question_answer 29)

The work function for the following metals is given : Na : 2-75 eV ; K : 2-30 eV ; Mo : 4-17 eV ; Ni : 5-15 eV. Which of these metals will not give photoelectric emission for a radiation of wavelength 3300 Å from a He- Cd laser placed 1 m away from the photocell ? What happens if the laser is brought nearer and placed 50 cm away ?  

Answer:

Here,    Distance,  and  Using the relation we get Since the energy E of the incident photon of light is greater than the work functions of all the metals given in the question, photoelectric emission will occur in all the metals. The distance of the source does not increase or decrease the energy of the photon of light incident, therefore, the energy of electrons ejected will not change but the intensity of ejected electrons will increase  and become four times.