Refraction – Slower to Faster Medium

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The diagram shows an incident ray of white light approaching the boundary between air and glass.

  • As the ray travels through the glass and then crosses the boundary and encounters air it bends away from the normal (the dotted green line).
  • The incident ray of light is refracted away from the normal because the ray travels from glass, the slower, more optically dense medium with a higher refractive index into the air, a faster, less optically dense medium with a lower refractive index.
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Description

Refraction - Slower to Faster Medium

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Give an example of a medium through which light travels more quickly?
The maximum speed of light occurs in a vacuum! Light travels in air at 99% of the speed of light in a vacuum.
Does light undergo refraction
Yes! When light leaves a vacuum or travels from one transparent medium into another, it undergoes refraction causing it to change both direction and speed.
If the refractive indices are known, can the angle of refraction be calculated for a ray of light travelling from one medium to another?
Yes! The angle of refraction for a ray of light travelling from one transparent medium to another can be calculated using the law of refraction (Snell's law) if both refractive indices are known.
How is the refractive index of a medium calculated?
The refractive index of any medium can be calculated by dividing the speed at which light in a vacuum by the speed of light in the medium.
How do the refractive indices of different media affect the direction in which light travels?
When light crosses a boundary into a medium with a high refractive index (eg. diamond = 2.42) there is a significant change in direction and speed. When light crosses a boundary into a medium with a low refractive index (eg. water = 1.333) there is a less significant change in direction or speed.

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 provides an introduction to refraction.
  • It looks at the path of white light rather than at the paths of the different wavelengths that white light contains.
  • Related topics including reflection and dispersion are covered on other pages of this series.
  • Introductions to the terms refractive index and the law of refraction (sometimes called Snell’s law) also appear on later pages in the series.

An overview of refraction

  • Refraction refers to the way that light (electromagnetic radiation) changes direction and speed 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.
  • When light is refracted its path bends and so changes direction.
  • When light undergoes refraction its speed (velocity) changes.
  • 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 (such as between air and glass) it changes speed.
  • Depending on the media through which light is refracted, its speed can increase or decrease.

About the diagram

  • This diagram shows an incident ray of white light approaching the boundary between glass and air.
  • The diagram shows that the angle of incidence and the angle of refraction of the ray of light are different as a result of refraction.
  • As the ray crosses the boundary from the glass and encounters the air, it bends away from the normal (the dotted green line) because air is an optically less dense medium (with a lower refractive index than glass) that causes it to increase in speed.
  • Imagine running into the wind. It’s always harder to run in water because it is a physically denser medium than air. The physical density of a substance is similar in that sense to the optical density of a transparent medium.

Refraction

  • When light crosses the boundary between two different transparent media it undergoes refraction.
  • The effect of refraction is that light changes speed along with its direction of travel.
  • As the speed of light changes so does its wavelength but the frequency and so the colour an observer sees remains the same.
  • The result of the change in direction is that rays either bend towards or away from the normal.
  • The normal is an imaginary line drawn on a ray diagram at right angles (perpendicular) to the boundary between two media.
  • The change between the angle of incidence and the angle of refraction of a ray of light is always measured between the ray and the normal.
  • Whether light bends towards or away from the normal depends on the difference in optical density of the new medium it encounters.
  • An incident ray of light is refracted towards the normal and slows down when it travels from air into glass. Compared with air, glass is a slower, more optically dense medium (with the higher refractive index).
  • An incident ray of light is refracted away from the normal and speeds up when it travels from glass into air. Compared with glass, air is a faster, less optically dense medium (with a lower refractive index).

Calculating the angle of refraction

  • The direction in which a ray bends, and the precise angle, can be calculated if the type and refractive indices of both media are known.
  • The effect of refraction can be calculated using a neat little equation called the law of refraction (also known as Snell’s law).
  • If three of the variables are known, the law of refraction can be used to calculate the fourth.
  • Tables of refractive indices are available for common materials so that the change in direction of a ray can be calculated.
  • Tables of refractive indices for common materials often provide both the refractive index for white light as well as indices for specific wavelengths.

For an explanation of the refractive index (index of refraction) of a medium see: Refractive Index Explained.

For an explanation of how to use the refractive index of a medium see: How to Use the Refractive Index of a Medium.

For an explanation of the Law of Refraction see: Snell’s Law of Refraction Explained.

Incident light

  • Incident light refers to incoming light that is travelling towards an object or medium.

White light

  • White light is the name given to visible light that contains all wavelengths of the visible spectrum at equal intensities.
  • The sun emits white light because sunlight contains equal amounts of all of the wavelengths of the visible spectrum.
  • As light travels through a vacuum or a medium it is described as white light if it contains all the wavelengths of visible light.
  • As light travels through a vacuum or the air it is invisible to our eyes.
  • White light is what an observer sees when all the colours that make up the visible spectrum strike a white or neutral coloured surface.

Visible spectrum

  • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
  • Human beings don’t see wavelengths of visible light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
  • The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.

Angle of incidence

  • 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.

Angle of refraction

  • The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.
  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.

Optical density

  • Optical density is a measurement of the degree to which a medium slows the transmission of light.
  • The more optically dense a material, the slower light travels.
  • The less optically dense a material, the faster light travels.

The normal

  • In geometry, the normal is a line that intersects another line.
  • In optics, the normal is an imaginary line drawn on a ray diagram at right angles (perpendicular) to the boundary between two media.
  • The normal is often used to measure angles against.

Some key terms

Angle of refraction

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

  • The angle of refraction is measured between a ray of 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.
  • See this diagram for an explanation: Refraction of a ray of light
  • If the boundary between the media is curved, the normal is drawn perpendicular to the boundary.
Refractive index

The refractive index (index of refraction) of a medium measures how much the speed of light is reduced when it passes through a medium compared to its speed in a vacuum.

  • Refractive index (or, index of refraction) is a measurement of how much the speed of light is reduced when it passes through a medium compared to the speed of light in a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • The refractive index can vary with the wavelength of the light being refracted. This phenomenon is called dispersion, and it is what causes white light to split into its constituent colours when it passes through a prism.
  • The refractive index of a material can be affected by various factors such as temperature, pressure, and density.
Medium

In physics and optics, a medium refers to any material through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.

  • Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
  • The properties of the medium, such as its density and composition, influence how light propagates through it.
  • Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
  • Examples of Mediums:
    • Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
    • Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
    • Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.
Index of refraction

The refractive index (index of refraction) of a medium measures how much the speed of light is reduced when it passes through a medium compared to its speed in a vacuum.

  • Refractive index (or, index of refraction) is a measurement of how much the speed of light is reduced when it passes through a medium compared to the speed of light in a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • The refractive index can vary with the wavelength of the light being refracted. This phenomenon is called dispersion, and it is what causes white light to split into its constituent colours when it passes through a prism.
  • The refractive index of a material can be affected by various factors such as temperature, pressure, and density.
White light

White light is the term for visible light that contains all wavelengths of the visible spectrum at equal intensities.

  • The sun emits white light because sunlight contains all the wavelengths of the visible spectrum in roughly equal proportions.
  • Light travelling through a vacuum or a medium is termed white light if it includes all wavelengths of visible light.
  • Light travelling through a vacuum or air is not visible to our eyes unless it interacts with something.
  • The term white light can have two meanings:
    • It can refer to a combination of all wavelengths of visible light travelling through space, regardless of observation.
    • What a person sees when all colours of the visible spectrum hit a white or neutral-coloured surface.
Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the boundary between one transparent medium and another.

  • Light bends towards the normal and slows down when it moves from a fast medium (like air) to a slower medium (like water).
  • Light bends away from the normal and speeds up when it moves from a slow medium (like diamond) to a faster medium (like glass).
  • These phenomena are governed by Snell’s law, which describes the relationship between the angles of incidence and refraction.
  • The refractive index (index of refraction) of a medium indicates how much the speed and direction of light are altered when travelling in or out of a medium.
  • It is calculated by dividing the speed of light in a vacuum by the speed of light in the material.
  • Snell’s law relates the angles of incidence and refraction to the refractive indices of the two media involved.
  • Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the refractive indices.
Angle of incidence

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.
  • See this diagram for an explanation: Reflection of a ray of light
  • 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.
Law of refraction

As light crosses the boundary between two transparent media, the law of refraction (Snell’s law) states the relationship between the angle of incidence and angle of refraction of the light with reference to the refractive indices of both media as follows:

When electromagnetic radiation (light) of a specific frequency 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 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 direction of a ray of light and the normal – where 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 wavelength of the incident light is accounted for.
    • The refractive indices used are selected for the pair of media concerned.
    • The speed of light is expressed in metres per second (m/s).
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.
  • For any specified angle, it’s sine is the ratio of the length of the side opposite that angle, to the length of the longest side of the triangle (the hypotenuse).
  • The mathematical notation for sine is sin.

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