Light with an energy flux of 25 x 10⁴ Wm^-2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15cm² the average force exerted on the surface is

(1)1.25x10^-6N

(2)2.5x10^-6N

(3)1.2x10^-6N

(4)3.0x10^-6N

When the energy of the incident radiation is increased by 20%, the kinetic energy of the photoelectrons emitted from a metal surface increases from 0.5 eV to 0.8 eV. The work function of the metal will be:
1. 0.65 eV

2. 1.0 eV

3. 1.3 eV

4. 1.5 eV

What will be the percentage change in the de-Broglie wavelength of the particle if the kinetic energy of the particle is increased to 16 times its previous value?
1. 25
2. 75
3. 60
4. 50

The wavelength \(\lambda_{e}\) of an electron and \(\lambda_{p}\) of a photon of the same energy E are related as:

1. ${\lambda }_p\propto {{\lambda }_e}^2$

2. ${\lambda }_p\propto {\lambda }_e$ 

3. ${\lambda }_p\propto \sqrt{{\lambda }_e}$

4. ${\lambda }_p\propto \frac{1}{\sqrt{{\lambda }_e}}$

A 200W sodium street lamp emits yellow light of wavelength $0.6 \mu m.$ Assuming it to be 25% efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is 

(1) $1.5\times {10}^{20}$                               

(2) $6\times {10}^{18}$

(3) $62\times {10}^{20}$                                 

(4) $3\times {10}^{19}$

Monochromatic radiation emitted when electron on hydrogen atom jumps from first excited to the ground state irradiates a photosensitive material. The stopping potential is measured to be 3.57 V.The threshold frequency of the material is:

(1)$4\times {10}^{15}Hz$                               

(2)$5\times {10}^{15}Hz$

(3)$1.6\times {10}^{15}Hz$                           

(4)$2.5\times {10}^{15}Hz$

An $\alpha ‐$particle moves in a circular path of radius 0.83 cm in the presence of a magnetic field of \(0.25 \mathrm{~Wb} / \mathrm{m}^2\). The de-Broglie wavelength associated with the particle will be:

1. $1 \stackrel{0}{A}$                                                   

2. $0.1 \stackrel{0}{A}$

3. $10 \stackrel{0}{A}$                                                 

4. $0.01 \stackrel{0}{A}$

If the momentum of an electron is changed by p, then the de-Broglie wavelength associated with it changes by 0.5%. What is the initial momentum of the electron?

1. 200p

2. 400p

3. $\frac{\mathrm{p}}{200}$

4. 100p

In the Davisson and Germer experiment, the velocity of electrons emitted from the electron gun can be increased by

1. increasing the filament current

2. decreasing the filament current

3. decreasing the potential difference between the anode and filament

4. increasing the potential difference  between the anode and filament

A radioactive nucleus of mass M emits a photon

of frequency $\nu$ and the nucleus recoils. The recoil

energy will be:

(1) ${\mathrm{h}}^2{\mathrm{\nu }}^2/2 {\mathrm{Mc}}^2$

(2) zero

(3) h$\nu$

(4) ${\mathrm{Mc}}^2-\mathrm{h\nu }$