Physics - AQA - Unit 3 - Lenses - Revision Cards in GCSE Physics

Principle Focus

There is a principle focus on each side of a lens.
The distance from the centre of the lens to the principle focus is called the focal length.
The principle focus of a converging lens is where the rays hitting the lens parallel to the axis all meet up.
The principle focus of a diverging lens is the point where rays hitting the lens parallel to the axis appear to all come from. You can trace them back back until they all appear to meet up at a point behind the lens.

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An incident ray parallel to the axis refracts through the lens and passes through the principle focus on the other side.
An incident ray passing through the principle focus refracts through the lens and travels parallel to the axis.
An incident ray passing through the centre of the lens carries on in the same direction.

A converging lens diagram following the above rules (http://www.physicsclassroom.com/Class/refrn/u14l5da2.gif)

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An incident ray parallel to the axis refracts through the lens and travels in line with the principle focus (so it appears to have come from the principle focus).
An indicent ray passing through the lens towards the principle focus refracts through the lens and travels parallel to the axis.
An incident ray passing through the centre of the lens carries on in the same direction.

(http://www.physicsclassroom.com/Class/refrn/u14l5ea4.gif)

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A real image is where the light from an object comes together (converges) to form an image on a 'screen'.
Like the image formed on the retina in the back of the eye.

A virtual image is when the rays are diverging, so the light appears to be coming from a completely different place.
When you look in a mirror you see a virtual image of your face, the object appears to come from behind the mirror.
You can get a virtual image when you look through a magnifying lens, the virtual image looks bigger than the object actually is.

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