51.

A wire carrying current I and other carrying 2I in the same direction produce a magnetic field B at the mid-point. What will be the field when 2I wire is switched off?

• B/2

• 2B

• B

• 4B

Magnification of a compound microscope is 30. The focal length of eye-piece is 5 cm and the image is formed at a distance of distinct vision of 25 cm. The magnification of the objective lens is:

• 6

• 5

• 7.5

• 10

A ray falls on a prism ABC (AB = BC) and travels as shown in the figure. The least value of the refractive index of the material of the prism should be;

• 1.5

• $\sqrt{2}$

• 1.33

• $\sqrt{3}$

In nuclear reaction

${}_{72}{}^{180}\mathrm{X} \stackrel{-\mathrm{\alpha }}{\to } \mathrm{Y} \stackrel{-\mathrm{\beta }}{\to } \mathrm{Z} \stackrel{-\mathrm{\alpha }}{\to } \mathrm{A} \stackrel{-\mathrm{\gamma }}{\to } \mathrm{P}$

the atomic mass and number of P are, respectively:

• 170,69

• 172,69

• 172,70

• 170,70

A radioactive substance has activity 64 times higher than the required normal level. If T_1/2 = 2h, then the time, after which it should be possible to work with it, is:

• 16 h

• 6 h

• 10 h

• 12 h

The slit width, when a light of wavelength 6500 Å is incident on a slit, if first minima for red light is at 30°, is :

• 1 × 10^-6 m

• 5.2 × 10^-6 m

• 1.3 × 10^-6 m

• 2.6 × 10^-6 m

A flat mirror revolves at a constant angular velocity making 2 revolutions/s. With what velocity will a light spot move along a spherical screen with a radius of 10 m if the mirror is at a centre of curvature of the screen?

• 251.2 m/s

• 261.2 m/s

• 271.2 m/s

• 241.2 m/s

A vessel consists of two plane mirrors at right angles (as shown in the figure). The vessel is filled with water. The total deviation in incident ray is

• 0°

• 90°

• 180°

• none of these

An object is placed at a distance equal to the focal length of a convex mirror. If the focal length of the mirror is f, then the distance of the image from the pole of the mirror is :

• less than f

• equal to f

• more than f

• infinity

The Fermi level of an intrinsic semiconductor is pinned at the centre of the bandgap. The probability of occupation of the highest electron state in valence band at room temperature will be

• zero

• between zero and half

• half

• one