61.

The intensity of gamma radiation from a given source is I. On passing through 36 mm of lead, it is reduced to l/8. The thickness of lead which will reduce the intensity to l/2 will be

• 6 mm

• 9 mm

• 18 mm

• 18 mm

The electrical conductivity of a semiconductor increases when electromagnetic radiation of wavelength shorter than 2480 nm is incident on it. The band gap in (eV) for the semiconductor is

• 1.1 eV

• 2.5 eV

• 0.5 eV

• 0.5 eV

A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed 1 /2 m away, the number of electrons emitted by photocathode would

• decrease by a factor of 4

• increase by a factor of 4

• decrease by a factor of 2

• decrease by a factor of 2

Starting with a sample of pure ⁶⁶Cu, 7/8 of it decays into Zn in 15 minutes. The corresponding half-life is

• 10 minutes

• 15 minutes

• 5 minutes

• 5 minutes

If radius of  nucleus is estimated to be 3.6 Fermi then the radius nucleus

• 6 fermi

• 8 fermi

• 4 fermi

• 4 fermi

A Young’s double slit experiment uses a monochromatic source. The shape of the interference fringes formed on a screen is

• hyperbola

• circle

• straight line

• straight line

A fish looking up through the water sees the outside world contained in a circular horizon. If the refractive index of water is 4/3 and the fish is 12 cm below the surface, the radius of this circle in cm is

Two point white dots are 1 mm apart on a black paper. They are viewed by eye of pupil diameter 3 mm. Approximately, what is the maximum distance at which these dots can be resolved by the eye? [ Take wavelength of light = 500 nm ]

• 5 m

• 1 m

• 6 m

• 6 m

A thin glass (refractive index 1.5) lens has an optical power of – 5D in air. Its optical power in a liquid medium with refractive index 1.6 will be

• 1 D

• 5/8 D

• -1 D

• -1 D

The diagram shows the energy levels for an electron in a certain atom. Which transition shown represents the emission of a photon with the most energy?

• lll

• IV

• I

• I