Two thin lenses of power – 4D and 2D are placed in contact coaxially. What is the focal length of the combination?

The power of a lens combination is the sum of the individual powers:
P total = P1+ P2
​Here, P1 = −4D and P2 = 2D :
P total = −4+2=−2D
The focal length f is related to the power by
f = 100/P (in cm):
f =100/−2 =−50 cm (or -0.5 m)
Thus, the focal length of the combination is -50 cm.

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