Photoelectric effect - PHOTONS AND ELECTRONS by Lizelle Swanepoel

Effect of wavelength of the incident photons on the kinetic energy of the ejected photoelectrons:

lizellexs — 2019-07-16 19:24:30 UTC

Recall:
v = fλ
So, for lightwaves:
c = fλ
f = c/λ

The energy of the photons:
E = hf
= hc/λ

E ∝ f
E ∝ 1/λ

lizellexs — 2019-07-16 19:34:12 UTC

ANALOGY FOR WORK FUNCTION AND THRESHOLD FREQUENCY - A ROCKET LAUNCH! Escape velocity & Launch energy

lizellexs — 2019-07-16 19:43:18 UTC

E_k= E_fuel- E_FgThe same happens with electrons in a metal surface when light falls onto it.

If the light is of sufficient energy, then the electrons can be ejected off the particular metal surface.
This is called the photoelectric effect.

Photoelectric Equation

lizellexs — 2019-07-16 20:15:46 UTC

E_hf = W₀ + E_k

E_k = hf - W₀E_k=hf - φ

1/2mv² = hf - hf₀

PROBLEM: Will electrons be ejected?

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DEMO VIDEO OF THE PHOTOELECTRIC EFFECT

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How can photons act as particles if they are massless?

lizellexs — 2019-07-16 21:47:42 UTC

Photons = packets (quanta) of light
Photons are massless; m = 0

So how can they act as particles (as if they have mass)?
• Photons have MOMENTUM due to Energy, and can impart their energy to electrons of any metal surface to eject them off of the surface.
• Mass of a photon comes from its energy divided by the speed of light squared:

m_photon= hf/c²

THE STORY BEGINS - Einstein Nobel Prize 1921

lizellexs — 2019-07-16 22:19:32 UTC

For discovering the Law of the photoelectric effect.

BACKGROUND HISTORY

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Isaac Newton
“Light is a not a wave, but made of particles called a corpuscles (small discreet particles as different parts of light”.

Discovered the spectrum of VIS light, as it disperses through a prism.

Christiaan Huygens

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Dutch physicist and astronomer famous for WAVE THEORY OF LIGHT.

James Clerk Maxwell

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EM Wave Theory of light.

Discovered the connection between light electromagnetic waves.
Light consists of:
An electric field
A magnetic field

Light is an EM Wave

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EM Spectrum

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Thomas Young

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Young’s Double slit experiment

Light has wave-particle duality.
It can act as particle or wave.

Finally Einstein

lizellexs — 2019-07-16 22:59:07 UTC

Max Planck

lizellexs — 2019-07-17 20:06:51 UTC

“The exchange of energy between light and matter only occurs in discrete amounts, called quanta.”

Planck constant, h = 6,62 ×10^-34 J
h is the quantum of electromagnetic action, which relates the energy carried by a photon to its frequency.
A photon's energy is equal to its frequency multiplied by the Planck constant. E = hf

Double slit experiment explained

lizellexs — 2019-07-17 20:32:50 UTC

ADDITIONAL QUESTIONS WITH ANSWERS

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PHOTOELECTRIC EFFECT & EMISSION SPECTRA

Definitions

lizellexs — 2019-07-18 06:21:54 UTC

Frequency = The number of complete waves that pass a point per second. (measured in hertz, Hz).
Wavelength = The distance between two consecutive points on a wave that are in phase. (measured in metres, m)
Photoelectric effect = The process that occurs when electromagnetic radiation shines on a certain metal surface and it ejects electrons from the metal.
Photon = The quantum of energy when the energy is in the form of electromagnetic radiation.
Photoelectron = electron ejected off a metal by a photon.
Threshold frequency (f₀) = The minimum frequency of radiation at which electrons will be ejected from the surface of a certain metal.
Work function (W₀) = The minimum energy needed to emit an electron from the surface of a certain metal.

Energy of a photon is always a “multiple” of h, depending on the frequency of light

lizellexs — 2019-07-18 07:03:57 UTC

PHOTOELECTRIC GRAPHS

lizellexs — 2019-07-18 08:18:29 UTC

You need to be able to:

Draw a graph from relevant data.
Read off/determine the x-intercept.
Extrapolate the graph.
Read off/determine the y-intercept.
Calculate the gradient.
Interpret what gradient represents.

Graph of Ek vs. f

lizellexs — 2019-07-18 08:20:04 UTC

BE = Binding energy or Work function

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Threshold frequencies for different metals

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The Photocell

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You must Explain its working

One electron is ejected per every one photon!

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Graph of Ek vs. 1/λ

lizellexs — 2019-07-23 16:26:05 UTC

TYPICAL EXAM QUESTION

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NOTES 2025

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ADDITIONAL QUICK EXERCISES

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EXERCISE

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ANSWERS

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ANSWERS

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EXERCISE 2

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