Question

A simple microscope is used to see the object first in blue light and then in red light. Due to the change from blue to red light, what is the effect on its magnifying power?

• A. decreases
• B. increases
• C. remains constant
• D. first increase and then decreases

Hint:

The focal length is longer for red light, so M = 1 + D/f is smaller.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The magnifying power of a simple microscope (image at the near point) is \(M = 1 + \dfrac Df\), where \(D\) is the least distance of distinct vision.

Step 2: Effect of colour:
The refractive index of glass is smaller for red light than for blue. By the lens maker's formula \(\dfrac1f = (\mu - 1)\left(\dfrac1{R_1} - \dfrac1{R_2}\right)\), a smaller \(\mu\) gives a larger \(f\).

Step 3: Result:
For red light \(f\) is larger, so \(\dfrac Df\) is smaller and \(M\) decreases.

Final Answer:
Red light gives a longer focal length, so M decreases. \[ \boxed{\text{(A) }\text{decreases}} \]