4.5 NOTES LIGHT and ENERGY

I. Light and Quantized Energy A. The nuclear atom and unanswered questions Rutherford found the nucleus, but how are the electrons arranged? Recall Bohr said e- are found in energy levels How do electrons behave? BIG IDEA: ELECTRONS BEHAVE LIKE WAVES AND PARTICLES

B. Wave nature of light - – to understand the nature of the atomic structure, we first need to understand light. 2. Electromagnetic spectrum - copy of spectrum; radio cosmic, energy increases, wavelength decreases; don’t need to memorize but be aware of which has more energy

B. Wave nature of light B. Wave nature of light 1. How can waves be measure? Waves’ characteristics: C = λ ν C = speed of light λ (lambda) = wavelength, in units of nm (1 x 10-9), m, Ǻ (1 x 10-10), etc. distance b/w 2 identical parts of wave; ν (nu) = frequency (energy) how fast wave oscillates up & down; # waves pass a point in a second;

Considering light as a wave can be helpful, but it does not fully explain how light interacts with matter. For instance, when heating an object, like iron, why does it emit specific frequencies of light? Or why do metals emit electrons when certain frequencies of light shine on them?

C. Particle nature of light 1. The quantum concept – Max Planck Discovery: Matter can only gain or lose specific amount of energy. Planck called these amounts quantum. A quantum is a little “bundle” of light Light has particles as well as wave Characteristics called bundles, Einstein later called particles photons

Calculations involving light waves Planck used this knowledge to develop an equation to related the frequency of a “packet of energy” to the amount of energy it possessed. Calculations involving light waves c = λν, where c = speed of light, 3.00 X 108 m/s c/λ = ν E = hν, where E = energy, h = Planck’s constant, 6.626 X 10-34 J.s Niels Bohr combined these two equations algebraically: E = hc/λ; J.s = joule x sec; unit of energy; can round to 6.63 (3 sig figs)

You will not be responsible for these equations or the math, but it is important to understand: BIG IDEA: We can use the observed frequency of light to determine how much energy it has.

Einstein 2. The photoelectric effect – Albert Einstein 1905 Defintion: evidence of Quantum Theory; If a light is shined at a piece of metal and the light has enough energy then an e- will be released it is a single beam of light that must have enough energy (2 beams of ½ strength will not work!); If not enough energy in the beam of light, no e-; more than enough energy then e- is released and will move

Planck’s proposal didn’t go well, but couldn’t explain why electrons were released when light was shined on various metals; wave model says continuous long enough time will still eject electrons  not true; each metal requires a specific frequency to release e-, Einstein called specific energies photons (Particle like nature of light) Einstein called a quantum of light a photon. Einstein’s only Nobel Prize :/

The Photoelectric Effect

Atomic emission spectra bright line spectrum  unique to each element, only certain frequencies are emitted, originally thought would be continuous, we know ν corresponds to energy; BIG IDEA – We can use the light frequencies (colors) that elements give off to determine their identity.

ACTIVITY!