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Investigating the Positioning of Real Images formed by a Convex Lens.

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Sam Turner        Page         01/05/2007

Investigating the Positioning of Real Images formed by a Convex Lens

  1. Scientific background to the investigation

I know that cameras can give us a sharp image, because of rays which start at one particular point go through the optical system at one point. This creates am image on the focal point. I know that every lens has a focal lens. Concave lenses are very similar to concave mirrors, in the way in which they form images, they have the same properties to form images. A concave lens forms an upright, virtual image of any object placed in front of it. The image is always smaller than the object and closer to the lens than the original image. Changing the position of the object changes the position and size of the image which is formed, but the basic form of the diagram stays unchanged.

        For distant objects, the film must lie at the principal focus of the lens if the image is to be in sharp focus. For closer objects, the distance between the lens and the film must be increased. Accurate focusing of the image is achieved by screwing the lens backwards or forwards in its holder to suit the particular object distance.

  1. Secondary sources of information

I consulted the text book (Explaining physics) where I was given an equation which was relevant to this investigation. I therefore predict that the investigation data will obey this equation.

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  1. Rough preliminary work done

The preliminary work that I have done, is to check the amount of light reaching the film. This is controlled by the diaphragm. This alters the aperture of the hole through which the light passes. Increasing the aperture lets more light into the camera, but it can also lead to some focusing problems. High quality cameras have expensive multi-element lenses which produce sharply focused images even at wide apertures.

When I used the camera I wondered how it worked and how the rays were formed on the screen, so I took the camera apart. I discovered that the rays which entered the camera went through a convex lens, this causes then to diverge, and be brought to a focus at a particular point called the focal point. The focal length is the distance from the lens to the focal point.

  1. Outcome of rough preliminary work done

I discovered form the preliminary work that I did,  all the rays from one point going through an optical system will meet at one point. To avoid blurring this is where we must put the film on the screen.

  1. Conclusions from rough preliminary work

From the preliminary work, I can conclude that a camera has a convex lens, which brings the rays to a focus. The rays going through the system, are all bought to a focus at the same time, and the distance is called the focal length.

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  1.  Special measurement routines for precise data

I will measure from the centre of the lens. I must allow for the lens thickness. I will repeatedly check that the screen is not crumpled, so that the image will be crumpled and will not look sharp. When I view the image I will make sure that I have one eye shut, this will make it easier to see the sharpness of the image.

  1. Repeated readings proposed

For each lens, I will record 10 pairs of readings. I will repeat them myself and invite people to do the same readings to check that mine are accurate. If there is agreement it suggests that the data I have recorded is reliable.

        I will look at my data graphically, so that I can check the data points on the graph. If they lie on a smooth line with little scatter, I will be inclined tot think that the reliability is good.

  1. Analysis proposed linked to theory

The analysis that I proposed to do is to, have a lens, with a screen of white paper, and a light. I will change the independent variables of u and f at separate times, this will ensure that I will be able to work out the dependant variable of v.

        When I draw the graph of my results I will use the formula u + v / uv = f . This will ensure that I will have a straight line, which makes it easier to check and evaluate.

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