But in our experiment we are shining the ray onto the curved surface and seeing how light goes from Perspex into air so we will attempt to work out sin I over Sin R and multiply it by the speed of light in air to get the speed of light in Perspex.
fig.1
I predict that the graph of sin I over sin r will be a straight line through the origin. As sin I over sin r is a constant.
Diagram
Methodology
- Place Raybox on a sheet of plain white a3 paper and drawaround it.
- Next shine 7 light rays using the collimator and glass lens) to get a sparse accurate ray of light) into the curved surface of the d-block at seven desired angles so we can measure how the ray of light will differ from travelling first through Perspex then into light.
- Next draw the light rays on to the paper. We will take two readings of the angles at which these rays hit the block to boost our accuracy.
- Next draw the seven refracted rays from the other side of the d-block onto the paper.
- Finally Measure the angles of incidence and reflection and note them down.
Obtaining
According to background research in order to find the spped of light we need to find out the value of Sine I and Sine R in order to get N.
Sine I
Sine r
I now have seven readings for N and I will take the average
=4.521487684 divide by 7 = 0.645926812
N= 0.645926812.
N=
Speed of light in Perspex = Speed of light in air x N
We know that light travels through air at a speed of 300000000 m/p/s
Therefore,
Speed of light in Perspex = 300000000 m/p/s x 0.64596812 = 193778043.6 m/p/s
Using the graph
By taking the gradient of the graph the refractive index of the boundary can be found.
Sine I = 0.174
Sine R = 0.280
Base of triangle – (b-a) = 0.415-0.280=0.135
Height of triangle – (c-b) =0.259-0.174= 0.085
When sine I over Sine R, (when light goes from Perspex to air) Gradient – 0.085 / 0.135 = 0.630
Refractive index of Perspex When sine R over Sine I (when light goes rom air to Perspex) Gradient – 0.135 / 0.085 = 1.58.
From my background research I knew that the refractive index of Perspex was 1.49, so my graph is reasonably accurate but not 100% accurate (see evaluation)
Analysis
Thegraph I drew is a graph to show Sine I against Sine R it is to be used as a guide to inform me of how accurate my results are, I have incorporated a best line to see how well accurate my results were by seeing how well they sat on the best line. The best line should be a straight line through the point of origin as according to my background information Sine I over Sine R is a constant.
My graph is telling me that the gradient of the straight line is Sine I over sine R which is the refractive index of the boundary.
My results tell me that the speed of light in Perspex is 193778043.6, I predicted using scientific background information that you should get the speed of light in air when you multiply the speed of light in Perspex by the refractive index of Perspex (1.49)
193778043.6 m/p/s x 1.49 = 28872928.5 m/p/s,
We know that the speed of light in air is 300000000 m/p/s.
I now know that the test I have conducted is accurate as the two figures for the speed of light in air are very similar. Although there is a difference this can be a result of not having 100% accurate results, as a number of points were not exactly on the line.
Evaluating
From conducting this experiment I can conclude that my prediction has been satisfactorily proved and my results are reliable to a certain extent despite the fact that not all of them sat on the line in the graph perfectly. Factors that may be responsible for this maybe the pencil line drawn on the paper was thick and when it came to measuring an angle not a 100% accurate reading could have been taken, and after each ray had been drawn the pencil may have got blunter thus producing rays of different thickness.. Another factor may be using the collimator to produce the light beam. The Collimator had a small slit in to allow light to travel through; I feel that the slit could have been a bit narrower to have produced a sparser ray hence so we could have drawn it a bit more accurately. As well as this the D-block occasionally got knocked and moved slightly off the point it was supposed to be so we had to place it back a number of times so it may have been in a slightly different place than it was when measuring the last angle.
If I were to repeat the experiment again improvements that I could make may be to have used a sharper pencil when drawing on the light rays and kept sharpening it after I had drawn each ray to get rays of consistent thickness. As well as this I would use A collimator with a smaller slit to get a sparser more accurate ray and finally I would secure the d-block down some how.
Another experiment I could do is measure the speed of light in water.
Measuring The Speed Of Light in Glass
Prediction
I predict that the speed of light in water multiplied by the refractive index of water (1.33) will be equal to the speed of light in air.
Firstly I would fill a fish tank half full of water; next I would place a long metal rod into the fish tank at an angle near the side of the tank and get someone to draw the refracted image on the side onto the fish tank, I would then place the rod in the tank at different angles and note down the angles of incidence and refraction, and work out the speed at which light travels in exactly the same way as the Perspex experiment.