Key Areas covered Photoelectric effect as evidence for the particulate nature of light Photons of sufficient energy can eject electrons from the surface of materials The threshold frequency is the minimum frequency of a photon required for photoemission The work function of the material is the minimum energy required to cause photoemission Determination of the maximum kinetic energy of photoelectrons

What we will do today: State what is meant by the photoelectric effect. State how frequency affects the photoelectric effect. State that a beam of radiation carries bundles of energy. Carry out calculations on the energy bundles using Equation E = hf.

Photoelectric effect

Photoelectric effect Sometimes, when electromagnetic radiation above a certain frequency strikes a surface, electrons are emitted. This can be used to detect radiation and is the basis on which solar cells and light dependent resistors operate.

Gold Leaf Electroscope The photoelectric effect can be demonstrated using a gold leaf electroscope:

Results: Only negatively charged electroscopes discharge. Even dim UV light is enough to discharge, this is because it has a high frequency White light does not work as it has a lower frequency

frequency We can see that photoelectric emission depends on frequency. Below a certain frequency, called the threshold frequency, f0, there is no photoelectric emission.

Intensity Increasing intensity at f < f0 will still have no effect. Increasing intensity at f > f0 will cause more photoelectric emission. They are directly proportional. A bigger intensity results in a bigger photoelectric current produced.

2012 Qu: 15

Quantum Theory Does light travel as waves or photons? If we increase intensity at frequencies above f0 will cause more photoelectric emission. But wave theory of light disagrees with this as the waves are continuous ie don’t change. It was suggested by scientist Max Planck in 1905 that light doesn’t travel as a continuous wave but travels in packets called photons.

Quantum Theory Does light travel as waves or particles? Modern physics now takes the view that light can act both like a wave and like a particle without contradiction. It depends on how we test it. If we look for evidence that it is a wave, we can find it. But also, if we look for evidence that it is a particle we can find that too. The universe seems to be made up of things that are both particle-like and wave-like. This is known as wave–particle duality.

Quantum Theory Does light travel as waves or particles? To eject an electron from a metal requires a precise amount of energy. A weak UV source has sufficient energy to do this for a clean zinc surface, but no matter how high the intensity of the white light is, no electrons are ejected. This is true even though, over a period of time, the ‘total’ energy of the white light is greater than that of the UV. In 1904 Einstein applied an earlier idea of Planck to the phenomenon and proposed that light was not a continuous wave, but existed as a stream of ‘packets’ or ‘quanta’. These quanta are called photons and are particles of light (and other electromagnetic radiation), although unlike other particles they have no mass.

Energy of Photons A photon has energy given by: E = h f E = energy (J) f = frequency (Hz) h = Planck’s constant (6.63 x 10-34Js) – given in data sheet

V = f λ Often only the wavelength of light is given. As we know that light travels at 3 x 108 ms-1 (known as c) we can use the equation v = f λ and our energy equation is then re-arranged: E = h c λ

Intensity of Photons If more photons are provided by a more intense beam of light then: I = N h f I – Intensity (Wm-2) N – no. of photons per second per square metre f = frequency (Hz) h = Planck’s constant (6.63 x 10-34 Js)

Work function When a photon is absorbed its energy is used to release an electron. The minimum energy needed by an electron to produce photoelectric emission (escape from a metal) is called the work function, which is dependent on frequency: Work function = h f0 Every metal has a different value for work function.

Any extra energy is kinetic energy, Ek Such an electron would escape but have no kinetic energy. If the energy of the incoming photon, E = hf (where f>f0), is greater than the work function, then the extra energy will appear as kinetic energy: Ek = E – E0 Ek = hf - hf0

Example Find threshold frequency The work function for Gold is 7.84 x 10-19 J A piece of Gold is illuminated with frequency of 1.5 x 1015 Hz Will photoelectrons be emitted from the Gold foil? Find threshold frequency E = hf0 F0 = E h = 7.84 x 10-19 6.63 x 10-34 F0 = 1.18 x 1015 Hz As f = 1.5x1015 Hz f > f0 therefore photoelectrons will be emitted.

Problem solving question: Intensity and current If intensity increases, so does the no. of photons per second per square metre. Therefore there is more photoelectric emission. This means that current (flow of electrons) also increases.

Example 2: 2007

2001 Qu: 19 B

2003 Qu: 18 A

2004 Qu: 15 D

2006 Qu: 17 A

Past Paper Questions 2008, Qu: 29 2005, Qu: 29 2000, Qu: 28