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Investigating the factors which affect the sideways displacement of a light ray through a glass block.

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Physics Coursework:

Investigating the factors which affect the sideways displacement of a light ray _                                 through a glass block


Refraction: Refraction occurs when light travels from one medium to another in which the speed of light is different. The speed in glass is slower than its speed in air so light is refracted when it travels for air to glass, or vice versa. This change in speed usually results in a change in the direction in which light travels.

Aim: I'm going to investigate the factors, which affect the path of a light ray through a glass block.

These factors are:        

  • Angle of incidence
  • Material
  • Thickness of material

The factor I have chosen is the angle of incidencebecause it is the easiest to carry out in the classroom since it would be difficult to find different materials that could be tested. Also it would be difficult to find one material with different thickness (to overcome that more than one of the materials could be put together however it would not be completely accurate since there will be a small gap between the two or more pieces.

The experiment I am going to carry out is an experiment to show the sideways displacement of a light ray through a Glass Block.


1)Glass block (60 mm wide)


2)Ray Box  (with as narrow ray as possible for more precision in measuring the angles)

3)Protractor  (with a small scale for more accuracy)

4)Ruler  ( a mm ruler to be more precise measuring)

5)Pencil  (with a sharp point for grater accurately when plotting points and drawing lines)


...read more.


angle of incidence

angle of refraction






























  •  Scientific explanation: When the angle of incidence is 0, there is no sideways displacement (as shown in diagram).  As I increase, d must also increase.

We can calculate the sideways displacement: I am going to calculate the sideways displacement for i=30 degrees:

1. To find out whatris. I know that the refractive index of glass is 1.52 from a preliminary experiment:

Sin i (angle of incidence)  =n (refractive index)

Sin r (angle of refraction)        image37.png

Sin 30   = 1.52 (what I got from preliminary experiment)                image38.png

Sin r        

Sin r = Sin 30  = 0.333image39.png


r = Sin-1(0.333) = 19.45

2. To get the distance AB:  (Shown on diagram 2)

6        =Cos. r


  6        =Cos. 19.45image41.png


AB=       6               = 6.36image39.png

           Cos. 19.45

3. To get distance D and therefore find sideways displacement:        

D      =  Sin (i-r)

AB image42.png

D      = Sin (30-19.45)


D= 6.36xSin10.55


I can do the same for an angle of incidence of 60 degrees:

Sin i (angle of incidence)  =n (refractive index)image37.png

Sin r (angle of refraction)        

Sin 60   = 1.52 (what I got from preliminary experiment)image38.png

Sin r        

Sin r = Sin 60  = 0.57image39.png


r = Sin-1(0.57) = 34.75

  6        =Cos. rimage40.png


  6        =Cos. 34.75image41.png


AB=       6               = 7.30image39.png

           Cos. 34.75

D      = Sin (i-r)

AB image42.png

D      = Sin (60-34.75)


D= 7.3xSin25.25


I have made a graph (in excel) showing my predicted values of sideways displacement against angle of incidence.  According to my calculations shown by the graph I predict that as the angle of incidence increases, the sideways displacement also increases if you shine a light ray through a glass block.  As shown by the graph, this displacement is proportional.


Obtaining Evidence

...read more.


Method: I am going to carry out an experiment similar to the one I originally did with the difference that this time I am going to have instead of one glass block, two joined together in different ways producing different thickness. For each thickness I will record the results. This time I will not change the angle of incidence but leave it constant all the way.


Fair Test, Accuracy and Precision: I will change nothing else but the thickness of the material. The angle of incidence will remain the same. I will repeat the experiment twice for each thickness and produce an average. I will use same methods of achieving precision as I did in the first experiment.


My prediction is that as the thickness of the glass block increases, the sideways displacement will also increases.

We can see that from the diagram. I used the yellow ray to show how the sideways displacement would have decreased as the thickness halves. We can conclude from the diagram that as the thickness doubles, the sideways displacement also doubles and therefore the two are directly proportional.

        As the thickness increases, there will be more space the ray has to travel through and because the original ray and the diffracted ray travel at different directions, they will get further away from each other causing the sideways displacement to increase.  



...read more.

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