Question

If the focal length of objective lens is increased then magnifying power of

• A. microscope will increase but that of telescope decrease
• B. microscope and telescope both will increase.
• C. microscope and telescope both will decrease
• D. microscope will decrease but that of telescope will increase.

Answer 1

The Correct Option is D

To understand the effect of increasing the focal length of the objective lens on the magnifying power of a microscope and a telescope, we need to examine how magnifying power in both instruments is related to the focal length of the objective lens.

• Microscope:
  The magnifying power of a microscope is given by the formula: M = \dfrac{D}{f_o} \times \dfrac{L}{f_e}, where M is the magnifying power, D is the least distance of distinct vision, f_o is the focal length of the objective lens, L is the length of the microscope tube, and f_e is the focal length of the eyepiece.
  As we can see from the formula, the magnifying power is inversely proportional to the focal length of the objective lens (f_o). Therefore, increasing the focal length of the objective lens will decrease the magnifying power of the microscope.
• Telescope:
  The magnifying power of a telescope is given by the formula: M = \dfrac{f_o}{f_e}, where f_o is the focal length of the objective lens and f_e is the focal length of the eyepiece.
  In contrast to the microscope, the magnifying power of a telescope is directly proportional to the focal length of the objective lens. Hence, increasing the focal length of the objective lens will increase the magnifying power of the telescope.

Therefore, the correct answer is: microscope will decrease but that of telescope will increase.