Refraction – Slower to Faster Medium

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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:

• Introducing reflection
• Types of reflection
• Introducing refraction
• Calculating the angle of refraction
• Introducing chromatic dispersion
• Introducing refraction and wavelength

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.