A 200W sodium street lamp emits yellow light of wavelength $0.6<mspace\; width="1em"></mspace>\mu m.$ 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. $3\times {10}^{19}$

Electron volt is a unit of 
(1) Potential                         

(2) Charge

(3) Power                             

(4) Energy

In an electron gun, the electrons are accelerated by the potential V. If e is the charge and m is the mass of an electron, then the maximum velocity of these electrons will be

(1) $\frac{2\mathrm{eV}}{\mathrm{m}}$                   

(2) $\sqrt{\frac{2\mathrm{eV}}{\mathrm{m}}}$

(3) $\sqrt{\frac{2\mathrm{m}}{\mathrm{eV}}}$                 

(4) $\frac{{\mathrm{V}}^2}{2\mathrm{em}}$

An AIR station is broadcasting the waves of wavelength 300 metres. If the radiating power of the transmitter is 10 kW, then the number of photons radiated per second is

(1) $1.5\times {10}^{29}$             

(2) $1.5\times {10}^{31}$

(3) $1.5\times {10}^{33}$             

(4) $1.5\times {10}^{35}$

The threshold wavelength for photoelectric emission from a material is 5200 Å. Photo-electrons will be emitted when this material is illuminated with monochromatic radiation from :

(1) 50-watt infrared lamp

(2) 50-watt ultraviolet lamp

(3) 1-watt ultraviolet lamp

(4) Both (2) and (3)

The threshold frequency of a metal is \(10^{15}~\text{Hz}.\) When light with a wavelength \(\lambda=4000~\mathring{{A}}\) is incident on its surface, which of the following statements is correct?

The photo-electrons emitted from a surface of sodium metal are such that 

(1) They all are of the same frequency

(2) They have the same kinetic energy

(3) They have the same de Broglie wavelength

(4) They have their speeds varying from zero to a certain maximum

In a photo cell, the photo-electrons emission takes place
(1) After ${10}^{-1}$ sec on incident of light rays

(2) After ${10}^{-3}$ sec on incident of light rays

(3) After ${10}^{-6}$ sec on incident of light rays

(4) After ${10}^{-8}$ sec on incident of light rays

Photocell is a device to: 

(1) Store photons

(2) Measure light intensity

(3) Convert photon energy into mechanical energy

(4) Store electrical energy for replacing storage batteries