6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light Did the Greeks Invent Everything?!? 6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light

585 BCE – 1900 CE Beginning with the Greeks and continuing until around 1900, the system of “Classical Physics” slowly emerged with these fundamental ideas at its core… Newton’s Laws define mechanics Light is a WAVE Atoms make up matter and are indivisible In around 1900, physicists were announcing that physics is a COMPLETE SCIENCE and can explain EVERYTHING!

Well… almost everything… Physicists in this era were still unable to explain THREE important phenomenon Blackbody Radiation Wave Theory: A blackbody will emit radiation that directly relates to its temperature. The higher the temperature, the more frequencies can be emitted. Experiment: The intensity of radiation does not keep increasing but forms a curve of intensities below the expected amount.

Well… almost everything… Physicists in this era were still unable to explain THREE important phenomenon Bright Line Spectrum Wave Theory: When you heat an element it should radiate a continuous range of colors related to that temperature (spectrum). Experiment: Each element emits only very specific colors in a ‘bright line spectrum’

Well… almost everything… Physicists in this era were still unable to explain THREE important phenomenon Photoelectric Effect Wave Theory: Increasing intensity should give the same number of faster electrons. Increasing frequency should give more electrons per second. Experiment: Increasing intensity gives more electrons per second. Increasing frequency gives faster electrons or makes the effect stop working.

Planck’s Constant Max Planck invented the concept of the QUANTUM to explain the problem of Blackbody Radiation. Derived the constant 6.63 x 10-34 J·s to describe the scale of energy increments allowed in atoms. Energy going into and out of atoms is QUANTIZED. By why?

Photoelectric Effect Albert Einstein uses the concept of the quantum to explain the Photoelectric Effect. Invents the idea of the PHOTON which has an energy related to Planck’s Constant and its frequency. Equation

Einstein’s Model So, we have phenomenon that show that light MUST be a wave… ex. DIFFRACTION and INTERFERENCE …but the PHOTOELECTRIC EFFECT shows that light MUST be made up of particles… SO WHICH IS IT?!? metal plate beam of light a brighter beam would mean MORE photons Each photon has a chance to liberate an electron but the photon must have enough energy (the right frequency) For Einstein’s explanation to be correct light is NOT a wave! It must be made of particles. The particles carry a certain amount of energy related to the light’s frequency. EM Radiation (light) is neither waves nor particles but has properties of BOTH.

Photon Properties Massless Carry energy and momentum Travel at the speed of light Have particle-like collisions – COMPTON EFFCT Before After e- electron gains KE electron incoming photon e- v v = 0 outgoing photon lower E, lower f longer λ

Try it before going to the side with answers! Example #1 What is the energy of a photon with a wavelength of 4.5 x 10-7 meters? What type of photon is it? Try it before going to the side with answers!

Example #1 What is the energy of a photon with a wavelength of 4.5 x 10-7 meters? What type of photon is it? Ephoton = ? λ = 4.5 x 10-7 m c = 3 x 10 8 m/s h = 6.63 x 10-34 J·s Ephoton = hf = hc/λ Ephoton = hc/λ Ephoton = (6.63 x 10-34 J·s)(3 x 108 m/s) / (4.5 x 10-7 m) Ephoton = 4.4 x 10-19 J Referencing the EM Spectrum table, we see that this photon’s wavelength corresponds to Visible Light

Try it before going to the side with answers! Example #2 If a photon has 7.0 x 10-3 joules of energy, what frequency does it have? What type of photon is it? Try it before going to the side with answers!

Example #2 If a photon has 7.0 x 10-3 joules of energy, what frequency does it have? What type of photon is it? Ephoton = 7.0 x 10-3 J f = ? h = 6.63 x 10-34 J·s Ephoton = hf = hc/λ Ephoton = hf f = (7.0 x 10-3 J)/ (6.63 x 10-34 J·s) f = 1.1 x 1031 Hz Referencing the EM Spectrum table, we see that this photon’s frequency corresponds to Gamma Ray

Wave/Particle Duality 1924 Louie de Broglie: because light has properties of particles, it might be possible for matter to exhibit the properties of waves. This was proven to be true by ELECTRON DIFFRACTION experiments demonstrated by Davisson and Germer in 1927. While particles do seem to have a particle nature, this is only evident for FAST-MOVING, SUB-ATOMIC PARTICLES.

ENERGY can be converted into MASS. Mass/Energy Duality In 1905, Einstein publishes his paper on Special Relativity. One of several proposals from this paper is that: MASS can be converted into ENERGY AND ENERGY can be converted into MASS. Equation

Mass/Energy Duality This conversion from mass into energy is how an ATOMIC BOMB functions.

Try it before going to the side with answers! Example #3 How much energy would be released if a 7.0 kilogram mass was completely converted into energy? Try it before going to the side with answers!

Example #3 How much energy would be released if a 7.0 kilogram mass was completely converted into energy? m = 7 kg E = ? c = 3.0 x 108 m/s E = mc2 E = (7 kg)(3.0x108 m/s)2 E = 6.3 x 1017 J (The amount of energy the entire USA uses in a day!)

End of 6.1.1 - PRACTICE