Goal: To understand how light acts like a particle

Objectives:1) To learn about Quantization

2) To understand Blackbody radiation3) To learn more about The photon4) To learn about the Photoelectric

Effect

Quantization

• Okay this is an ugly looking word.• All it means is that energy is restricted to specific

values.• Think of a staircase.• Each stair has a difference potential energy.• However, you are only allowed to have the

potential energies that are on the stairs.• Any potential energy between two stairs is not

allowed.

Blackbody Spectrum – continuous

• All objects which absorb most of the light which passes through them emit energy as a “blackbody”. The shape of the blackbody spectrum is always the same.

• The strength of the spectrum (i.e. how much light it emits) and the peak wavelength of the spectrum depend on the temperature of the object.

http://theory.uwinnipeg.ca/users/gabor/foundations/quantum/slide6.html

The photon

• A single light “particle” is called a photon.• Photons have energy which depend on wavelength or

frequency.• E = hf = hc / λ• h is a constant called Planck’s constant• h = 6.626 * 10-34 J * s• Or, h = 4.136 * 10-15 eV * s

• Often times though photons are measured in units of eV (electron-Volts).

• Note that eVs are energies.

What did Einstein win a noble prize for?

Photoelectric Effect

• When light is shined onto a metal electrons can be stripped from that metal because of the impact of the photons.

• Some specific amount of the photon gets absorbed and the rest may be used on the electron.

• The absorbed amount is called the Work Function (Φ).

• The maximum energy the electron can leave the metal with is called Kmax

• So, Kmax = hf - Φ

Example

• An alloy has a work function of 1.2 eV (1 eV = 1.602 * 10-19 J).

• A photon of light with a frequency of 2 * 1015 Hz strikes the alloy.

• What is the maximum kinetic energy the emitted electron can have?

• Give answer in terms of eV

Example 2

• A laser is shined onto a metal strip.

• The work function of the strip is 2.3 eV.

• If the highest energy electron emitted has an energy of 3.5 eV then what is the frequency of the laser beam (hint, to get values not in energy you need to convert somewhere back to Joules)?

But what happens if:

• What happens if the energy of your light photon is less than the Work Function?

• Well if that happens you don’t emit electrons.

• Therefore there is a minimum photon energy to knock loose photons.

• This energy of course corresponds to some frequency.

• This frequency is called the threshold frequency (fo).

Threshold Frequency

• Kmax = hf – Φ • At the threshold frequency Kmax is 0, that is you

barely get an electron.

• So, 0 = hfo – Φ

• Or, fo = Φ / h

• Example:• If the work function of a metal is 4.2 eV then

what is the Threshold Frequency?

A few other topics

• Compton Scattering:• When a photon hits an electron the electron can absorb

the photon.• Since it gains energy the photon will move off (just like in

P218 when you had collisions).• Energy and momentum are conserved so a photon is

emitted.• However, it is emitted at some angle – determined by the

recoil of the electron.• Also, since the electron steals some of the energy from

the photon, the new photon will have less energy than the old photon.

Compton Shift

• This causes a Compton Shift:

• λnew – λold = (h/mec)(1-cos(θ))

• me is the mass of the electron

• θ is the angle between the direction of the old photon and the new one.

Conclusion

• We learned about the particle nature of light.

• We saw how light is emitted form most objects.

• We learned about the Photoelectric effect and how to calculate work function and threshold frequency.