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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.

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

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.

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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# Related AS and A Level Microscopes & Lenses essays

1. ## Physics coursework; Finding the focal length of a lens using a graphical method.

4 star(s)

Possible Modifications: * using more precise equipment, * Repeating the experiment more times increases the accuracy, * As does conducting the experiment in a completely dark room with a brighter light source which makes the image easier to focus. * Using a brighter light source also means it could be diffused more decreasing the depth of focus further.

2. ## In this experiment I will be investigating the efficiency of a motor. I hope ...

Prediction: The fatter in the middle, out of the two lenses that I decide to use, the smaller the focal length. Therefore if one of the lenses is twice as fat in the middle compared to the other. Its focal length will be twice as short, because it will refract light at a doubly greater angle.

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

These percentage errors can be worked out using the equation percentage error = (error x 100)/reading. To find out the largest amount these could have affected my results I will find the percentage error for the longest length measured, which was when I was measuring using the object distance of 20cm with a red filter.

2. ## Relationship Between U and V For a Convex Lens

The red lens was chosen firstly it was a thin lens, and not a fat lens. This is because the fatter the lens the more rays of light refract as they go through, the more the rays of light bend the nearer they will cross over.

1. ## The focal length of a convex lens.

11.76 cm % Error = (11.76 - 10)/10 X 100% = 17.6 % Experiment 4: Table 2 d (cm) 49 47 45 43 41 39 a (cm) 22 20 16 12 8 4 d2 - a2 (cm2) 1917 1809 1769 1705 1617 1505 Plot of d2 - a2 against d Slope of graph = (400 - 200)/(10-5)

2. ## To investigate the relationship between the distance between a lens and an object, and ...

The object must be kept constant, as varying this could have a serious effect on the experiment. Some objects may appear more focused than others, depending on their colour, texture and size and therefore the object must be the same throughout the experiment The eyesight of the person observing is

1. ## Lenses experiment

This is also a diagram of how a magnifying glass works. Ray Diagram 2 shows the object on the focal point, when constructing a ray diagram of this 2 parallel lines are given this then means that the image is magnified, but to an amount that one would find it immeasurable.

2. ## I intend to show how the position and nature of the image can be ...

As soon as I have all the measurements I will then use the following formula to find the focal lengths: F =(u * v) / (u + v) Another thing which I will record is whether the image is virtual or real.

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