Which substance is used as a smoke screen in warfare?

In the following common emitter circuit,if $\beta = 100$,$V_{CE} = 7\,V$,$V_{BE} = 0$,and $R_C = 2\,k\Omega$,then find the value of $I_B$ in $mA$.

For the reaction $2NO_2 + F_2 \to 2NO_2F$,the following mechanism has been provided:
$NO_2 + F_2 \xrightarrow{slow} NO_2F + F$
$NO_2 + F \xrightarrow{fast} NO_2F$
Thus,the rate expression of the above reaction can be written as:

The velocity at the mean position of a particle executing $S.H.M.$ is $v$. What is the velocity of the particle at a distance equal to half of the amplitude?

Two lenses in contact made of materials with dispersive powers in the ratio $2 : 1$,behave as an achromatic convex lens of focal length $10\, cm$. The individual focal lengths of the lenses are

If the imaginary part of $\frac{2z+1}{iz+1}$ is $-2$,then the locus of the point representing $z$ in the Argand plane is