1. Index Unit 03 Electron Configuration Module 02: Light as a Particle Based on the PowerPoints By Mr. Kevin Boudreaux, Angelo State Univerisity U03Mod01 Light as a Wave1

2. Index Module 02: Student Expectations Be able to: – state how we know light behaves like a particle. – identify a description/diagram of Blackbody radiation. – identify a description/diagram of photoelectric effect – identify a description/diagram of atomic line spectra – calculate energy, wavelength, or frequency using Planck’s equation or Planck’s equation combined with the speed of light equation.

3. Index Light is Not Just a Wave It was discovered that there are several natural phenomena that could not be explained using the wave models of classical physics. 1.blackbody radiation 2.the photoelectric effect 3.atomic line spectra Ultimately, an entirely new way of looking at energy was required to understand these phenomena. This explanation – quantum mechanics – also led to our modern understanding of the structure of the atoms.

4. Index Problem 1: Blackbody Radiation Blackbody radiation is the visible glow that solid objects give off when heated, as a result of the vibration of electrons within the solid (e.g., the heating element of a metal stovetop, the hot metal filament in a light bulb). When iron is heated, it first glows a dull red color, becomes orange as the temperature increases, then a blinding white glare, and eventually a dull blue. If light were only a wave you would get the expected curves (dashed lines) shown below and on the next slide. The longer wavelengths (red light) have a lower intensity while the shorter wavelengths (blue light) have a higher intensity. If this trend were to continue, the intensity would be even higher in the UV range. Instead, the intensity drops off in the UV range so that almost no light in the UV range is produced. This is known as the UV catastrophe.

5. Index Problem 1: Blackbody Radiation

6. Index Blackbody Radiation

7. Index Solution: Max Planck In 1900, Max Planck (Nobel Prize, 1918) concluded that the energy radiated by a heated object can’t be continuously variable; instead the energy is emitted only in packets known as quanta (singular, quantum). Quantum is a packet of energy The amount of energy, E, associated with each quantum of energy is given by the Planck equation:

8. Index Problem 2: The Photoelectric Effect It was discovered that cathode ray tubes with the voltage turned down would not conduct electricity unless light was shining on the cathode. Photoelectric tube – a vacuum tube where electrons will not flow through the tube unless light is shining on the metal of the cathode.

9. Index Problem 2: The Photoelectric Effect The photoelectric effect occurs when light shines on a metal, causing it to emit electrons. – The frequency of the light must be above some threshold frequency, which is different for each metal. If the frequency is below the threshold no electrons are emitted, no matter how bright the light is. – Low frequency light will not eject electrons from the metal, no matter how bright the light or how long it shines, but high frequency light will eject electrons, even if the light is dim.

10. Index Solution: Einstein Albert Einstein (1905, Nobel Prize, 1921) explained the photoelectric effect by proposing that light behaves as if it were a steam of small, massless packets of energy, called photons, having an energy given by the Planck equation: The energy of a photon of EM radiation is proportional to its frequency (color, wavelength); while the intensity (brightness) of the light is due to the to the number of photons – When a low-energy (low ν) photon strikes the metal, nothing happens, but a high-energy (high ν) photon strikes with enough force to knock an electron loose.

11. Index So, Is Light a Wave or a Particle? Yes In classical electromagnetic theory, light was picture as purely a wave phenomenon, with an energy that was continuously variable. – The wave model explains phenomena such as refraction, diffraction, etc., that don’t make sense by treating light as a particle. Now we are suddenly picturing light as a shower of particles, each having an energy of hν. – The particle model explains blackbody radiation and the photoelectric effect, that don’t make sense by treating light as a wave. It is now known that in addition to behaving as waves, light can also behave as small particles. It has a dual wave- particle nature.

12. Index Energy Equations

13. Index Energy Example What is the energy of electromagnetic radiation that has a frequency of 4.295 x 10 14 Hz?

14. Index Energy Example What is the energy of electromagnetic radiation that has a frequency of 4.295 x 10 14 Hz?

15. Index Energy Example What is the energy of electromagnetic radiation that has a frequency of 4.295 x 10 14 Hz?

16. Index Energy Example What is the energy of electromagnetic radiation that has a frequency of 4.295 x 10 14 Hz?

17. Index Energy Example What is the energy of electromagnetic radiation that has a frequency of 4.295 x 10 14 Hz?

18. Index Energy Example What is the energy of electromagnetic radiation that has a frequency of 6.4 x 10 5 Hz?

19. Index Energy Example What is the energy of electromagnetic radiation that has a wavelength of 523 nm?

20. Index Energy Example What is the energy of electromagnetic radiation that has a wavelength of 344 m?

21. Index Energy Example What is the frequency of electromagnetic radiation that has an energy of 4.55 x 10 -18 J? What type of radiation is it?

22. Index Energy Example What is wavelength of electromagnetic radiation that has an energy of 4.6 x 10 -20 J? What type of radiation is it?

23. Index Energy Example When electron falls from energy level 3 to energy level 2, red visible light with a wavelength of 656nm is produced. What is the energy difference between energy levels 3 and 2?

24. Index Energy Example If the difference in energy between energy levels 6 and 2 is 4.8482x10 -19 J (∆E 6-2 =4.8482x10 -19 J), what type of electromagnetic radiation is produced?