31.

Assertion: Magnetic Resonance Imaging (MRI) is a useful diagnostic tool for producing images of various parts of human body. 

Reason:  Protons of various tissues of human body play a role in MRI.

• If both assertion and reason are true and reason is the correct explanation of the assertion

• If both assertion and reason are true but reason is not the correct explanation of the assertion

• If assertion is true, but reason is false

• Both assertion and reason are false statements

Assertion: Diamagnetic materials can exhibit magnetism.

Reason: Diamagnetic materials have permanent magnetic dipole moment.

• If both assertion and reason are true and reason is the correct explanation of the assertion

• If both assertion and reason are true but reason is not the correct explanation of the assertion

• If assertion is true, but reason is false

• Both assertion and reason are false statements.

Assertion: An emf  $\overline{\mathrm{E}}· \overline{\mathrm{dl}}$  is induced in a closed loop where magnetic flux is varied. The induced E is not a conservative field. 

Reason:  The line integral $\overline{\mathrm{E}}·\overline{\mathrm{dl}}$ around the closed loop is non-zero.

• If both assertion and reason are true and reason is the correct explanation of the assertion

• If both assertion and reason are true but reason is not the correct explanation of the assertion

• If assertion is true, but reason is false

• Both assertion and reason are false statements

In refraction, light waves are bent on passing from one medium to the second medium, because in the second medium

• the frequency is different

• the coefficient of elasticity is different

• the speed is different

• the amplitude is smaller

A wire mesh consisting of very small squares is viewed at a distance of 8 cm through a magnifying converging lens of focal length 1O cm, kept close to the eye. The magnification produced by the lens is

• 5

• 8

• 10

• 20

Hard X - rays for the study of fractures in bones should have a minimum wavelength of 10^-11 m. The accelerating voltage for electrons in X-ray machine should be

• < 124.2 kV

• > 124.2 kV

• between 60 kV and 70 kV

• = 100 kV

In photoelectric effect, the electrons are ejected from metals if the incident light has a certain minimum

• wavelength

• frequency

• amplitude

• angle of incidence

A lens is made of flint glass (refractive index= 1.5). When the lens is immersed in a liquid of refractive index 1.25, the focal length

• increases by a factor of 1.25

• increases by a factor of 2.5

• increases by a factor of 1.2

• decreases by a factor of 1.2

When a p-n diode is reverse biased,then

• no current flows

• the depletion region is increased

• the depletion region is reduced

• the height of the potential barrier is reduced

The operation of a nuclear reactor is said to be critical, if the multiplication factor  ( K ) has a value

• 1

• 1.5

• 2.1

• 2.5