A simple microscope has a magnifying power of 3.0 when the image is formed at the near point (25 cm) of a normal eye. (a) What is its focal length? (b) What will be its magnifying power if the image is formed at infinity?

Sol. The simple microscope has, m = 3, when image is formed at D = 25 cm a) m = 1 + D/f ⇒ 3 = 1 + 25/f ⇒ f = 25/1 = 12.5 cm b) When the image is formed at infinity (normal adjustment) Magnifying power = D/f = 25/12.5 = 2.0