In telescopes for a given wavelength, the objectives with large aperture are used for

In Young's double slit experiment, width of the second slit is double the width of first slit, consequently the amplitude of the light from two slits. '$I_m$' is the maximum intensity. The resultant intensity '$I$' when they interfere with the phase difference of $\phi$ is given by

In a single slit diffraction experiment, for wavelength '$\lambda$', half angular width of the principal maxima is '$\theta$'. Also for wavelength of light '$p\lambda$', the half angular width of the principal maxima is '$q\theta$'. The ratio of the half angular widths of the first secondary maxima in the first case to second case will be

Three identical polaroids $P_1$, $P_2$ and $P_3$ are placed one after another. The pass axis of $P_2$ and $P_3$ are inclined at angle of $60^\circ$ and $90^\circ$ with respect to axis of $P_1$. The source has an intensity $I_0$. The intensity of light finally coming out is