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When a ray parallel to the optic axis (normal to the plane of the lens) is incident on the lens, it is bent so that it crosses the optic axis at the focal point of the lens. In fact, this defines the focussing power of the lens.
Focal Point of Lens
A feature of the lens is that the direction the lens is travelling in makes no difference. We therefore see that any ray originating from the focal point of the lens is bent so that it leaves travelling parallel to the optic axis.
Consistent with these rules is that any ray travelling through the centre of the lens is not affected.
To see how the lens magnifies an object, three rays are drawn from the point on the object furthest from the optic axis.

The image appears in focus where the rays meet on the opposite side of the lens.
Object Image
The relationship between the focal length, f, of the lens and the distance, v, at which the image of a point source at distance u in front of the lens is in focus is given by the thin lens equation as follows:
1/u + 1/v = 1/f
The resulting image will be at a magnification of M, where
M = u/v
We shall explore the behaviour of a lens according to the above equations in the following simulation.
We see that the magnification of the object is dependent on its position relative to the focal point of the lens. Use the slider to move the object relative to the lens.

When is the magnification greatest? What would you have predicted from the lens equation?

Alternatively we can alter the strength of the lens - this moves the focal point of the lens

.In the electron microscope we may easily change the strengths of the lens by altering the amount of current flowing through the coils.
Move object
Move image