Question

(i) Draw a neat labelled ray diagram of a compound microscope. Explain briefly its working. 

(ii) Why must both the objective and the eye-piece of a compound microscope have short focal lengths?

Answer 1

Working: Suppose a small object AB is placed slightly away from the first focus F₀' of the objective lens. The objective lens forms the real, inverted and magnified image A'B', which acts as an object for eyepiece. The eyepiece is so adjusted that the image A'B' lies between the first focus F_e' and the eyepiece E. The eyepiece forms its image A'' B'' which is virtual, erect and magnified. Thus the final image A'' B'' formed by the microscope is inverted and magnified and its position is outside the objective and eyepiece towards objective lens. Magnifying power of compound microscope is

 for final image at distance of distinct vision

 for final image at infinity

(ii) For large magnifying power, f₀ and f_e both have to be small.