Exploring Photoelectric Effect & Quantum Theory

1. 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

2. 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.

3. Photoelectric effect

4. 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.

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

6. 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

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

8. 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.

9. 2012 Qu: 15

10. Quantum TheoryDoes 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.

11. Quantum TheoryDoes 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.

12. Quantum TheoryDoes 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.

13. 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

14. 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

15. 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)

16. 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.

17. 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

18. 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. Example

19. 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.

20. Example 2: 2007

21. 2001 Qu: 19 B

22. 2003 Qu: 18 A

23. 2004 Qu: 15 D

24. 2006 Qu: 17 A

25. Past Paper Questions • 2008, Qu: 29 • 2005, Qu: 29 • 2000, Qu: 28