Theory
I know that when light goes through a different medium from which it is already in (a vacuum for example) it slows down, this is caused by there being more particles in the way. Light refracts because the change in speed, hitting the medium at an angle, causes one side of the ray to slow down first, and so it turns, towards the normal. The normal is a line at right angles to the medium. If a ray went down the normal, no refraction would occur, as both sides of the ray would slow down at the same time. If light went through from a medium into a vacuum, the ray would speed up.
The value of the ratio Sini/Sinr indicates the amount of bending occurs when a ray passes from one medium to another, for two media it will be the relative refractive index, as it depends on both media, but as air is so close to a vacuum we can assume it is, giving us an absolute refractive index, from a vacuum to a medium. Dividing the speed of light in a vacuum, – the place where light travels fastest, by this ratio (from a different medium), we get the speed of light in that medium. Snell’s law states that “the refractive index of the medium light is passing into/ the refractive index of the medium light is passing out of = Sini/Sinr”. As air has a refractive index of 1, it simplifies to - the refractive index of the medium light is passing into
= Sini/Sinr *
Prediction
I think that the speed of light in glass will be less of that than in air because the speed of light in air is so close to that of a vacuum, which is where light travels fastest. It also stands to reason that a solid is denser that air, and so light will have to travel through more particles.
Variables
The independent variable is the angle of incidence; I will vary this from 10o till 80o at 10o intervals. At 0o there will be no refraction as explained, and at 90o the D-Block will be missed.
The dependant variable is the angle of refraction; I will repeat the results at least twice and take a wide range of readings so that the experiment will be fair, and reliable. I will read the protractor to within 0.5o so the readings will be precise.
Other variables will be kept constant so that any change in the dependant variable will likely be caused by a change in the independent variable. Such factors include the media and the entrance point.
Results
Mean of Sini/Sinr 24.214 = 1.51
16
Speed of light in air/vacuum = 300,000,000m/s
Speed of light in glass = 300,000,000
1.51
=199,000,000m/s
Conclusions
- The speed of light in glass (199,000,000m/s) is less than that of air (300,000,000m/s).
- The graph shows that Sini/Sinr is constant and proportional, and can be used as the refractive index, as does the results table.
- The prediction has been fully supported though there is no proof of particles getting in the way as the cause of the light slowing down.
- This proves that light slows down from air into glass, and in doing so refracts. This refraction stays constant, as expected and the level of refraction-(which is constant, so works) can be obtained using Snell’s law. This refractive index can be used to obtain the speed of light in that particular medium, by using the speed of light in air/a vacuum as a basis.
Evaluation
- The results are firm enough to draw accurate conclusions, and test the prediction because the points on the graph stick very close to the line of best fit.
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The second set of results are almost all above the first set by 0.5o suggesting that one variable was overlooked, but didn’t change, possibly the D-Block was off the normal slightly.
- Also saying that the refractive index of air is 1; when it is 1.0003, may render the results slightly out if repeating the experiment more accurately, though here it does not make a difference.
- You could do the experiment backwards, from glass into air, to prove that Snell’s law still works. If the refractive index turned out the same then it should be proven.
* Extracts from web-site: (Preliminary work)
