• Photoelectric Effect

Question 1

What is the photoelectric effect?

Answer 1

• If you shine a light of high enough frequency onto the surface of a metal, the metal will emit electrons ( usually the frequency falls within the UV range ).

Question 2

What occurs in the photoelectric effect?

Answer 2

• Free electrons n the surface of the metal absorb energy from the light
• If an electron absorbs enough energy, the bonds holding it to the metal break and the electron is released.
• Emitted electrons are then known as photo-electrons.

Question 3

What are the various conclusions drawn from the photoelectric effect discovery?

Answer 3

• Photoelectric emission of electrons from a metal surface does not take place if the frequency of the incident electromagnetic radiation is below a certain value ( threshold frequency ). Threshold frequency is dependent on the type of metal.
• Number of electrons emitted per second is proportional to the intensity of the incident radiation, provided the frequency is greater than the threshold frequency. If the frequency of the incident radiation is less than the threshold frequency, no photoelectric emission from that metal surface can take place, no matter how intense the incident radiation is.
• Photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface, provided the frequency of the radiation exceeds the threshold frequency, regardless of intensity.
• Photo-electrons are emitted with a variety of kinetic energies, ranging from zero to a maximum value, this value of max Ke increases with the frequency of the radiation, unaffected by the intensity of the radiation.

Question 4

What can wave theory not explain about the photo electric effect?

Answer 4

• Existence of a threshold frequency
• Why photoelectric emission occurs without delay.
• The higher the intensity of the wave, the more energy it should transfer to each electron, therefore Ke should increase with intensity, however there is no reason for Ke depending only on the frequency.
• According to wave theory, the electrons should be emitted eventually no matter the frequency.

Question 5

How does wave theory attempt to explain the photo electric effect?

Answer 5

• For a particular frequency of light, the energy carried is proportional to the intensity of the beam.
• The energy carried by the light would be spread evenly over the wavefront
• Each free electron on the surface of the metal would gain a bit of energy from each incoming wave.
• Gradually each electron should gain enough energy to leave the

Question 6

What did Einstein suggest?

Answer 6

Light is composed of photons ( wave packets ), the suggestion was that EM waves exist in discrete packet called photons.

Question 7

What is the formula for energy of a photon?

Answer 7

E = hf= hc/λ

Question 8

How did Einstein explain the photoelectric effect?

Answer 8

• When light is incident on a metal’s surface, a free electron on the surface will absorb a single photon from the incident light, and thus gain an energy ( E = hf is the energy of a light photon ).
• An electron can leave the metal surface if the energy gained from a single photon exceeds the work function of the metal Φ. Excess energy gained by the photons becomes kinetic energy.

Question 9

What is the work function,Φ?

Answer 9

Minimum energy needed by an electron to escape from the metal surface.

Question 10

What happens to excess energy gained by an electron?

Answer 10

It becomes the electron’s kinetic energy.

Question 11

What is the formula for the maximum kinetic energy of an emitted electron?

Answer 11

• Ekmax = hf - Φ
Ekmax = energy of photon - work function
Re arranging for energy of a photon =
• hf = Ekmax + Φ

Question 12

What is required for emission from a metal surface?

Answer 12

Ekmax > 0 or hf > Φ

Question 13

What is the formula for the threshold frequency of a metal?

Answer 13

• Emission can take place provided:
Ekmax > 0 or hf > Φ

Threshold freqeuncy = fmin = Φ / h

Question 14

What is the stopping potential?

Answer 14

• Minimum potential needed to stop photoelectric emission.
• Electrons that escape the metal plate can be attracted back by providing the metal plate with a sufficient positive charge.

Question 15

What happens to the kinetic energy at the stopping potential, and why does this occur?

Answer 15

• It becomes zero.
• This is because each emitted electron must do extra work equal to e x Vs ( stopping potential ) to leave the metal surface

Question 16

What is the formula for maximum kinetic energy?

Answer 16

• Work done = potential difference x charge
• Ekmax = e x Vs
e = charge on the electron = 1.6 x 10 ^ -19
Vs = stopping potential in Volts
Ekmax = Maximum Kinetic Energy, measured in joules.

Question 17

What is the formula for kinetic energy?

Answer 17

Ek(max) = 1/2mv(max velocity)^2

Question 18

How does intensity of light affect kinetic energy?

Answer 18

• Kinetic energy of the electrons is independent of the intensity, as they can only absorb one photon at a time.
• Increasing the intensity only means more photons per second on an area, each photon will still have the same energy.

Question 19

What happens when a conduction electron absorbs a photon?

Answer 19

• Its kinetic energy increases by an amount equal ot the energy of the photon.
• If the energy of the photon exceeds the work function of the metal, the conduction electron can leave the metal

Question 20

What happens if an electron does not leave a metal following photoelectric emission?

Answer 20

collides repeatedly with other electrons and positive ions and it quickly loses its extra kinetic energy.