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?

The magnifying power of a simple microscope is given by $M = 1 + \dfrac{D}{f}$ or $M = \dfrac{D}{f}$.

The focal length of a lens is related to the refractive index by the lens maker's formula: $\dfrac{1}{f} = (\mu - 1)\left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right)$.

According to Cauchy's relation, the refractive index decreases with an increase in wavelength: $\mu = A + \dfrac{B}{\lambda^2}$.

Since the wavelength of red light is greater than that of blue light ($\lambda_{\text{red}} > \lambda_{\text{blue}}$), the refractive index for red light is less than that for blue light ($\mu_{\text{red}} < \mu_{\text{blue}}$).

Consequently, the focal length of the lens is greater for red light ($f_{\text{red}} > f_{\text{blue}}$).

Since magnifying power is inversely proportional to focal length, the magnifying power decreases when the light is changed from blue to red ($M_{\text{red}} < M_{\text{blue}}$).

Answer: decreases