If the focal length of the objective lens is increased:
- AThe magnifying power of both the microscope and the telescope will decrease
- BThe magnifying power of the microscope will increase but that of the telescope will decrease
- CThe magnifying power of the microscope will decrease but that of the telescope will increase
- DThe magnifying power of both the microscope and the telescope will increase
Show answer and explanation
Correct answer: (c) The magnifying power of the microscope will decrease but that of the telescope will increase
Explanation
The objective lens works in opposite ways in the two instruments:
- Compound microscope: magnifying power is about (L/fₒ) × (D/fₑ), so it is inversely proportional to the focal length of the objective. A longer fₒ gives less magnification. That is why a microscope uses an objective of very short focal length.
- Telescope: magnifying power = fₒ/fₑ, so it is directly proportional to the focal length of the objective. A longer fₒ gives more magnification.