9.4 - Focus 2

1. 9.4 - Focus 2 The reconceptualisation of the model of light led to an understanding of the photoelectric effect and black body radiation.

2. From his equations linking electricity, magnetism and light, Maxwell predicted the existence of an em spectrum (different waves that travel at the speed of light). Hertz was the first scientist to verify Maxwell’s prediction. Like others he investigated whether em radiation was associated with electrical discharge. Maxwell’s Equations

3. Hertz’s Experiments • Hertz used an oscillator (plates with brass knobs), a high voltage induction coil and two loops that contained small gaps across which a spark could jump.

4. Hertz’s Experiments • Hertz produced standing waves of a known frequency (from the oscillator) by reflecting his generated waves off a metal sheet. • He calculated wavelength by doubling the distance between consecutive positions of maximum interference (ie. largest sparks in the detector loop). • v=f a value close to c = 3.0 x 108 ms-1

5. Activity 11.1 (p.207 JAC) provides a method for the production and reception of radio waves (as do walkie talkies). If a radio is not tuned to an AM station, static/ interference is heard when an induction coil is switched on nearby. The induction coil produces radio waves which are then received by the radio. Radio Waves in the Lab

6. Planck’s Hypothesis • To explain the shape of the black body radiation curve (experimentally determined), Planck had to hypothesise that the radiation emitted and absorbed by the walls of a black body is quantised.

7. Quantum Theory • Classical (wave) theory could not explain the black body radiation curve or the photoelectric effect. • Planck’s introduced the concept of quantum theory with his hypothesis. • He suggested that the discrete nature, E=nhf, was due to properties of the atoms rather than the energy itself.

8. As a result of the shortcomings of the classical theory, light (an example of em radiation) is explained in terms of photons with particular energy and frequency. You are usually asked to calculate the energy of a photon with a particular wavelength in nm. 1 nm = 1 x 10-9 m c = f E = hf = hc/  Photon model of light

9. The Photoelectric Effect • Hertz was the first to observe the photoelectric effect in his radio wave experiments but did not investigate it. • He observed a stronger spark when uv light was incident on the detector loop and a weaker spark when glass was used to prevent uv radiation from reaching the detector loop.

10. The photoelectric effect refers to the emission of electrons from the surface of a metal by em radiation of sufficient energy (ie. frequency). Uv light incident on a clean piece of zinc resting on top of a negatively charged electroscope allows the photoelectric effect to be observed. The uv light causes the electroscope to discharge quickly. The Photoelectric Effect

11. Lenard’s Observations • Lenard used the apparatus to investigate how the energy of the emitted electrons (photoelectrons) varied with the intensity of light.

12. Lenard’s Observations • Lenard used light of different frequencies and intensities. • He found that the intensity affected the number of photoelectrons and that frequency affected the kinetic energy of the photoelectrons (if high enough).

13. Einstein’s Explanation • Einstein gave a simple explanation of Lenard’s results by extending Planck’s hypothesis to the radiation itself being quantised rather than what was allowed by the atomic oscillators. • KE of photoelectron = Energy of photon (hf) - metal’s work function (W)

14. Possible Calculations • Although it is unlikely, you could be asked to calculate the maximum kinetic energy of particular photoelectrons, photon energies and/or work functions. • KE = hf - W • Electron volts (eV) may be used as the unit of work function instead of joules. • 1 eV = 1.602 x 10-19 J.

15. Einstein’s Contribution • describe black body radiation curve and Planck’s hypothesis in terms of oscillators and quanta, E=hf • distinguish between quantum and classical theory • give results of photoelectric effect experiments and Einstein’s explanation in terms of quanta (+ve) • distinguish between Planck and Einstein’s quanta and the relationship to wave-particle duality (+ve) • refer to further progress of quantum theory being hindered by Einstein’s lack of belief in it (-ve) • make an appropriate judgement of his contribution • see question 26 of SCEGGS Trial 2004

16. Planck was a nationalist. Planck saw scientific research unconnected to social and political forces and stayed in Germany with his research funded by a government whose policies he did not support. Einstein was a pacifist but his theories led to the development of the atomic bomb. Einstein thought that scientific research should be removed from political forces but understood that they were not. As a Jew he was ridiculed and fled Nazi Germany. Planck versus Einstein

17. Photocells and Solar Cells • The photoelectric effect liberates electrons and enables a current to flow in both photocells and solar cells. • The vacuum tube is the main component of a photocell (focus 1). • Semiconductors are the main component of solar cells (focus 3).

18. Photocells • Photons of light strike the concave photoemissive surface (cathode). • Electrons liberated by the photoelectric effect move towards the focal point (anode). • Electrons move through the external circuit to balance the charges on the electrodes.

19. Solar Cells • Solar cells are made of a p-n diodes. • Each p-n diode has an intrinsic E-field in the depletion layer. • The E-field causes the electrons liberated by photons of light to only flow in one direction. • This movement of charge causes electrons to flow in the circuit.

20. Similarities: photoelectric effect involved in the flow of current neither device can operate with light below a threshold frequency. Differences: structure and function: vacuum tube versus solid state (operation, size and ruggedness) uses: automatic doors/ ‘magic eyes’ compared with solar electricity Comparison