Question

a) Describe in brief one experimental evidence by giving the name of the experiment and

the phenomenon it exhibited, that shows light has: [3]

(i) wave property.

(ii) particle property.

b) The Einstein’s equation for the photoelectric effect is,

hf = KEmax + W0

where W0 is the work function of the metal.

(i) By referring to the terms involved in the equation, explain the physical process this

equation represents. [3]

When the surface of a metal is illuminated with monochromatic light, photoelectrons are

emitted. How will the number of photoelectrons emitted and the maximum speed of the

photoelectrons are affected when: [4]

(ii) the light intensity is increased but its frequency remains constant? Explain.

(iii) the light frequency is increased but the intensity remains constant? Explain.

Answer 1

a i) wave property

The single slit experiment led t the Huygen's principlie, which evidently shows that the light has wave like properties.

when light was made to fall on the slit, light was diffracted and wavefronts like ripples in water were seen that showed wave like property.

a ii) particle property

photoelectric effect, When light fell on the surface of a metal, the photon in the light would strike an electron in the metal leading to its ejection.

b i)

The Einstein’s equation for the photoelectric effect is,

hf = KEmax + W0

When a photon of high energy (hf) would fall on a metal surface that has a work function(Wo), which is the mimimum energy requried to remove the electron from the metal surface, the electron would get ejected if and only if hf > Wo and the electron would have a maximum kinetic energy of KEmax.

=> hf > Wo

=> hf = Wo + KEmax

b ii)

when the light intensity is increased but its frequency remains constant, the number of photoelectrons emitted increased leading to a current rise, as increasing the light intensity leads to the increase in the energy of the incoming photons while the frequency is kept constant.

b iii)

when the light frequency is increased but the intensity remains constant, the kinetic energy of the photoelectrons increased. We know, E = hf, hence, energy and frequency are directly proportional.