The aperture of the objective lens of a telescope is made large so as to

How to increase the resolving power of a telescope and a microscope?

The human eye has an approximate angular resolution of $\theta = 5.8 \times 10^{-4} \text{ rad}$ and a typical photo printer prints a minimum of $300 \text{ dpi}$ (dots per inch, $1 \text{ inch} = 2.54 \text{ cm}$). At what minimal distance $d$ should a printed page be held so that one does not see the individual dots (in $\text{ cm}$)?

The separation between two microscopic particles is measured as $P_A$ and $P_B$ by two different lights of wavelength $2000 \; \mathring{A}$ and $3000 \; \mathring{A}$ respectively. Then:

The ratio of resolving powers of an optical microscope for two wavelengths $\lambda_1 = 4000\,\mathring{A}$ and $\lambda_2 = 6000\,\mathring{A}$ is

Our eye is most sensitive for which of the following wavelengths in $\mathring{A}$?