An object is placed at a distance of 10 cm from a co-axial combination of two lenses A and B. The combination forms a real image three times the size of the object. If lens B is concave with a focal length of 30 cm, what is the nature and focal length of lens A?
Convex, 6 cm
or v = – 3 u
Now u = –10 cm, therefore v = +30 cm.
If F is the focal length of the combination, we have
Focal length of the concave lens B = f1 = –30cm. If f2 is the focal length of lens A, we have
Which gives f2 = 6 cm.
Since f2 is positive, the lens is convex.
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