Question

A 5-W point-source emits monochromatic light of wavelength 5000 Å. How many photons per second strike a unit area placed 5 m away from the source and illuminated by it? What should be the work function of the metal from whose surface this light can liberate photoelectrons? 

  

Solution

Correct option is

The energy of an emitted photon is 

     

             

           .

The power of the source is 5 W, that is, the rate of emission of light is 5 Js–1. Since the energy of one light-photon is , the number of photons emitted per second from the source is         

      

          

The number of photons striking per second per unit area placed 5 m (= R, say) away from the source is 

       

                

Now, 

      Work function of the metal = energy of incident photon  

                                                    

                                                  

                                                  = 2.475 eV.

SIMILAR QUESTIONS

Q1

Calculate the energy of a photon, in electron-volt (eV) and in watt-hour (W h), of wavelength 6600 Å. 

Q2

An X-rays tube produces a continuous spectrum of radiation with its short wavelength end at 0.45 Å. What is the maximum energy of a photon in the radiation? What is the order of accelerating voltage (for electrons) required in such a tube? 

     

Q3

 

In an accelerator experiment on high-energy collisions of the electrons with positrons, a certain event is interpreted as the annihilation of an electron-positron pair of total energy 10.2 BeV into two -rays of equal energy. Find the wavelength associated with each -ray. 

Q4

Energy from Sun is received on earth at the rate of . If average wavelength of solar light is taken as 550 nm, then how many photons are received on earth per meter2 per second? 

Q5

Monochromatic light of frequency  is produced by a laser. The power emitted is . What is the energy of a photon in the light beam?

Q6

A beam of green light  gives energy at the rate of 1 Js–1 to a surface where it is fully absorbed. How many photons reach the surface per second? If every 100 photons emit one electron, how much current will flow from the surface? 

Q7

 

Calculate the number of photons emitted per second by a transmitter of 10 kW power, emitting radio waves of wavelength 500 m. 

Q8

Calculate the number of photons emitted per second by a transmitter of 10 kW power, emitting radio waves of wavelength 500 m. 

Q9

The minimum light intensity that can be perceived by human eye is about 10–10 Wm–2. Find the number of visible-light photons that must enter the pupil of our eye per second for vision. Take the area of the pupil to be about 0.4 cm2 and the average wavelength of visible light to be 5000 Å. 

Q10

The wavelength of a photon is 1.4 Å. It becomes 2.0 Å after a collision with an electron. Calculate the energy of the scattered electron.