31.

A radioactive element  ${}_{90}\mathrm{X}^{238}$ decays into ${}_{83}\mathrm{Y}^{ 222}$. The number of β-particles emitted are 

• 1

• 2

• 4

• 6

Minimum excitation potential of Bohr's first orbit in hydrogen atom is

• 3.6 V

• 10.2 V

• 13.6 V

• 3.4 V

A luminous object is placed at a distance of 30 cm from the convex lens of focal length 20 cm.  On the other side of the lens, at what distance from the lens a convex mirror of radius of curvature 10 cm be placed in order to have an upright image of the object coincident with it

• 30 cm

• 60 cm

• 50 cm

• 12 cm

A point source of light is placed 4 m below the surface of water of refractive index 5/3. The minimum diameter of a disc which should be placed over the source on the surface of water to cut-off all light coming out of water is

• 6 m

• 3 m

• 4 m

• 2 m

Ionisation potential of hydrogen atom is 13.6 eV. Hydrogen atom on the ground state rarely excited by monochromatic radiation of photon 12.1 eV. The special line emitted by a hydrogen atom according to Bohr's theory will be

• one

• two

• three

• four

The binding energy per nucleon is maximum in the case

• ${}_{92}{}^{235}\mathrm{U}$

• ${}_{56}{}^{141}\mathrm{Ba}$

• ${}_{26}{}^{56}\mathrm{Fe}$

• ${}_4{}^2\mathrm{He}$

Two coherent light beams of intensities I and 41 are superposed. The maximum and minimum possible  intensities in the resulting beam are

• 5I and I

• 5I and 3I

• 9I and I

• 9I and 3I

The half-life of radio isotope is 4 h. If initial mass of the isotope was 200 g, then mass remaining after 24 h will be

• 1.042 g 

• 2.084 g

• 3.125 g

• 4.167 g

Which logic gate is represented by the following combination of logic gates?

• OR

• NOR

• AND

• NAND

The work function for metals A, Band C are respectively 1.92  eV,  2.0 eV and 5 eV. According to Einstein's equation the metals which will emit photo, electrons for a radiation of wavelength 4100 A^o is/are

• none

• A only

• A and B only

• All the three metals