interface of any given pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.

  • Snell’s law deals with the fact that for an incident ray of light approaching the boundary of two media, the sine of the angle of incidence multiplied by the index of refraction of the first medium is equal to the sine of the angle of refraction multiplied by the index of refraction of the second medium.
  • Snell’s law deals with the fact that the sine of the angle of incidence to the sine of the angle of refraction is constant when a light ray passes across the boundary from one medium to another.
  • Snell’s law can be used to calculate the angle of incidence or refraction associated with the use of lenses, prisms and other everyday materials.
  • When using Snell’s law, the angles of incidence and refraction are measured between the path of a ray of light and the normal.
  • The normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • The speed of light in a vacuum expressed in metres per second = 299,792,458 m/sec. So = 299,792 km/sec.

Follow the blue links for definitions . . . . or check the summaries of key terms below!

Some Key Terms

Move to the next level! Check out the following terms.

Angle of incidence

The angle of incidence measures the angle at which incoming light strikes a surface. The angle of incidence is measured ...
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Angle of refraction

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media ...
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Medium

Any material through which an electromagnetic wave propagates (travels) is called a medium (plural media). In optics, a medium is ...
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Nanometre

A nanometre is a unit of measurement of the wavelength of electromagnetic radiation ...
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optical density

Optical density is a measurement of the degree to which a refractive medium slows the transmission of light. The optical ...
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Sine

In mathematics, the sine is a trigonometric function of an angle. The sine of an acute angle is defined in the context of a right-angle triangle ...
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Speed of light

The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time ...
Read More

Wavelength

Wavelength is a measurement from any point on the path of a wave to the same point on its next ...
Read More

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