51.

When LED is forward biased, then

• electrons from the n-type material cross the p-n junction and recombine with holes in the p-type material

• electrons and holes neutralise each other

• at junction electrons and holes neutralize each other

• None of the above

High-frequency waves are

• affected by the solar cycle

• reflected by ${\mathrm{D}}_2$ layer

• absorbed by the ${\mathrm{F}}_2$

• capable of use for long-distance communication on the moon

Maximum velocity of photoelectron emitted is 4.8m/s. If e/m ratio of the electron is $1.76 \times {10}^{11} {\mathrm{CKg}}^{-1}$ then stopping potential is given by

• 5 $\times$ 10^-10 J/C

• 3 $\times$ 10^-7 J/C

• 7 $\times$ 10^-11 J/C

• 2.5 $\times$ 10^-2 J/C

The de-Broglie wavelength of an electron having 80 eV of energy is nearly (1 eV = 1.6 × 10^-19J, mass of electron = 9.1 × 10^-31 kg, Planck's constant = 6.6 × 10^-34 J-s)

• 140 Å

• 0.14 Å

• 14 Å

• 1.4 Å

The distance between two coherent sources is 1 mm. The screen is placed at a distance of 1 m from the sources. If the distance of the third bright fringe is 1.2 mm from the central fringe, the wavelength of light used is

• 4000 Å

• 5000 Å

• 6000 Å

• 7200 Å

Two polaroids are crossed. If now one of them is rotated through 30° and unpolarized light of intensity IO is incident on the first polaroid, then the intensity of transmitted light will be

• I_o/4

• 3I_o/4

• 3I_o/8

• I_o/8

A wavefront is represented by the plane y = 3-x. The propagation of the wave takes place at

• 45° with x direction

• 135° with x direction

• 60° with x direction

• No sufficient data

The intensity ratio of two coherent sources of light is P. They are interfering in the same region and produce an interference pattern. Then the fringe visibility is

• $\frac{1+\mathrm{P}}{2\sqrt{\mathrm{P}}}$

• $\frac{2\sqrt{\mathrm{P}}}{1+\mathrm{P}}$

• $\frac{\mathrm{P}}{1+\mathrm{P}}$

• $\frac{2\mathrm{P}}{1+\mathrm{P}}$

The electrical conductivity of semiconductor increases, when electromagnetic radiation of a wavelength shorter than 24800 Å is incident on it. The bandgap for the semiconductor is

• 0.9 eV

• 0.7 eV

• 0.5 eV

• 1.1 eV

The wavefront of a wave has direction with wave motion

• parallel

• perpendicular

• opposite

• at an angle of θ