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To find the best position of a lens to give the best magnification.

Extracts from this document...

Introduction

Mukwinda Phiri                                                                                                                                  11Wn

GCSE Physics Coursework: Lenses Experiment

Aim

To find the best position of a lens to give the bestmagnification.

Theory

There are two basic types of lenses, convex lenses and concave lenses.  A convex lens is also called a converging lens and a concave lenis is also called a diverging lens.  Convex lenses are thicker in the middle than at the edges and concave are thinner in the middle than at the edges, which is exactly the opposite.  The diagram below shows some examples of both types of lenses.

Diagram 1

If a beam of light is directed parallel at a convex lens then the light is brought to focus by the lens.  If the beam of light is directed along the lens axis (the line through the centre of each surface), the rays are brought to a focus on the axis at the focal point of the lens.  The focal length (f) of a convex lens is the distance from the lens to the point where the rays are brought to a focus.

Diagram 2

If a beam of light is directed parallel at a concave lens then the light rays are made to diverge from the lens.  

...read more.

Middle

Test

To make sure the experiment is fair a few factors have to be taken into consideration.

To make it a fair test then I will use the same ray box, same metre ruler, same slide, same lens and the same screen.  

To make the experiment accurate and to prevent anomalies I will repeat the whole experiment at least three times and take an average reading.

Equipment

Lens

Screen

Ray box

Slide

Metre Ruler

Method

  1. Set up the diagram as shown below.
  2. Move the object closer to the lens and measure the image distance as well as the object distance.
  3. Calculate the magnification.
  4. Repeat step 2 several times and repeat the experiment three more times and take an average reading.

Diagram

Preliminary work

My preliminary work was done using a java applet that can be found on the following website:http://members.nbci.com/-XMCM/surendranath/Applet.html.

My results are shown below.

OBJECT DISTANCE

(mm)

IMAGE DISTANCE

(mm)

MAGNIFICATION

(mm)

200

200

1

180

225

1.25

160

266.7

1.7

140

350

2.5

120

600

5

100

10100

101

80

-400

-5

60

-150

-2.5

Actual Experiment results.

OBJECT DISTANCE

(Cm)

IMAGE DISTANCE

(Cm)

MAGNIFICATION

(Cm)

10

Infinity (INF)

Infinity (INF)

15

Infinity (INF)

Infinity (INF)

16

200

12.5

18

118

6.5

20

65

3.25

22

53

2.409

24

41

1.7083

25

40

1.6

26

37

1.42

28

33

1.179

30

30

1

45

22

0.48

45

22.5

0.5

...read more.

Conclusion

The object distance against magnification graph forms a curve.  This curve represents exactly what I had predicted that the nearer the object is to the lens the better the magnification.

The graph that shows the relationship between 1/u and 1/v shows a negative correlation of points and this means that as the object distance decreases the image distance increases.  

I had one difficulty when doing the experiment and that was the judgment in measuring the distance from the lens to the image (1 cm error) and from the object to the lens (0.5 cm error).

Theoretically the best magnification should be when the object distance is nearest to the lens but before the focal point.  This is represented in the experimental results above.

Evaluation

I think my plan worked out because I got the results I expected to get.

My results for my experiment are quite accurate and reliable but there is a marginal error.  My experiment could be improved if I used a tape measure instead of a ruler because there was a marginal error in measuring the object distance and image distance.

The first two points were anomalous.

I think that if I did the experiment again I could change the focal length.

...read more.

This student written piece of work is one of many that can be found in our AS and A Level Microscopes & Lenses section.

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