Photoelectric Effect

1. Photoelectric Effect Quantum Physics Lesson 1

2. Comment made circa 1900 (Believed to be from Lord Kelvin) • "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement"

3. Learning Objectives • Describe the three main conclusions of the photo-electric effect. • Define the Electron Volt, Work Function & Threshold Frequency • State and use the photoelectric equation.

4. What is Light? • In the late 19th Century, scientists thought it was a wave and had lots of experimental evidence to back it up. • … but there was a problem…

5. Photoelectric Effect First observed in 1839, by Alexandre Edmond Becquerel. If you shine light on to a metal it can emit electrons.

6. Safety • Don’t look directly into the laser! • Put signs on the door. • UV only shines on zinc plate.

7. Photoelectric Effect Try this experiment and observe what happens when you shine a normal light, laser light and UV light on to the negatively charged metal plate. Will the result be the same each time? Make notes of your observations. How can you explain your observations?

8. or No effect No effect With U.V. leaf falls immediately (Diagrams: resourcefulphysics.org) Photoelectric Effect What if 100 lasers were directed onto the plate? Would that have an effect?

9. Homework • Complete Past Paper Question – may need to look up answer to part (b)! • Complete worksheet but not questions that are crossed out – don’t need to know that bit! • I will post a link to some useful online notes over the weekend on Unit 1 page. • I will collect and mark next Thursday.

10. Significant Figures • 1. All non-zero digits are significant. • 2. In a number without a decimal point, only zeros BETWEEN non-zero digits are significant. E.g. sig in 12001 but not in 12100 • 3. In a number with a decimal point, all zeros to the right of the right-most non-zero digit are significant. 12.100  5 s.f.

11. Use a Reasonable Number of S.F. • Try to use the same s.f. as those provided in the question or just one more. • Example: • A man runs 110 metres in 13 seconds, calculate its average speed. • Speed = Distance/Time = 110 metres / 13 seconds • =8.461538461538461538461538461538 m/s • =8.46 m/s seems acceptable. • If in doubt use 3 s.f. – can’t be too far wrong.

12. Recap • What is the photo-electric effect?

13. For a given metal there is a minimum frequency called the threshold frequency below which there is no emission. Photoelectrons are emitted with a range of KE from 0 up to a maximum which increases as the frequency increases. Nothing to do with intensity. Number of photoelectrons emitted per second is proportional to the intensity of the incident radiation. Three Main Conclusions

14. Analogies If you’re stuck down a well you can’t get out unless you have enough energy to jump out in one go – same for an electron. Coconut Shy – can fire 1,000 ping pong balls at a coconut – but they’re just ping pong balls, not going to knock the coconut off! It only takes one bullet though...that does have enough energy and momentum

15. What’s going on?

16. Why is this important? Only conclusion 3 can be explained if light is a only a wave. The other 2 conclusions can explained by thinking of light as arriving in discrete packets of energy called quanta. Evidence that light consists of tiny particles called photons!

17. Definitions (From Past Papers) • The Work Function:- • minimum energy to remove an electron from the surface of a metal • The Threshold Frequency:- • minimum frequency of electromagnetic radiation required to eject photoelectrons from a metal surface

18. THE ‘ULTRAVIOLET CATASTROPHE’ 1900 - Rayleigh This was a CLASSICAL prediction, first made in the late 19th century, that an IDEAL BLACK BODY at thermal equilibrium will emit radiation with INFINITE POWER. Max Planck resolved this issue by postulating that electromagnetic energy did not follow the classical description, but could only oscillate or be emitted in DISCRETE PACKETS OF ENERGY proportional to the frequency. He called these packets ‘QUANTA’. Note:

19. Photon Energy We can work out the energy of an incoming photon:- Symbol Equation:- Where E is the Energy of Photon in Joules (J) f is the Frequency of the Radiation in Hertz (Hz) h is Planck’s constant = 6.63 x 10-34 Js Word Equation:-

20. Photon Energy Recall from GCSE that f = c/λ so we can substitute this into the photon energy equation E=hf to get: Or in words:-

21. Worked Example • Q: What is the photon energy for UV radiation with a wavelength 400 nm?

22. Worked Example • Q: What is the photon energy for UV radiation with a wavelength 400 nm? • λ = 400 nm = 400 x 10-9 m • E = ? • h = 6.63 x 10-34 J s • c = 3 x 108 ms-1

23. The Electron Volt Defined as:- The amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt. Energy = Charge × Potential Difference = 1.602×10−19 C × 1 V = 1.602×10−19 J Note that 1 V = 1 JC −1

24. THE PHOTOELECTRIC EFFECT 1905 - Einstein The emission of electrons from a surface (usually metallic) upon exposure to, and absorption of, electromagnetic radiation. The photoelectric effect was explained mathematically by Einstein who extended the work on QUANTA as developed by Planck.

25. More Equations • The process of tearing an electron loose takes up an amount of energy called the work function,Φ, and the rest is converted into kinetic energy, EK(max) • So when emission occurs we use Einstein’s equation:- • Or in Symbols:-

26. More Equations • When the light incident on the metal is at exactly the threshold frequency the photons have just enough energy to free the electrons (i.e. the work function) • In Symbols:- • where f0 is the threshold frequency.

27. Learning Objectives Describe the three main conclusions of the photo-electric effect. Define the Electron Volt, Work Function & Threshold Frequency State and use the photoelectric equation.

28. Einstein ‘All the fifty years of conscious brooding have brought me no closer to the answer to the question, “What are light quanta?”. Of course, today every rascal thinks he knows the answer, but he is deluding himself.’

29. Physics Workshop Every Wednesday 3.40pm-4.40pm in O8 Is this time good for most people?

30. Definition Capello verb To expect the very best. To not accept excuses. To get the job done. Get Lampard and Gerrard playing well in the same team. I’m going to Capello you! (well, apart from number 4)

31. Today’s Objectives Explain the occurrence of line spectra (e.g. of atomic hydrogen) as evidence of transitions between discrete energy levels in atoms Calculate the energy of emitted photons using the equation hf = E1 - E2. Explain what is meant by the terms ionisation and excitation Enter and store numbers in your calculator.

32. Gas Discharge Lamp When the gas-discharge lamp is switched on the gas is ionised. Free electrons, accelerated by the electrical field in the tube, collide with gas and metal atoms. Some electrons circling around the gas and metal atoms are excited by these collisions, bringing them to a higher energy state. When the electron falls back to its original state, it emits a photon, resulting in visible light or ultraviolet radiation.

33. Quantisation A quantum mechanical system or particle that is bound, confined spatially, can only take on certain discrete values of energy, as opposed to classical particles, which can have any energy. These values are called energy levels. The term is most commonly used for the energy levels of electrons in atoms or molecules, which are bound by the electric field of the nucleus. The energy spectrum of a system with energy levels is said to be quantized.

34. Emission Spectra Each element has its own specific set of lines.

35. Spectra

36. Emission spectra An energy input raises the electrons to higher energy levels. This energy input can be by either electrical, heat, radiation or particle collision. When the electrons fall back to a lower level there is an energy output. This occurs by the emission of a quantum of radiation. When ever possible, electrons occupy the lowest energy level called the ground state.

37. Electron in orbit round a nucleus in an atom Energy input. The electron is excited and rises to a higher energy level (shell). The electron falls back to its original energy level and energy is emitted in the form of radiation. The bigger the drop the greater the energy emitted and the shorter wavelength the radiation has (blue light).

38. Equation In words:- In Symbols:- Note that E1 and E2 refer to the energies of the energy levels and hf is the energy of the photon emitted when an electron falls from the higher level to the lower level.

39. Energy Levels • “Why do the states have negative energy?” • This is because the zero of energy is considered to be that of a free electron 'just outside' the atom. • All energy states 'below' this – i.e. within the atom are therefore negative. • Energy must be put into the atom to raise the electron to the 'surface' of the atom and allow it to escape.

40. The Electron Volt (Again) An electron volt is the kinetic energy gained by an electron when it is accelerated through a potential difference of 1V. W=QV W=1.60 x 10-19 C x 1 V (V=JC-1) W=1.60 x 10-19 J = 1eV Let’s try convert an energy (eV  J)

41. Definitions Ionisation When an atom loses an orbiting electron (and becomes charged)  from exam. An electron is removed from an atom (making it a positive ion).  from exam. Comment: Would gaining an electron also count as ionisation? Excitation An electron is raised to a higher energy level but remains within the atom.

42. Definitions Ground state – the lowest energy level of an atom. Excited State – when one or more of an atom’s electrons moves to an outer shell at higher energy. An energy input raises the electrons to higher energy levels.

43. Absorption spectra When light of all frequencies is passed through a gas then the gas absorbs light of the same frequency as it would emit. The light is radiated in all directions causing a reduction of intensity in the direction of the observer (dark lines). And so is seen when emitted energy is absorbed by a medium.

44. How do we get emission spectra?

45. Quick Question • Evaluate:- • using your calculator.

46. Physics Workshop Every Wednesday 3.40pm-4.40pm in O8 Is this time good for most people?

47. Homework • For Friday 18th September:- • Research using books & the internet • How do fluorescent tubes work? • Why are they used? • Use the words ionisation and excitation in your answer. • Don’t copy and paste  put in your own words!

48. Today’s Objectives Understanding of ionisation and excitation in the fluorescent tube  Homework. Calculator (Order of Operations, BODMAS). Re-arrange equations. Use prefixes & Converting between unit magnitudes.

49. Extra point to note • One photon releases one photo-electron.

50. Order of Operations • B - Brackets first • O - Orders (ie Powers and Square Roots, etc.) • DM - Division and Multiplication (left-to-right) • AS - Addition and Subtraction (left-to-right) • 30 ÷ 5 × 3 =6 × 3= 18    Left to Right is the conventional order and is what your calculator does. • 30 ÷ 5 × 3 =30 ÷ 15= 2  For this to be the case brackets would have to go around the (5 × 3) • Note that the fact that the 5 and 3 are put on the bottom on the previous slide implies they should be multiplied first.