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# Sideways Displacement of a Light Ray

Extracts from this document...

Introduction

Yulia Leyko                                                                                                                                                    Manor House

IVTH FORM INVESTIGATION - SIDEWAYS DISPLACEMENT OF A

LIGHT RAY

Planning experimental procedures.

The aimof this investigation is to find out how the sideways displacement of a light ray depends on various factors.

When you do an experiment, you should pay attention on following factors:

• the angle of incidence
• type of material (glass, plastic, water, etc.)
• thickness of the material

I will change just one of those factors - the angle of incidence. I chose it, because there are also some problems with the equipment and I can not get 6 or 7 different types of material and the angle of incidence is the easiest factor to change in words of practice.

The following equipment should be used:

• 1 glass block (the material, through which the ray light will go)
• 1 ray box ( to produce a ray light, which should be as thin as possible)
• 1 protractor ( to measure the angles)
• 1 ruler (to measure the dimensions of the glass block)
• 1 sharp pencil
• 1 piece of blank white paper

Below is the diagram of the experiment:

The experiment step-by-step: instructions:

1. fix the glass block on the blank piece of paper
2. draw a line in pencil around the glass block to make sure that the glass block will stay on the same place during the experiment
3. draw a perpendicular line("the normal") to the side of the glass block
4. fix the ray  box  near the glass block
5. draw a line to show the position of the ray box to make sure that the ray box will stay on the same position (same distance away from the glass block) during the experiment
6. measure the angle of incidence ( how far away is the ray of light from "the normal")  (i)
7. measure the sideways displacement
8. record the results into the appropriate table
9. repeat the experiment for the same angle of incidence

Middle

i -r

15

12

3

30

20

10

45

30

15

60

35

25

75

40

35

To assume that angle (i) = 15, then (r) = 12 and (i-r) = 3.

If we increase (i) - the angle (r) and therefore angle (i-r) increase as well, for example:

(i) = 45 , (r) = 30, (i-r) = 15.

This happens, because:

Those two angles are equal (the ones that marked with         ) .

I predict, according to my scientific knowledge, that the sideways displacement of the light ray depends on the angle of incidence and therefore, if the angle of incidence will increase, the sideways displacement will increase as well.

(i) - ( r) shows  that light is changing it's path, but it doesn't mean that sideways displacement is  proportional to the angle of incidence. The preliminary results show the difference between the angle of incidence and the refracted angle, which might help to explain, that even at that stage, when the light ray is going through the glass block there is a difference, so that when the ray of light actually does go out of the glass there is  difference between where the light actually goes and how it should have gone without the glass, which is called sideways displacement. This is shown on the diagram below :

Using my preliminary experiment I can actually make a prediction on what the sideways displacement will be. If I take a table of results and draw a diagram with all the appropriate information and then plot a graph.

 angle of incidence ( i ) refracted angle ( r ) i -r 15 12 3 30 20 10 45 30 15 60 35 25 75 40 35

So that my diagrams would look like this:

As I can clearly see from my hand-drawn diagrams, the sideways displacement increases as the angle of incidence gets bigger. I also plotted a graph, basing on the results that I have got from the diagrams, in order to underline my prediction.

It is important to bear in mind, that the diagrams are all of the same size (i.e. the rectangles that represent glass blocks are all of the same size), and the results that I got are not proportional, i.e. sideways displacement of a light ray is not proportional to the angle of incidence.

Obtaining evidence.

I have carried out the experiment carefully, following my plan. I changed the table, where I recorded the results, because the table, that I decided to use in my plan was not suitable for this particular experiment. This can be explained by the fact that, for example, my last angle of incidence was meant to be 105°, but this can not be done on practice, because otherwise the ray of light would have to go through the glass block, entering it from another side. It would happen, because the "normal" is perpendicular to the glass block, therefore to make sure, that it would be a fair test, I could allow the ray of light to go in the glass block just in the angle not greater than 90°. And if I would take the results, that were in my previous table, I could not make a fair test, because I would be able to take just 5 results. The evidence of it, is the fact that, it would not be possible to take two last results ( 90° and 105° ), because if I would put the ray of light at the angle of 90°, the ray of light would just go by the side of the glass block. This reasons resulted in fact, that I have changed my table for results completely, changing the angles of incidence, that I used in practice.

To record my results in an adequate way I have a table, to show my practical observations in a "numerous" way.

Table of results:

 angle of incidence ( i ) sideways displacement / cm / degrees result 1 result 2 result 3 average 10 0.3 0.2 0.4 0.3 20 0.7 0.8 0.7 0.7 30 1.2 1.3 1.1 1.2 40 1.8 1.9 1.7 1.8 50 2.6 2.4 2.3 2.4 60 3.4 3.2 3.1 3.2 70 4.4 4.3 4.2 4.3

Conclusion

Instructions -

• fix the glass block on the blank piece of paper
• draw a perpendicular line("the normal") to the side of the glass block
• fix the ray  box  near the glass block
• draw a line to show the position of the ray box to make sure that the ray box will stay on the same position (same distance away from the glass block) during the experiment
• measure the angle of incidence ( how far away is the ray of light from "the normal")  (i)
• measure the sideways displacement
• record the results into the appropriate table
• repeat the experiment for the same thickness of a glass block
• if needed, repeat the experiment for the same thickness of a glass block one more time ( if the difference between result 1 and result 2 is big)
• record the results into the appropriate table
• repeat steps 6-10 for 5-6 different thickness' of glass blocks

Table for the results-

 thickness of a sideways displacement/cm glass block / cm result 1 result 2 result 3 average

Safety -

• the whole experiment should be done in the free area (without any external objects: books, pencil-boxes, etc.)
• experiment should be done on straight horizontal surface, so that any parts of the equipment will not go over the edge (e.g. of the table, where the experiment is carried out)
• I must be careful with the equipment, because :
• the glass is easy to break or scratch, so I will not keep it near the edge of the table and I will make sure that it is in a safe position.
• the ray box is connected to the plug, so I have to be careful with the electricity, also during the experiment the ray box might get hot, so I will be careful in touching it.
• I will be careful in moving the equipment, so that everything will be safe
• if something  goes wrong - ask the teacher

Prediction -

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