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To investigate how the angle of deviation in a glass block depend on the angle of incidence.

Extracts from this document...

Introduction

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Planning:

Simple procedure:

Make the light ray from the ray box pass through the glass block by changing the angle of incidence several times and measure the angle of deviation for each angle of incidence.

Apparatus:

Reading ranges: The range of angle of incidence will from 0° to 89°and  10°difference between them.

Safety: Wood board will be needed to prevent any fires by placing the bulb in the ray box too long on the table and heat the table up. Avoid our hands with touching the bulb directly and scald the skins.

Method: Use the above apparatus to see what the angle of deviation is when we have a particular angle of incidence. Repeat this by using different value of angles i.

Steps:

  • Place a board on the table and put a piece of A3 paper on it.
  • Place a glass block on the paper and draw its shape along its edges.
  • Draw a normal line on the block frame on the paper.
  • Draw the angle of incidence with a particular value.
  • Switch on the ray box and make the light ray overlap the incidence ray.
  • Draw the refracted ray that overlaps the light ray which is bent away the normal.
  • Connect the incidence ray and refracted ray.
  • Measure each angle of deviation which is relative each angle of incidence.
  • Repeat the experiment and record different results.

(Measure all the angles by using a protractor.)

Valid Evidence:

  • Vary angle of incidence (independent variable) by moving the ray box further away form the normal. After that I need to measure the angle of deviation by using a protractor.
  • Control variables for  a fair test, I will have to keep the following factors the same:
  1. Same glass block          
  2. Ray box distance from block (This can affects the width of light ray and the brightness as well)
  3.  Use the same light box (Different light box may have different colors and brightness)
  4. Ensure apparatus not knocked/moved during measurement (This makes our results will not be accurate)  
  • Simple Result Table & Simple diagram of general theory:                                                                                                              
...read more.

Middle

°

               89°

Theory:                                                                                                                                              

When the light ray travels from the fast medium (air) to the slow medium (glass block) its speed has decreased, wave frequency stays the same and the wavelength decreases. So parts of wave reaches boundary first and slows down which causes the change in direction. See diagram above.

Preliminary work:

Use a ray box with single slit to make the light ray through a rectangular glass block by changing the angle of incidence and measure the angle of relative refractions. (See the relative graph on the left and simple diagram on the right). My preliminary work also helped me to plan an accurate procedure: Avoid too many lines on the paper to prevent lines overlap each other. Avoid the range between each reading is too small to prevent the change of refraction angle is too small.Minimized the error by using long ruler to draw the lines.

From the graph, I can see the general pattern that as the angle of incidence increases, the angle of refraction increases but slowly at the large value of i. This enables me to make the following prediction.

Prediction:

As the angle of incidence increases, the angle of refraction increases, but more slowly. From the formula (d= i – r), i rises, r rises slowly, so d rises rapidly at large i.                                                                                                      

Analysis:

...read more.

Conclusion

Suitability of procedure:

I think my procedure is suitable, because we could make it fair test by keeping other variables constant. The results are enough and reliable.

Improvements:

  • To do the experiment more times and gain more repeats to decrease the error
  • To use narrower slit that makes the beam narrower to go through the block (incorporating focusing lens)
  • To use larger glass block giving more accuracy on where light emerges, so the measurement error of angle deviation falls.

Accounting for anomaly:

The causes of anomaly in my results might be affected by this factor. Parallax error is caused especially wider beams at large angle of incidence. Possible chips on block can affect the track of refracted ray.

Even through my results have one anomaly, doesn’t affect the general conclusion. Because it still agreed with my prediction and the line on the graph is good enough. So the evidence is reliable enough to firmly definitely support my conclusion that as i increases d increases at an even greater rate.

Further Works:

We can investigate the same problem by using different ways.

  1. Investigate by using single colour (Get d vs I graph for different colours of spectrum). Simple apparatus seen below

See how angle of deviation varies with angle of incidence each time.

  1. Investigate deviation going from slow medium to fast medium (Simple apparatus below).
  1. Use different material of block (E.g. Perspex)

...read more.

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