Q21 of 805 Page 1

(a) In photoelectric effect, do all the electrons that absorb a photon come out as photoelectrons irrespective of their location? Explain.

(b) A source of light, of frequency greater than the threshold frequency, is placed at a distance ‘d’ from the cathode of a photocell. The stopping potential is found to be V. If the distance of the light source is reduced to d/n (where n>1), explain the changes that are likely to be observed in the (i) photoelectric current and (ii) stopping potential.

a) It is not necessary that if the energy supplied to an electron is more than the work function it will come out. It comes out of the metal atom only when it absorbs a single photon with energy greater than or equal to the work function. The electron after receiving energy may loose its energy due collisions with other metal atoms. So most of them get scattered into the metal. Only a few electrons near the surface may come out of the surface of the metal for whom the incident energy is greater than the minimum threshold energy to loose an electron (work function) of the metal. So, it is not necessary that all the electrons that absorb a photon come out as photoelectrons irrespective of their location.


b) When the light source is brought near the photocell, the intensity of the light increases, which doesn’t effects the stopping potential but the photo-current increases, the following diagram justifies the above statements,


In the plot, I denotes the intensity of the light.



More from this chapter

All 805 →
19

Draw a labelled ray diagram to show the image formation in a refracting type astronomical telescope in the normal adjustment position. Write two drawbacks of refracting type telescopes.

OR


(a)Define resolving power of a telescope. Write the factors on which it depends.


(b) A telescope resolves whereas a microscope magnifies. Justify the statement.

 20

A jar of height h is filled with a transparent liquid of refractive index μ. At the center of the jar on the bottom surface is a dot. Find the minimum diameter of a disc, such that when it is placed on the top surface symmetrically about the centre, the dot is invisible.

 22

A monochromatic radiation of wavelength 975 Å excites the hydrogen atom from its ground state to a higher state. How many different spectral lines are possible in the resulting spectrum? Which transition corresponds to the longest wavelength amongst them?

 23

Binding energy per nucleon versus mass number curve is as shown. and are four nuclei indicated on the curve


Based on the graph:


(a) Arrange X, W and S in the increasing order of stability.


(b) Write the relation between the relevant A and Z values for the following nuclear reaction.


S → X + W


(c) Explain why binding energy for heavy nuclei is low.


OR


How are protons, which are positively charged, held together inside a nucleus? Explain the variation of potential energy of a pair of nucleons as a function of their separation. State the significance of negative potential energy in this region?