First slide

Ray optics

Question

A simple microscope consists of a concave lens of power –10D and a convex lens of power +20D in contact. If the image is formed at infinity, then the magnifying power is (D = 25 cm)

Moderate

A

2.5
B

3.5
C

2.0
D

3.0

Solution

P = Power of combination = $P_{1} + P_{2} = (+ 20) + (- 10) = + 10 D$
$∴ F = \frac{1}{P} = \frac{1}{10} m = 10 cm .$
$For normal vision m = \frac{D}{F} = \frac{25}{10} = 2 .5$

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