What is the focal length of a convex lens of focal length 30cm incontact with a concave lens of focal length 20cm? Is the system aconverging or a diverging lens? Ignore thickness of the lenses.
Given: The focal length of concave lens is 20 cm , and the focal length of convex lens is 30 cm .
The equivalent focal length is given as,
1 f = 1 f 1 + 1 f 2
Where, the focal length of convex lens is f 1 and the focal length of concave lens is f 2 .
By substituting the given values in the above expression, we get,
1 f = 1 30 − 1 20 1 f = 2 − 3 60 f = − 60 cm
Thus, the focal length of the combination of the lenses is 60 cm and the negative sign indicates that the system of lenses is a diverging lens.
Given: The focal length of concave lens is
The equivalent focal length is given as,
Where, the focal length of convex lens is
By substituting the given values in the above expression, we get,
Thus, the focal length of the combination of the lenses is
