The Photoelectric Effect

Question 1

On what factors do the magnitude of photoelectric current, the velocity or ejected electrons depend during photoelectric emission?

Answer 1

Photoelectric current depends on the intensity of the incident light, and the velocity of the emitted photo electrons depends on the frequency of the incident light. Both are directly proportional to their respective factors.

Question 2

What is meant by threshold wavelength?

Answer 2

The maximum wavelength beyond which photo electrons cannot be emitted from a meta surface.

Question 3

Two metals A and B have work functions 4 eV and 6 eV respectively. Which metal has lower threshold wavelength for photoelectric effect?

Answer 3

Metal B, because energy is inversely proportional to wavelength. The lower the wavelength, the higher the energy.

Question 4

Does ‘threshold frequency’ depend upon the frequency of the incident light?

Answer 4

No, it depends on the source of the light and is a factor in determining the velocity of the velocity and kinetic energy of the emitted photo electrons.

Question 6

What is cut-off or stopping potential?

Answer 6

It is the ‘negative’ potential applied to the second plate of the metal surface from which photo electrons are emitted. At this value, photoelectric current completely stops as electrons do not possess enough kinetic energy after emission from the first plate to reach the second plate due to the high difference in negative potential.

Question 7

What is meant by the kinetic mass of a photon? What about momentum?

Answer 7

The mass produced due to the motion of the photon (=hf/c^2). However, rest mass is 0.
The momentum of a photon is the product of the Kinetic mass of a photon and its velocity.

Question 10

How many photons are effective in the emission of one photo electron? Does every photon eject a photo electron?

Answer 10

One photon is effective enough for emission.
Not every photon may eject a photo electron, depending on the distance between the electron and the metal surface and the amount of energy it loses in collisions.

Question 11

Radiation of frequency 10^15 Hz falls on surfaces P, Q and R. P does not emit electrons, Q emits electrons with zero KE and R emits electrons with KE. What can you say about each surface?

Answer 11

P has the greatest work function, Q requires exactly 10^15 Hz for emission of electrons and so cannot provide KE to its electrons, R has the least work function with threshold frequency below the given amount.

Question 12

How is Stopping Potential related to Kinetic Energy?

Answer 12

eVo=0.5mv^2

Question 13

Photons of energies twice and five times the work function fall upon a metal surface. What will be the ratio of their maximum velocities?

Answer 13

Using the equation E=hf+1/2mv^2,
Substitute 2hf and 5hf respectively and divide the two equations to obtain the answer:
1:2

Question 14

Which of these will be most effective for electron emission: microwave, infrared, ultraviolet?

Answer 14

Ultraviolet, having the largest frequency and the smallest wavelength of all the mentioned rays.

Question 15

Why are alkali metals most suited for photoelectric emission?

Answer 15

Alkali metals have comparatively lower work functions. So, visible light which is readily available is able to eject photo electrons from them.

Question 16

If the maximum kinetic energy of electrons emitted by a photocell is 4 eV, what is its stopping potential?

Answer 16

Using the formula KE=eVo,
Vo=4 Volts
(Remember, if KE is in eV, it is the same as the stopping potential in terms of value).

Question 17

A mirror reflects about 80% of visible light incident on it. How will you find out whether 20% of the photons have not been reflected at all or all the photons have been reflected but their energy has been reduced by 20%?

Answer 17

The first part of the question is true, and the second false.
If the energies were reduced by 20%, then the wavelength would have increased (inversely proportional) and the colour would have changed. However, this does note occur.

Question 18

Calculate the momentum of a photon of energy 6*10^-19.

Answer 18

p=E/c
=(610^-19)/(310^8)
=2*10^-27 kg m/s

Question 19

If the intensity of light falling on the emitting substance of a photocell is increased, what will be the effect on its stopping potential?

Answer 19

The maximum KE, and thus the Vo required to stop the current completely depends on the frequency of the light, and so will remain unchanged.

Question 20

Explain the variation of photoelectric current in a tube with a. Voltage across the tube and b. Intensity of incident radiation.

Answer 20

At all positive voltages of the anode (incident light falls upon the cathode not the anode), current remains maximum because photo electrons emitted from the cathode reach the anode. As the anode is made more and more negative, current slows further because less electrons are reaching it.
Current is directly proportional to the intensity of the incident light.

Question 21

As the anode potential difference is made slightly more positive, why does the current not immediately reach its maximum value?

Answer 21

All photo electrons are not emitted from the surface of the metal, and some are emitted from the interior and required more attraction to generate current by flowing from the cathode to the anode.

Question 22

Does the distance of the source of light from the cathode matter? If so, how?

Answer 22

Intensity is dependent on distance of the source. The closer it is, the greater the intensity. If current has been generated in the photoelectric cell, the distance will play a role in determining its magnitude.

Question 23

Electrons are emitted from a surface when green light falls on it but not when yellow light does. Will I. Red light emit electrons, or II. Blue light emit electrons?

Answer 23

Blue light will but red light won’t because the wavelengths are lesser and greater than green respectively.

Question 24

It is harder to remove a photo electron from copper than sodium. Which has a greater work function?

Answer 24

Copper, because the threshold frequency is much higher.

Question 25

Is photoelectric emission possible at all frequencies?

Answer 25

No, if the energy produced by that frequency is less than the required work function of the metal surface.