31.

A battery consists of a variable number 'n' of identical cells (having internal resistance 'r' each) which are connected in series. The terminals of the battery are short-circuited and the current I is measured. Which of the graphs shows the correct relationship between I and n?

• 

• 

• 

• 

The magnetic potential energy stored in a certain inductor is 25 mJ, when the current in the inductor is 60 mA. This inductor is of inductance

• 0.138 H

• 138.88 H

• 13.89

• 1.389 H

In Young's double slit experiment the separation d between the slits is 2 mm, the wavelength λ of the light used is 5896 Å and distance D between the screen and slits is 100 cm. It is found that the angular width of the fringes is 0.20°. To increase the fringe angular width to 0.21° (with the same λ and D) the separation between the slits needs to be changed to

• 1.8 mm

• 19 mm

• 1.7 mm

• 2.1 mm

An astronomical refracting telescope will have large angular magnification and high angular resolution when it has an objective lens of

• Small focal length and large diameter

• Large focal length and small diameter

• Small focal length and small diameter

• Large focal length and large diameter

Unpolarised light is incident from air on a plane surface of a material of refractive index 'µ'. At a particular angle of incidence 'i', it is found that the reflected and refracted rays are perpendicular to each other. Which of the following options is correct for this situation?

• Reflected light is polarised with its electric vector parallel to the plane of incidence

• Reflected light is polarised with its electric vector perpendicular to the plane of incidence

• i = tan^-1( 1/μ )

• i = sin^-1( 1/μ )

An em wave is propagating in a medium with a velocity $\overrightarrow{V} = V\hat{i}$ . The instataneous oscillating electric field of this em wave is along +y axis. Then the direction of oscillating magnetic field of the em wave will be along

• -z direction

• +z direction

• -x direction

• -y-direction

The refractive index of the material of a prism is √2 and the angle of the prism is 30°. One of the two refracting surfaces of the prism is made a mirror inwards, by a silver coating. A beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface) if its angle of incidence on the prism is

• 60°

• 45°

• zero

• 30°

An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be

• 30 cm away from the mirror

• 36 cm away from the mirror

• 36 cm towards the mirror

• 30 cm towards the mirror

For a radioactive material, the half-life is 10 minutes. If initially there are 600 number of nuclei, the time is taken (in minutes) for the disintegration of 450 nuclei is

• 20

• 10

• 15

• 30

The ratio of kinetic energy to the total energy of an electron in a Bohr orbit of the hydrogen atom, is

• 1:1

• 1:-1

• 1:-2

• 2:-1