91.

Li nucleus has three protons and four neutrons. Mass of lithium nucleus is 7.016005 amu. Mass of proton is 1.007277 amu and mass of neutron is 1.008665 amu. Mass defect for lithium nucleus in amu is

• 0.04048 amu

• 0.04050 amu

• 0.04052 amu

• 0.04055 amu

Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index is $\frac{3}{2}$ is

• 2 × 10^-4 s

• 2 × 10⁴ s

• 2 × 10^-11 s

• 2 × 10¹¹ s

Two thin lenses of focal length 20 cm and 25 cm are in contact. The effective power of the combination is

• 4.5 D

• 18 D

• 45 D

• 9 D

The magnification of the image when an object is placed at a distance x from the principal focus of a mirror of focal length f is

• $\frac{\mathrm{x}}{\mathrm{f}}$

• $1 + \frac{\mathrm{f}}{\mathrm{x}}$

• $\frac{\mathrm{f}}{\mathrm{x}}$

• $1 - \frac{\mathrm{f}}{\mathrm{x}}$

In the Young's double slit experiment, the central maxima is observed to be I_0.  If one of the slits is covered, then the intensity at the central maxima will become

• $\frac{{\mathrm{I}}_0}{2}$

• $\frac{{\mathrm{I}}_0}{\sqrt{2}}$

• $\frac{{\mathrm{I}}_0}{4}$

• ${\mathrm{I}}_0$

The ratio of the de-Broglie wavelength of an $\mathrm{\alpha }$-particle and a proton of same kinetic energy is

• 1 : 2

• 1 : 1

• 1 : $\sqrt{2}$

• 4 : 1

Which of the following is not conserved in nuclear reaction ?

• Total energy

• Mass number

• Charge number

• Number of fundamental particles

The number of $\mathrm{\alpha }$-particles and β-particles respectively emitted in the reaction ₈₈A¹⁹⁶ →  ₇₈B¹⁶⁴ are

• 8 and 8

• 8 and 6

• 6 and 8

• 6 and 6

The counting rate observed from a radioactive source at t = 0 was 1600 count/s and at t = 8 s it was 100 counts/s. The counting rate observed as counts per second at t = 6 s, will be

• 400

• 300

• 250

• 200

If n₁ and n₂ are the refractive indices of the core and the cladding respectively of an optic fibre, then

• n₁ = n₂

• n₁ < n₂

• n₂ < n₁

• n₂ = 2 n₁