NEET UG 2013 — Optics

A parallel beam of fast moving electrons is incident normally on a narrow slit. A fluorescent screen is placed at a large distance from the slit. If the speed of the electrons is increased, then which of the following statements is correct?

A parallel beam of light of wavelength $\lambda$ is incident normally on a narrow slit. A diffraction pattern formed on a screen placed perpendicular to the direction of the incident beam. At the second minimum of the diffraction pattern, the phase difference between the rays coming from the two edges of slit is:

A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices $\mu_1$ and $\mu_2$ and $R$ is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is

In Young's double slit experiment the distance between the slits and the screen is doubled. The separation between the slits reduced to half. As a result the fringe width:

In Young's double slit experiment, the slits are $2 \mathrm{~mm}$ apart and are illuminated by photons of two wavelengths $\lambda_1=12000 Å \quad$ and $\lambda_2=10000 Å$. At what minimum distance from the common central bright fringe on the screen $2 \mathrm{~m}$ from the slit will a bright fringe from one interference pattern coincide with a bright fringe from the other?

The reddish appearance of the sun of sunrise and sunset is due to :

Two plane mirrors are inclined at $70^{\circ}$. A ray incident on one mirror at angle, $\theta$ after reflection falls on second mirror and is reflected from there parallel to first mirror. The value of $\theta$ is: