# Deviation of Light by a Prism.

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Introduction

Nigel Evans

Physics Coursework – “Deviation of Light by a Prism”

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Deviation of Light by a Prism

Aim

The aim of this investigation is to test using a prism how the angle of deviation (how far the light ray is deviated from its original position) is affected by varying the angle of incidence (where the light ray goes into the prism), and whether this has any relation to the angle of emergence (where the light actually comes out of the prism).

## Planning

I firstly need to conduct theoretical work and a preliminary investigation to test what is going on and to get a clearer view of the aspects of the investigation. In my theoretical work I will have to use the geometry of triangles and Snells Law. For Snells law to work, I will need to know the refractive index of the glass that I am using. To do this I will use a glass block of the same glass as the prism, and then use sighting pins (and light) to pinpoint the incident ray and the emergence ray and then find the angle of refraction. A more detailed description of how this experiment is going to work can be seen below.

To find the refractive index of the glass that I will be using in the actual experiment, I set up a glass block that was of the same type of glass as the prism, and lined up sighting pins through it. This enables me to draw the angle of incidence (measured from the Normal (dotted line below), a line at 90° to where the incident ray strikes the glass block) and this also enables me to draw the angle of emergence (where the light exits the prism).

Middle

Sin R2 x n = Sin E

Sin 29.5° x 1.51 = 0.744

Sin-1 0.744 = 48.1°

Therefore E = 48.1°

Now that I have all for values need to calculate the deviance I again have to use the fact that X + Y = δ:

(I – R1) + (E – R2) = δ

(50 – 30.5) + (48.1 – 29.5) = δ

19.5 +18.6= 38.1°

Below is a results table of the answers that I have gained for the angles sizes using the two above methods.

Results Table for Theoretical Modelling:

Incident Ray, I (°) | Refracted Ray, R1 (°) | Refracted Ray, R2 (°) | Emergence Ray, E (°) | Deviation, δ (°) |

20 | 13.0913 | 46.9087 | Not Possible | Not Possible |

25 | 16.2530 | 43.7470 | Not Possible | Not Possible |

30 | 19.3371 | 40.6629 | 79.71 | 49.71 |

35 | 22.3245 | 37.6755 | 67.35 | 42.35 |

40 | 25.1942 | 34.8058 | 59.53 | 39.53 |

45 | 27.9229 | 32.0771 | 53.31 | 38.31 |

50 | 30.4851 | 29.5149 | 48.06 | 38.06 |

55 | 32.8530 | 27.1470 | 43.55 | 38.55 |

60 | 34.9965 | 25.0035 | 39.66 | 39.66 |

65 | 36.8845 | 23.1155 | 36.36 | 41.36 |

70 | 38.4852 | 21.5148 | 33.63 | 43.63 |

75 | 39.7684 | 20.2316 | 31.48 | 46.48 |

80 | 40.7070 | 19.2930 | 29.93 | 49.93 |

85 | 41.2794 | 18.7206 | 28.99 | 53.99 |

90 | 41.4718 | 18.5282 | 28.67 | 58.67 |

These are the calculated values for the prism, using the refractive index for the glass as 1.51. Any values before 30° are not possible because the refracted ray will appear at the wrong side of the prism. This factor makes the SINE of the angle bigger that 1 and therefore it is not possible to calculate a value for the emergence ray and also therefore not for the deviance either (as there is no emergence ray to calculate it from). The Incidence angle of 90° is also highlighted because although this value and other values above it are possible in calculation in practice the 90° angle would actually miss the prism and the values of the incidence ray above 90° would have to come from inside the prism itself. I have therefore chosen to take readings in the range of 30° - 85°. I will take one reading below 30° but this will only be taken to prove right my initial theory work and will not be taken into consideration in any conclusions that I draw.

Over the next two pages is two graphs plotted in the incident angle range of 30° - 85°, which show the predicted trend in

Conclusion

I would also maintain a sharper pencil at all times as this would make angle measuring easier, and maintain a high degree of accuracy in my investigation.

I would try and obtain a angle measuring tool that is of a better standard that a protractor and measures to a higher degree of accuracy, to ensure that all my results were very reliable.

Reliability

Overall I think my results were very reliable, and they linked back very well to my initial calculations. I think there is room for improvement as there is in any case but overall I think that my investigation was conducted reliably and to as high a degree of accuracy as the involved apparatus permitted.

Anomalous results

There were no major anomalies in my results, but having said this the two graphs that I have prepared from my results show some points that do not fit the exact trend, but they are still acceptable. An example of this is on the graph of the angle of emergence plotted against the angle of incidence, the two values of incidence angles 35° and 60° show slight bumps, even thought he line should be a perfect curve. These two could have arisen to inaccuracy on my part or for one of the reasons stated in the above sections.

The second graph of angles of deviation plotted against the angles of incidence show no real anomalous results with all points fitting the general trend of the graph in the same way that my initial calculations did (i.e. a smooth curve).

To conclude I think that given the situation and the equipment provided, I made the best use of it and even through there was room for improvement, my collected results were of a high standard as shown when comparing to my calculated values.

This student written piece of work is one of many that can be found in our GCSE Waves section.

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