Radii of curvature of a converging lens are in the ratio 1 : 2. Its focal length is 6 cm and refractive mdex is 1.5. Then its radii of curvature are respectively

• 9 cm and 18 cm

• 6 cm and 12 cm

• 3 cm and 6 cm

• 4.5 cm and 9 cm

Light emitted during the de excitation of electron from n = 3 to n = 2, when incident on a metal, photoelectrons are just emitted from that metal. In which of the following de excitations photoelectric effect is not possible ?

• From n = 2 to n = 1

• From n = 3 to n = 1

• From n = 5 to n = 2

• From n = 4 to n = 3

The additional energy that should be given to an electron to reduce its de-Broglie wavelength from 1 nm to 0.5 nm is

• 2 times the intial kinetic energy

• 3 times the initial kinetic energy

• 0.5 times the initial kinetic energy

• 4 times the initial kinetic energy

The ionisation energy of an electron in the ground state of helium atom is 24.6 eV. The energy required to remove both the electron is

• 51.8 eV

• 79 eV

• 38.2 eV

• 49.2 eV

Maximum velocity of the photoelectron emitted by a metal is 1.8 x 10⁶ ms^-1. Take the value of specific charge of the electron is 1.8 x 10¹¹ C kg^-1. Then the stopping potential in volt is

• 1

• 3

• 9

• 6

λ₁ and λ₂ are used to illuminate the slits. β₁ and β₂ are the corresponding fringe widths. The wavelength λ₁ can produce photoelectric effect when incident on a metal. But the wavelength λ₂ cannot produce photoelectric effect. The correct relation between β₁ and β₂ is

• β₁ < β₂

• β₁ = β₂

• β₁ > β₂

• β₁ ≥ β₂

47.

Pick out the correct statements from the following

1. Electron emission during β-decay is always accompanied by neutrino.
2. Nuclear force is charge independent.
3. Fusion is the chief source of stellar energy.

• I, II are correct

• I, III are correct

• Only I is correct

• II, III are correct

A nucleus ${}_{\mathrm{Z}}{\mathrm{X}}^{\mathrm{A}}$ emits an α-particle with velocity v. The recoil speed of the daughter nucleus is

• $\frac{\mathrm{A} - 4}{4\mathrm{v}}$

• $\frac{4\mathrm{v}}{\mathrm{A} - 4}$

• v

• $\frac{\mathrm{v}}{4}$

A radioactive substance emits 100 beta particles in the first 2 s and 50 beta particles 1n the next 2 s. The mean life of the smaple is

• 4 s

• 2 s

• $\frac{2}{0.693} \mathrm{s}$

• 2 × 0.693 s

In which of the following statements, the obtained impure semiconductor is of p-type ?

• Germanum is doped with bismuth

• Silicon is doped with antimony

• Germanium is doped with gallium

• Silicon is doped with phosphorus