The two variables that could easily be altered in a school laboratory situation would be the shape of the median and the colour of the light entering the median. I have chosen to change the colour of the light because I feel that this would give me a wider set of results.
Apparatus:
Power Pack- supplies power to the Ray Box (set to 12 volts)
Ray Box- generates a beam of light
Perspex Semi Circle- this is the median used to create total internal reflection
Coloured light filter- used to generate the coloured light of your choice
Protractor- used to measure any desired angles
Paper- an A4 sheet to mark any angles on from the experiment
Apparatus Diagram:
Method: Set the apparatus up as shown in the diagram and select the desired colour to test. Insert the filter into the ray box. Engage the ray box; the position of the filter may need to be manipulated to achieve the optimum setting for the experiment. Direct the beam of light to the rounded face of the Perspex semi circle. Move the beam of light to increase the angle until there is no angle of refraction and all the light is reflected back out of the Perspex. This is when the critical angle or the angle of total internal reflection has been reached. This angle must now be recorded.
Fair Testing: To keep this experiment a fair test it is important to keep all the variables the same and only change the one that is being tested (colour of the light). Ensure this by keeping the median the same throughout the investigation, always use Perspex. Use a semicircular block for all experimentation. Keep the size of the light beam the same by using the same light source in each experiment carried out. The light intensity can be kept constant by setting the voltage the light supply the same and by using the same ray box to supply it.
Safety: Bright light can be dangerous; do not look directly at the bulb when it is on because this could cause irreversible eye damage. Do not exceed the stated voltage of the bulb this could cause the bulb to ‘blow’ and possibly overload the power pack. Do not short the terminals of the power supply, this is dangerous and causes damage to the power pack.
Prediction: 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.
This diagram shows the wavelengths of the different colours of light, which is the basis of my investigation. (Source:
The tables above show the wavelengths of each and every colour in the visible spectrum
Preliminary Results:
I collected some preliminary results to narrow the range in which I had to search for the critical angle. This was to save time and give me an idea of the critical angle with white light, the control. Here are my results:
These results show that I should look for the critical between the angle of 35˚ and 45˚. (See also results sheet)
This is a graph to show a prediction of the critical angle of coloured light:
Results: I chose to use red, green and blue light because they are the primary colours of light
The result I have highlighted in yellow is an anomalous result and have decided to exclude it from my final average because it does not fit the pattern of the other results and my preliminary results. I thought it would throw my average out and would not be a true representation of the rest of the results.
Conclusion: From the results I have collected I have concluded that the colour of light does effect the critical angle. This also supports my prediction because the light that had the smallest critical angle was the one that had the largest wavelength (red) and the light with the largest critical angle had the smallest wavelength. This occurs because the smaller the wavelength the less easily the light is refracted, therefore the blue light entered the median at a larger critical angle to achieve total internal reflection. The red light had a smaller critical angle because of its longer wavelength. A longer wavelength means that it can refract easier and reach the critical angle sooner.
This diagram shows the colours of the spectrum as they disperse when the exit a prism. It shows that red light is bent the most, because of its small wavelength. It has a lower critical angle because it needs less bending to reach the critical angle because of its ability to bend. This is why it first in the spectrum because it disperses easily so it’s the first to leave the prism. This can also be applied to blue light, but in reverse. Blue can be seen bending the least so it needs a larger critical angle to get it to the point when it reaches total internal reflection.
I have one result that did not fit the pattern among the results I collected. I tried to keep these types of results to a minimum but it was inevitable that some could slip through. This result was probably caused by human error, for a full breakdown of possible causes see my Evaluation.
Evaluation: Not every result I took for each colour of light was the same for the three experiments I performed. This means, that I have some anomalous results. The following section explains possible reasons for these.
- The measuring equipment that I had access to could only measure accurately to the nearest half degree. This could have affected the accuracy in which I was able to measure the critical angle.
- Slight imperfections in the Perspex could affect the way that the light entered the median by slightly changing the angle that the light is bent through. In turn the critical angle could have become altered, but this would be marginal.
- The light beam that was provided by the ray box could be affected by the bulb in the ray box, how old it is, resistance in the wires connecting it the power pack etc. Providing a dimmer or brighter light may have led to angles of refraction disappearing before they are supposed to because they were too dim to be seen.
- The power packs that I have used could have differed slightly, supplying a brighter or dimmer light ray causing the same problems as described above.
- Using certain colour filters that were different thicknesses could have affected the angle of entry to the median and the critical angle because the light would have diffracted slightly differently when passing through the colour filter. This would be marginal, however.
If I repeat this experiment I will try to control the above areas of error by:
- Devising a way to measure more accurately, possibly by using aids such as computers to help me obtain more accurate results for my experiments.
- Using the newest block of Perspex to minimize the imperfections in it. This is not always practical in a school laboratory.
- Using the same ray box for every experiment will ensure the imperfections in the ray box will be applied to all the results.
- 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.
