In a Young's double slit experiment, an angular width of the fringe is 0.35$$^\circ$$ on a screen placed at 2 m away for particular wavelength of 450 nm. The angular width of the fringe, when whole system is immersed in a medium of refractive index 7/5, is $${1 \over \alpha }$$. The value of $$\alpha$$ is ___________.

In Young's double slit experiment the two slits are 0.6 mm distance apart. Interference pattern is observed on a screen at a distance 80 cm from the slits. The first dark fringe is observed on the screen directly opposite to one of the slits. The wavelength of light will be ____________ nm.

A small bulb is placed at the bottom of a tank containing water to a depth of $$\sqrt7$$ m. The refractive index of water is $${4 \over 3}$$. The area of the surface of water through which light from the bulb can emerge out is x$$\pi$$ m². The value of x is __________.

A light ray is incident, at an incident angle $$\theta$$₁, on the system of tow plane mirrors M₁ and M₂ having an inclination angle 75$$^\circ$$ between them (as shown in figure). After reflecting from mirror M₁ it gets reflected back by the mirror M₂ with an angle of reflection 30$$^\circ$$. The total deviation of the ray will be _____________ degree.