Focal Length
Key Stage 4 Higher
Meaning
The focal length of a lens is the distance between the lens and the focussed image of a "distant object".
About Focal Length
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| A convex lens focuses parallel rays to a focal point. |
- The focal length of a convex lens is the distance between the focal point and the centre of the lens.
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| A diagram showing how to find the focal length of a convex lens. |
To find the Focal Length of a convex lens:
- Focus the image of a "distant object" onto a screen.
- Measure the distance between the lens and the screen.
- The image distance (the distance between the focussed image and the lens) is the same as the focal length for a "distant object".
- If the object distance (the distance between an object and a lens) is too small, then the focal length is not the same as the image distance.