51.

An α -particle of energy 5 MeV is scattered through 180° by gold nucleus. The distance of closest approach is of the order of

• 10^-14 cm

• 10^-10 cm

• 10^-16 cm

• 10^-12 cm

Find the de-Broglie wavelength of an electron with kinetic energy of 120 eV.

• 112 pm

• 95 pm

• 124 pm

• 102 pm

Light of two different frequencies whose photons have energies 1 eV and 2.5 eV respectively, successively illuminate a metallic surface whose work function is 0.5 eV. Ratio of maximum speeds of emitted electrons will be

• 1 : 2

• 1 : 5

• 1 : 1

• 1 : 4

The polarising angle of glass is 57°. A ray of light which is incident at this angle will have an angle of refraction as

• 43°

• 25°

• 38°

• 33°

To observed diffraction, the size of the obstacle

• should be much larger than the wavelength

• has no relation to wavelength

• should be of the order of wavelength

• should be $\frac{\mathrm{\lambda }}{2}$ where λ is the wavelength

A radioactive decay can form an isotope of the original nucleus with the emission of particles

• one α and one β

• one α and four β

• four α and one β

• one α and two β

The half-life of a radioactive substance is 20 min. The time taken between 50 % decay and 87.5 % decay of the substance will be

• 25 min

• 30 min

• 10 min

• 40 min

A nucleus at rest splits into two nuclear parts having radii in the ratio 1 : 2. Their velocities are in the ratio

• 4 : 1

• 8 : 1

• 2 : 1

• 6 : 1

What is the wavelength of light for the least energetic photon emitted in the Lyman series of the hydrogen spectrum ? (take, hc = 1240 eV nm)

• 122 nm

• 82 nm

• 150 nm

• 102 nm

If an electron in hydrogen atom jumps from an orbit of level n = 3 to an orbit oflevel n =2, the emitted radiation has a frequency (R = Rydberg constant, c = velocity of light)

• $\frac{8\mathrm{Rc}}{9}$

• $\frac{3\mathrm{Rc}}{27}$

• $\frac{5\mathrm{Rc}}{36}$

• $\frac{\mathrm{Rc}}{25}$