Physics
posted by Help please .
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 
Jennifer
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 
Elena
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 =
=582.5+7 = 3.5
F=  0.286 m 
Physics 
Elena
Jenifer is right
1/F = 1/0.2 – 1/0.125 – 1/0.4 +1/0.1429 =
=582.5+7 = +1.5
F= + 0.667 m 
Physics 
Help please
thank you!
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