When light passes through a converging lens, light refracts. If the incident rays are parallel to the principal axis, in this case the line that passes through the center of the lens, the refracted rays will all meet in one point known as the principal focus.

When several rays of light that are parallel to the principal axis pass through a diverging lens, the refracted rays will seem to come from the same point known as the principal focus.

Converging lenses are usually used as magnifying glasses. However, if the object is place more than one focal length (distance from the principal focus to the lens or mirror) away from the lens, a real image will be produced where all the refracted rays meet.

The virtual images produce by a diverging lens are smaller than the object. Diverging lenses never produce real image.

As you can see, when a ray passes through the center of the lens (C) it is not refracted.

There are a couple of formulas that can be used with lenses,

• object distance (from lens) is always positive.

• image distance is positive for real images and negative for virtual images.

• focal length is positive for converging lenses and negative for diverging lenses.

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