G

Energy of a photonYou should be able to:• describe the particulate nature (photon model) of

electromagnetic radiation• state that a photon is a quantum of energy of

electromagnetic radiation• select and use the equations for the energy of a

photon: E = hf and E = hc / λ• define and use the electronvolt (eV) as a unit of energy• use the transfer equation eV = ½ mv2 for electrons and

other charged particles• describe an experiment using LEDs to estimate the

Planck constant h using the equation eV = hc / λ

The Planck Constant

Energy (of a photon) is proportional to frequency

E α f

E = h f

h = 6.626 068 76 x 10-34 Js

The electronvolt

1eV = 1.60 x 10-19J

Qelectron = -1.60 x 10-19C

1V = 1JC-1

The electronvolt

One electronvolt is the energy change of an electron when it

moves through a potential difference of one volt. Its value is

1.60 x 10-19J

The photoelectric effectYou should be able to:• describe and explain the phenomenon of the photoelectric effect• explain that the photoelectric effect provides evidence for a

particulate nature of electromagnetic radiation while phenomena such as interference and diffraction provide evidence for a wave nature

• define and use the terms work function and threshold frequency• state that energy is conserved when a photon interacts with an

electron• select, explain and use Einstein’s photoelectric equation

hf = ϕ + KEmax• explain why the maximum kinetic energy of the electrons is

independent of intensity and why the photoelectric current in a photocell circuit is proportional to intensity of the incident radiation.

The photoelectric effect

Energy of a photon landing on the surface

Energy needed by an electron to escape the atom

Kinetic energy of the electron

Imagine an electron is like a ball stuck in a deep hole. Energy is given to it by a light particle (a photon), if there is enough energy it can get out of the hole (or escape the atom) and any energy left

over is the kinetic energy of the electron.

The photoelectric effect

Energy of a photon landing on the surface

Electron does not have enough energy to escape the atom

If the photon does not have enough energy then the electron cannot escape the atom.

The photoelectric effect

Energy of a photon landing on the surface.

E = hf

Energy needed by an electron to escape the atom.

The work function, ϕ

Kinetic energy of the electron.

KEmax

hf = ϕ + KEmax

The photoelectric effect

hf = energy of a photon landing on the surface. Since ‘h’ is a constant, this is dependent of the frequency – if the frequency is too low then there may not be enough energy to release a photoelectron.

ϕ = the work function. The amount of energy needed for an electron to escape, this is dependent on the type of metal (e.g it is lower for sodium than for zinc).

hf = ϕ + KEmax

Threshold Frequency

-ϕ

f

KEmax

fo

hf = ϕ + KEmax

KEmax = hf – ϕ

y = mx + c

Threshold Frequency

The lowest frequency of radiation that will result in the emission of electrons

from a particular metal surface.

Work function

The minimum energy required to release an electron from a material.

V

μA

We can vary the frequency of the incident light.

p.d between plates

photo current

Wave-particle dualityYou should be able to:• explain electron diffraction as evidence for the

wave nature of particles like electrons• explain that electrons travelling through

polycrystalline graphite will be diffracted by the atoms and the spacing between the atoms

• select and apply the de Broglie equation λ = h / mv

• explain that the diffraction of electrons by matter can be used to determine the arrangement of atoms and the size of nuclei

Wave-particle duality

This proves the wave-like properties of light (but in this section we have been looking at its particle like properties)

Light has wave-particle duality.

This kind of pattern should be familiar to you. If we shine green monochromatic light through a diffraction grating we see an interference pattern caused by the constructive and destructive interference of waves.

Wave-particle dualitySo if something we used to think of as a wave can behave like a particle, can something we think of as a particle behave like a wave?

Yes!

Electrons (the particles that orbit the nucleus) can also interfere with electrons to make a diffraction pattern.

Wave-particle duality

Thin piece of polycrystalline

graphite

Electron diffraction

pattern

Electron gun

Vacuum

De Broglie Equation

A scientist by the name of De Broglie came up with the following equation linking the momentum of a particle with its wavelength:

mv = h / λ

Or λ = h / mv

i.e. The faster the electron the shorter the wavelength, and the smaller the object it diffracts around (that is why scanning electron microscopes are used to look at very small objects – smaller than the wavelength of visible light)