The modern man differs from the apes in

In an astronomical telescope in normal adjustment,a straight black line of length $L$ is drawn on the objective lens. The eyepiece forms a real image of this line. The length of this image is $l$. The magnification of the telescope is

If $\int \sec ^2 x \operatorname{cosec}^4 x \, dx = -\frac{1}{3} \cot ^3 x + k \tan x - 2 \cot x + C$,then $k$ is equal to

The wavelengths of light used in an optical instrument are ${\lambda _1} = 4000\,\mathring{A}$ and ${\lambda _2} = 5000\,\mathring{A}$. The ratio of their respective resolving powers (corresponding to ${\lambda _1}$ and ${\lambda _2}$) is:

Assertion : Bond angle of $H_2S$ is smaller than $H_2O$.
Reason : Electronegativity of the central atom increases,bond angle decreases.

Wood charcoal is used in gas masks because ........