Question Bank

UNIT 7: DUAL NATURE OF MATTER & RADIATION

VERY SHORT ANSWER TYPE QUESTIONS 1 Mark

1. With what purpose was famous Davisson- Germer experiment with

electrons performed?

2. De- Broglie wavelength associated with an electron accelerated

through a potential difference V is λ. What will be its wavelength when

the accelerating potential is increased to 4 V?

3. Electrons are emitted from a photosensitive surface when it is

illuminated by green light but electron emission does not take place by

yellow light . Will the electron be emitted when the surface is

illuminated by : (i) red light , and (ii) blue light ?

4. Ultraviolet light is incident on two photosensitive materials having

work functions W1 and W2 ( W1 > W2). In which case will the kinetic

energy of the emitted electrons be greater ? Why ?

5. Write the name given to the frequency νc, in the following graph (

Showing the variation of the stopping potential ( V0) with the frequency

( ν ) of the incident radiation ) for a given photosensitive material. Also

name the constant , for the photosensitive material , obtained by the

multiplying νc with Planck’s constant.

6. Two metals A and B have work functions 2 eV and 4 eV respectively.

Which of the two metals has a smaller threshold wavelength ?

7. In an experiment on photoelectric effect , the following graphs were

obtained between the photoelectric effect ( I ) and the anode

potential ( V). Name the characteristic of the incident radiation that was

kept constant in this experiment.

8. Name the experiment for which the following graph, showing the

variation of intensity of scattered electrons with the angle of

scattering , was obtained . Also name the important hypothesis

that was confirmed by this experiment .

9. What is the de-Broglie wavelength ( in angstrom ) associated with an

electron accelerated through a potential of 100 V ?

SHORT ANSWER TYPE QUESTIONS: 2 & 3 MARKS

1. An electron and a proton have the same kinetic energy. Which of the

two has a greater wavelength? Explain.

2. Given below is the graph between frequency (ν 0) of the incident light

and maximum kinetic energy ( E k) of emitted photoelectrons. Find the

values of (i) threshold frequency and (ii) work function from the graph.

3. In a photoelectric effect experiment, the graph between the stopping

potential ‘V’ and frequency ‘ν’ of the incident radiations on two

different metal plates P and Q are shown in the figure.

a) Which of the two metal plates, P and Q has greater value of work

function?

b) What does the slope of the lines depict?

4. Define the terms threshold frequency and stopping potential in

relation to the phenomenon of photoelectric effect. How is the

photoelectric current affected on increasing the (i) frequency (ii)

intensity of the incident radiations and why?

5. Sketch the graphs showing the variation of stopping potential with

frequency of incident radiations for two photosensitive materials A and B

having threshold frequencies vo > vo respectively.

(i) Which of the two metals, A or B has higher work function?

(ii) What information do you get from the slope of the graphs?

(iii) What does the value of the intercept of graph ‘A’ on the

potential axis represent?

6. Sketch a graph between frequency of incident radiations and stopping

potential for a given photosensitive material. What information can be

obtained from the value of the intercept on the potential axis?

7. Draw the graphs showing the variation of photoelectric current with

anode potential of a photocell for (i) the same frequencies but different

intensities I1 > I2 > I3 of incidents radiation, (ii) the same intensity but

different frequencies v1 > v2 > v3 of incident radiation. Explain why the

saturation current is independent of the anode potential.

8. Explain the laws of photoelectric emission on the basis of Einstein’s

photoelectric equation. Write one feature of the photoelectric effect

which cannot be explained on the basis of wave theory of light.

9. Mention the significance of Davission – Germer experiment. An α –

particle and a proton are accelerated from rest through the same

potential difference V. Find the ratio of de – Broglie wavelengths

associated with them.

10. Draw a schematic experimental arrangement used by Davisson and

Germer to establish the wave nature of electrons. Describe briefly how

the de – Broglie relation was experimentally verified in the case of

electrons.

Plot a graph showing variation in intensity of the diffracted beam with

scattering angle Ө for a typical accelerating voltage where the

constructive interference in this experiment occur.

11. Define the term ‘ work function’ of a metal. The threshold

frequency of a metal is fo. When the light of frequency 2f0 is incident on

the metal plate, the maximum velocity of electrons emitted is vi . When

the frequency of the incident radiation is increased to 5f0, the maximum

velocity of electrons emitted is v2. Find the ratio of v1 to v2.

12. Obtained the expression for the wavelength of de- Broglie wave

associated with an electron accelerated from rest through a potential

difference V. The two lines A and B shown in the graph plot the de –

Broglie wavelength (λ) as a function of 1/ √V (V is the accelerating

potential) for two particles having the same charge. Which of the two

represents the particle of heavier mass?

13. Red light, however bright it is , cannot produce the emission of

electrons from a clean Zinc surface . But even weak ultraviolet radiation

can do so. Why?

X – rays of wave length ‘λ’ fall on photosensitive surface, emitting

electrons. Assuming that the work function of the surface can be

neglected, prove that the de – Broglie wavelength of electrons emitted

will be √ hλ/ 2mc.

1/√V

λ

B

A

LONG ANSWER TYPE QUESTIONS

11. Derive the expression for the de- Broglie wavelength of an

electron moving under a potential difference of V volt.

Describe Davisson and Germer experiment to establish the wave nature

of electrons. Draw a labeled diagram of the apparatus used.

12. Red light, however bright, cannot cause emission of electrons from

a clean zinc surface. But even weak ultraviolet radiations can do so.

Why?

Draw the variation of maximum kinetic energy of emitted electrons with

the frequency of incident radiation on a photosensitive surface. On the

graph drawn, what do the following indicate (i) slope of the graph and

(ii) intercept on energy axis?

13. Explain the term : ‘Stopping Potential’ and ‘Threshold frequency’

in photoelectric emission. Draw a graph showing the variation of stopping

potential with frequency of incident light in relation to photoelectric

effect. Deduce an expression for the slope of this graph using Einstein’s

photoelectric equation.

14. Define the term:

(a) (i) Work function, (ii) Threshold frequency and (iii) Stopping potential,

with reference to photoelectric effect.

(b) Calculate the maximum kinetic energy of electrons emitted from a

photosensitive surface of work function 3.2 eV, for the incident radiation

of wavelength 300nm.