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)