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There are 2 ways to find characteristics of images formed by lenses.
Can use either ray diagrams or algebra.
You might have used ray diagrams previously.
Terminology
Terminology
Object distance is always positive
Height are
always downward - negative
always upward - positive
Converge lens - positive
Diverge lens - negative
Virtual image - negative
Real image - positive
f : focal length of lens; distance from optical centre to principal focus
Note: focal length (f) is same distance whether it's F or F'.
Equations
Equations
i = image
o = object
Focal Length Equation
Focal Length Equation
Examples
Examples
A converging lens has a focal length of 23 cm.
Object is 32 cm from the lens. Use thin lens equation calculate where the image is from the frog.
First we should have:
1/f = 1/do + 1/di
We need to find "di" for image distance. So we rearrange the equation:
1/di = 23-32 / 23x32
1/di = 9 / 736
We can get rid of 1 on the left side causing 9/736 to become 736/9
di = 736/9
di = 81.78
The image is 81.78 cm from the mirror
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