# Physics

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The following four lenses are placed together in close contact. Find the focal length of the combination.
Lens 1- F'= +200mm
Lens 2- F'= -125mm
Lens 3- F'= -400mm
Lens 4- F'= +142.9mm

• Physics -

For two lenses:

1/f = 1/f1 + 1/f2

where f is the focal length of the lens combination, f1 is the focal length of the first lens, and f2 is the focal length of the 2nd lens

Extending this to 4 lenses:

1/f = 1/f1 + 1/f2 + 1/f3 + 1/f4

or

1/f = 1/200 - 1/125 -1/400 + 1/142.9

1/f = 0.0014979

f = 667.6 mm

• Physics -

Lens 1- F'= +200mm=+0.2 m
Lens 2- F'= -125mm = - 0.125 m
Lens 3- F'= -400mm =- 0.4 m
Lens 4- F'= +142.9mm= +0.1429 m

1/F = 1/0.2 – 1/0.125 – 1/0.4 +1/0.1429 =
=5-8-2.5+7 = -3.5
F= - 0.286 m

• Physics -

Jenifer is right
1/F = 1/0.2 – 1/0.125 – 1/0.4 +1/0.1429 =
=5-8-2.5+7 = +1.5
F= + 0.667 m

• Physics -

thank you!

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