Law of Refraction Explained
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The 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.
Description
Law of Refraction Explained
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About the diagram
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.
Some key terms
The angle of incidence refers to the angle at which incoming light strikes a surface and is measured between a ray of incoming light and an imaginary line called the normal.
- In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
- The angle at which incident light from the Sun or a light bulb strikes a surface can affect the outcome. For instance, when incident light hits a mirror, the angle of incidence determines the angle of reflection.
The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time.
- The speed of light is measured in metres per second (m/s).
- Light travels through a vacuum at 300,000 kilometres per second.
- The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
- Light travels through other media at lower speeds.
- A vacuum is a region of space that contains no matter.
- Matter is anything that has mass and occupies space by having volume.
- When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
- In other contexts empty space is not considered to be a medium because it does not contain matter.
The angle of refraction measures the angle to which light bends as it crosses the boundary between different media.
- The angle of refraction is measured between the bent ray and an imaginary line called the normal.
- In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
- Snell’s law is a formula used to describe the relationship between the angle of incidence and the angle of refraction when light crosses the boundary between transparent media, such as water and air or water and glass.
The angle of incidence measures the angle at which incoming light strikes a surface.
- When light is travelling towards something it is said to be incident to that surface or object.
- The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
- In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
- Incident light may have travelled from the Sun or a man-made source or may have already been reflected off another surface such as a mirror.
- When incident light strikes a surface or object it may undergo absorption, reflection, refraction, transmission or any combination of these optical effects.
The angle of incidence measures the angle at which incoming light strikes a surface.
- The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
- In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
- If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.
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