To investigate the relationship between the distance between a lens and an object, and the distance a screen must be from the lens in order that it displays a focused image of that object on the screen.

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Lenses

“An investigation into the factors affecting a lens.”

Aim: To investigate the relationship between the distance between a lens and an object, and the distance a screen must be from the lens in order that it displays a focused image of that object on the screen.

Background knowledge:

In order to plan and my investigation, I am going to research the different types of lenses and the factors affecting them, and their uses. I hope that this will help me to plan and carry out a more successful investigation, and also help me formulate a hypothesis and understand my results better.

Lenses are optical devices that affect the passage of light through them by refraction. They can be made from almost any transparent materials, but the most common is glass. Lenses are used extensively in optical instruments such as cameras, telescopes and all sorts of projectors.

There are two main types of lens, convex (converging), and concave (diverging). Convex lenses are thicker in the centre, thinning out towards the edges, while concave lenses have thick edges and thin centres. A concave lens will refract light inwards, thus creating a smaller image while a convex lens will do the opposite, creating a larger image.

All lenses have an optical centre, where light can pass without being refracted and the principal axis of the lens passes though this point. At this point on the lens, the glass (or other material) is flat on both sides of the lens. When light passes through the lens anywhere except the optical centre, the rays are refracted, either towards the principal axis, in the case of a convex lens, or away from it, in concave lenses.

Lenses also have a focal point, which is the point at which the refracted rays converge with one another. In a convex lens this is on the opposite side of the lens to the object, but in a concave lens, where the refracted rays are directed away from the principal axis and will therefore never meet, the focal point is on the same side of the lens as the object, and is located by following the lines of the refracted light rays. This is a virtual focal point.

Because of this, a convex lens can be used to project an image onto a screen, the sort that would be used in a cinema projector, forming a “real image” which can be seen. A concave lens does not form any such image, it makes only a “virtual image” which cannot be displayed on a screen as it will be on the same side of the lens as the object.

In order to construct a ray diagram, you need to know the focal length of the lens you are using, as it will tell you where the rays must meet. To roughly estimate the focal length of a lens you can project an image of a light source, say a window, through a lens onto a card. The distance from the lens to the card, when the image is in focus gives the focal length. Ray diagrams are a simple way of demonstrating how light passes through a lens, and also why the effects happen.

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 Ray diagrams:

Convex Lens

Concave Lens

Considering the facts outlined above, I will have to use a convex lens in this investigation, as I require a real image – I must be able to see the image in order to investigate it. A concave lens would not create a real image and using one would make my investigation impossible to conduct.

I will be using a 10cm lens in ...

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