Ray coming from an object situated at infinity, fall on a convex lens and an image is formed at a distance of 16 cm from the lens. When a concave lens is kept in contact with the convex lens, the image is formed at a distance of 20 cm from the lens combination. Calculate the focal length of the concave lens.
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1' = - 2 cm h = ?
f1 = + 30 cm v1 = +30cm
f2 = -20cm h = 2 cm
u2 = 30 - 26 = 4 cm
1/v2 = 1/f2 + 1/u2 = -1/20 + 1/4 = 1/5
v2 = 5 cm from concave lens.
magnification = v2/u2 = 5/4 = 1.25
New image size = h" = h' * 1.25 = 2.5
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