Draw a labelled ray diagram of a compound microscope with the final image at the near point. Derive its magnifying power. How can the magnifying power be increased?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define magnifying power. The ratio of the angle subtended at the eye by the final image to the angle subtended by the object placed at the near point.
2 · Draw the ray diagram. [Objective and eyepiece; AB just beyond Fₒ; real, inverted A′B′ just inside Fₑ; virtual final image A″B″ at distance D; label Fₒ, Fₑ, L, D and the eye]
3 · Magnification by the objective. tan β = h/fₒ = h′/L, so mₒ = L/fₒ.
4 · Magnification by the eyepiece. It acts as a simple microscope: mₑ = 1 + D/fₑ.
5 · Total magnifying power.
m = mₒ × mₑ = (L/fₒ)(1 + D/fₑ) with the final image at the near point
m = (L/fₒ)(D/fₑ) with the final image at infinity
6 · Say how to increase it. Use an objective and an eyepiece of short focal lengths, and a longer tube.
7 · Real life. Lab technicians use it to spot malaria parasites in blood smears.
8 · Close in one line. The objective makes a magnified real image, and the eyepiece magnifies it again.
Scoring tip: For the near-point case, draw A′B′ inside the eyepiece's focus; for normal adjustment, draw it at the focus. Always draw the eye.
