Question

Define the magnifying power of a compound microscope when the final image is formed at infinity. Why must both the objective and the eyepiece of a compound microscope has short focal lengths? Explain.

Answer 1

When the final image is formed at infinity.
When the final image is formed at infinity, the angular magnification due to the eyepiece is

me = D/f_e

Thus, the total magnification when the image is formed at infinity can be defined as the product of magnification of objective lens and eyepiece. i.e

m = m_om_e = (L/f_o) (D/f_e)

From the above the equation, we can see that to achieve a large magnification of a small object, the objective and eyepiece should have small focal lengths.