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# Does the focal length of a lens depend on the colour of light used?

Extracts from this document...

Introduction

Physics coursework PSA3 experiment

Does the focal length of a lens depend on the colour of light used?

Aim

The aim is to investigate the Focal lengths of light in the visible spectrum.

Introduction

I intend on using light with as much difference in wavelength as possible so that I can compare my results with more ease and so that errors in my experiment do not lead to overlapping results that have no distinct difference in them. However as I am restricted to a school laboratory I shall be using light on the part of the electromagnetic spectrum visible to the human eye.  This is because filters for red and blue light can be found and used easily and have a large enough wavelength difference (red light with a wavelength of around 700nm whilst blue light is nearly 400nm) as they are on opposite parts of the visible spectrum to each other. Before I started the experiment I was given the lens I was going to use in the experiment so that I could work out the rough focal length of the lens.  This meant that I

Middle

25.0

78.5

81.5

80.5

88.8

79.2

88.9

82.9

28.5

62.0

64.0

60.0

63.0

61.3

63.5

62.3

33.3

48.1

49.5

46.0

48.3

47.0

48.3

47.9

40.0

39.5

40.0

38.7

39.4

38.6

40.0

39.4

50.0

33.0

33.4

32.7

33.0

32.9

33.3

33.1

66.7

28.5

29.0

28.6

28.9

28.6

28.9

28.8

100.0

25.1

25.2

25.1

25.0

25.1

25.1

25.1

 blue filter U (cm) V (cm) V (1st repeat) (cm) V (2nd repeat) (cm) average V (cm) minimum maximum minimum maximum minimum maximum 20.0 210.0 235.0 212.0 230.0 215.4 230.0 222.1 22.0 140.0 162.0 145.0 165.0 144.0 161.0 152.8 25.0 75.5 84.3 78.5 82.0 79.7 83.0 80.5 28.5 60.3 62.0 59.1 62.2 59.5 62.5 60.9 33.3 47.5 48.2 45.7 47.6 47.0 48.1 47.4 40.0 39.2 38.7 38.3 39.1 38.0 38.7 38.7 50.0 33.0 33.4 32.3 32.6 33.0 32.6 32.8 66.7 28.5 28.7 28.2 28.7 28.4 28.8 28.6 100.0 25.4 25.5 25.0 24.7 25.2 25.4 25.2
 red light reciprocal

Conclusion

My aim was achieved but the conclusion is not very reliable.

Modifications

To make my results more accurate I could use auto focus technology that can be found in some projectors and cameras rather than the human eye (which has questionable reliability) to determine the focal point.  This will ensure that I have the smallest possible error in the perception of the best (most in focus) image distance.  And in my experiment I found the percentage error in the determining of the focal point to be over 10%.  Without this error my conclusion would still have been the same.  I could also use a see through lens holder because it was hard to determine the object distance using the lens holder provided, and because I was using reciprocal values a small difference is magnified when dealing with small object distances. I would also measure more distances to increase the reliability.  I would also use different lenses so that I could compare that affect to gain a more reliable conclusion.  And I would also a transparent lens holder designed to slide over a meter rule to increase he accuracy of my results.

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