What is light? What is Light? Energy. Clearly, Light Can Be Described as a Wave 2/6/2013. The photoelectric effect

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1 What is light? What is Light? The photoelectric effect Is there any evidence that light is a form of energy? Energy Can be carried as a waves (continuous) Can be carries by baseballs (discrete) Which is more like light? Light travels in a Straight Line Light Reflects Consider how light behaves. Light Refracts Clearly, Light Can Be Described as a Wave Light Diffracts Light Interferes 1

2 Waves Seem to Work By the 1850s the wave model of light was the generally accepted one. Light as a Wave Also Explains Doppler Shift We are Familiar With the Shift in Sound Waves Doppler Shift in Light Waves (a) Sound of a train moving towards us is higher pitched (b) Sound of a train moving away from us is lower pitched If source approaches, light appears bluer. If source recedes, light appears redder. But Waves Cannot Explain ALL of Lights Behavior 2

3 When Don t Waves Work? Electron transitions in the Bohr model of the atom and their emission of light (spectral lines) provides an example of when light should be viewed as a packet of energy called a photon. Waves also cannot explain photographic film exposures. There are two further pieces of evidence of this packet-like nature of light: blackbody radiation photoelectric effect Photoelectric Effect the emission of electrons from a metal surface upon exposure to light. (How solar cells work.) This effect cannot be explained using a wave model of light. The Photoelectric Effect Why is this surprising? How can electrons get free of their material? What is the energy used for? If light behaves like other wave phenomena, what effect would changing the color and brightness of the light be expected to have on the ejected electrons (sometimes called photoelectrons )? Do you understand and agree with these predictions? When the experiments are carried out, the results DO NOT agree with the predictions. In fact, they are nearly the opposite. What should a scientist do when the results of an experiment are not as expected? Light as a particle 3

4 Unexpected results can cause a researcher to tweak an existing model or scrap it altogether in favor of a completely different idea. Perhaps light is not behaving as a wave at all. Instead it behaves in this experiment as small independent bits of discrete energy amounts called photons. Summary of particle observations Light Packets! Let s see if you got it The next slides will review the previous drawings. Test yourself to see if you got it. Don t got it? Keep reviewing until you do. Each electron can absorb one and only one photon. If the photon is energetic enough, the electron can escape with KE of motion. This is medium brightness blue light. The photons are energetic enough that the electrons can escape and have some KE. How would the picture look different if the blue light were brighter? How will the behavior of the electrons be different with brighter light? Bright light is light that has MORE photons. More electrons can absorb a photon and escape. Since the energy of the photons remains the same, the KE of the e- stays the same. What if the blue light is exchanged for higher frequency purple light (or UV)? How will the picture look different? How will the behavior of the electrons be different with higher frequency light? 4

5 Higher frequency light has more energetic (larger) photons. Each electron absorbs a larger packet of energy and therefore has more KE (travels faster) as it flies away. Electrons absorbing purple light (or UV) fly off with great energy (speed). What if the blue light is exchanged for lower frequency red light? How will the picture look different? How will the behavior of the electrons be different with lower frequency light? Lower frequency light has less energetic (smaller) photons. Each electron absorbs a smaller packet of energy. It cannot even escape. Electrons absorbing red light NEVER fly off no matter how bright the light is. Why doesn t red light work, no matter how bright it is? Red photons have very little energy. Each electron only absorbs one photon. Red photons do not provide enough energy to even escape the material, let alone have speed. No matter how bright the red light (how many red photons there are), no electrons escape. Make Sense! Red light is used in photographic darkrooms because it is not energetic enough to break the halogen-silver bond in black and white films Ultraviolet light causes sunburn but visible light does not because UV photons are more energetic Our eyes detect color because photons of different energies trigger different chemical reactions in retina cells Light has a dual nature. In some experiments it behaves like a wave. In other experiments, it behaves like a photon. It never acts like both at the same time. It always acts like one or the other. What is light, wave or particle? 5

6 End of Photoelectric Effect Assign: Worksheet 27W 6