# System focal length for thin lenses

## Homework Statement

Three “thin” lenses, each with focal length 25 mm, are placed 5 cm from each other. What is system focal length?

## Homework Equations

Lens equation: (1/oxd) + (1/ixd) = i/fx
Oxd = object distance
Ixd = image distance
fx = focal

## The Attempt at a Solution

Lens1:
(1/o1d) + (1/i1d) = 1/f1
(1/inf) + (1/i1d) = 1/25
i1d = 25 mm

Lens2:
O2d = 50-25 = 25 mm
(1/o2d) + (1/i2d) = 1/f2
(1/25) + (1/i2d) = 1/25
i2d = inf

Lens3:
O3d = inf
(1/o3d) + (1/i3d) = 1/f3
(1/inf) + (1/i3d) = 1/25
i3d = 25

So if I draw a line from i3d to a line parallel to the optic axis (see attachment), I’ll get system focal length = 25 mm. Is this right?

#### Attachments

• 3_lenses.png
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