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Ray Diagrams for Lenses
 When we use lenses (and mirrors), the images formed are of two types, real images or virtual images. Real Images A real image is one which can be projected onto a screen. It does not have to be on a screen but is formed at a place to which light from the object is converged. A real image is always inverted (relative to the object). Virtual Images A virtual image cannot be projected onto a screen. A virtual image is formed at a place where there is no light from the object; the lens (or mirror) modifies the trajectory of the light to make it appear to have come from a different place. The place where the light appears to come from is the position of the virtual image. A virtual image is always the same way up as the object. A carefully drawn ray diagram can be used to find - the position - size - type of image (real or virtual) formed by a lens (or mirror). In order to do this, at least two rays of light have to be drawn. We must find two rays, the trajectories of which we know completely. The two most obvious ones are illustrated below: Ray Representing Light Passing Through the Optical Centre of the Lens This ray is useful because light passing through the optical centre* of the less is not deviated. This is also true for diverging lenses. The points F are called principal focal points The distances f represent the focal length of the lens A line through the optical centre of the lens, normal to the plane of the lens, is called the principal axis of the lens. Ray Representing Light Moving Parallel to the Principal Axis By definition, a ray parallel to the principal axis is refracted (by a converging lens) so as to pass through the principal focal point of the lens. For a diverging lens we can use the same ray. However, in this case the light is refracted so as to appear to be coming from the principal focal point. This reminds us that the focal point of a diverging lens is virtual. When drawing ray diagram for lenses, for simplicity, we imagine that the refraction takes place at the centre of the lens. In reality, of course, the light changes direction at the surfaces but this makes no difference to the results taken from our diagrams. See here for examples of ray diagrams for lenses. * What is the optical centre of a lens? Well, it's the point near (or at) the geometric centre through which light passes undeviated! If the lens surfaces are not symmetrical the optical centre might not coincide exactly with the geometrical centre.

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