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An Investigation into the Effect on the Critical Angle by Changing the Colour of Light

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An Investigation into the Effect on the Critical Angle by Changing the Colour of Light

Aim: To investigate the factors affecting the size of the critical angle through a median of Perspex

Background Information: The critical angle of light is when it hits a different median from the one it had been travelling in, for example glass to air at a certain angle that causes total internal reflection. This angle is different for all lights and medians. Total internal reflection is when a beam of light travelling through a certain median is reflected back at an angle that is equal to its incidence instead of just being refracted and then passing out the other side.

This phenomenon is used to transmit information through fibre optic cables. Fibre Optic cables have a beam of light sent down them in which the information is encoded. The beam of light is angled to hit the side of the cable at an angle greater than medians critical angle (42˚). The beam then reflects off one side of the cable then to the opposite side. Again the angle of incidence is greater than the critical angle. This is then repeated all the way to the end of the cable where the information is needed.

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I predict that the critical angle for red light will have the smallest critical angle and blue light to have the largest critical angle due to their position in the spectrum, red being on the outside and blue on the inside. The light colours on the outside are bent the most because of their larger wavelengths, the light colours on the inside are bent the least because of their smaller wavelengths. Light is a type of electromagnetic radiation and is in the electromagnetic spectrum between Infra Red Light and Ultra Violet Light (see diagram). This means they travel in waves, waves have a particular wavelength, and the wavelength determines how easily the light rays are bent.Red light is bent quite easily and blue is not bent so easily because of their different wavelengths. To reach the critical angle the light has to bend sufficiently to achieve total internal reflection. If the light is not bent in easily it will take a larger angle of incidence to reach total internal reflection.image02.png

This diagram shows the wavelengths of the different colours of light, which is the basis of my investigation. (Source: http://imagers.gsfc.nasa.gov/ems/visible.html


Wavelength (nm)










Wavelength (nm)







The tables above show the wavelengths of each and every colour in the visible spectrum

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Using the same power pack for each experiment will ensure that none of the other experiments will have a different brightness of light ray.Using colour filter with the same thickness and make sure that they are as thin as possible all the way through the experiment to minimise the refraction that could occur while the light is passing in and out of the colour filter.

I think the main reason for my errors was how accurately I could measure the angles that were taken during the experiment. This is because with the equipment I had access to, I was unable to measure anything more accurate than half a degree.

To extend the experiment and enforce my conclusion I would increase the range of colours I tested to see if the trend is the same all the way through the spectrum. It would also be interesting to try mixes of colours (for example magenta and cyan) to see if there is any correlation between the critical angle of the colours that make up the secondary colour (two primary colours mixed together= a secondary colour).

Bibliography: Sources that I used are described as follows:

Physics for You, revised national curriculum edition for GCSE by Keith Johnson

Essay Bank. co.uk- written by Richard Gilberto, 1999 Essay reference: “The  electromagnetic spectrum”

BBC.co.uk homework helps Physics pages written by The BBC Homework Advice Team.[1]

[1]Patrick Hosford 10AG

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