6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light

Did the Greeks Invent Everything?!?

The Birth of Classical Physics• The ancient Greeks were certainly not the first to

wonder about and investigate nature, but they were the first to leave written records of their ideas.

• They recorded ideas regarding a vast number of subjects from Astronomy to Zoology.

• They conceptualized the building blocks of matter – which they called the ‘atom’ -- literally means ‘cannot be cut’.

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 RadiationWave 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 SpectrumWave 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 EffectWave 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.

hc

hfEphoton

Equation

Einstein’s Model

• 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.

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)

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?!?

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

e-

e-

v

v = 0

Before After

incoming photon

electron electrongains KE

outgoing photonlower E, lower f

longer λ

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 = hf = 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

Visible Light

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 = hf = hc/λ

7.0 x 103 J = (6.63 x 10-34 J·s) f

f = 1.1 x 1031 Hz

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.

Mass/Energy Duality• In 1905, Einstein publishes his paper on Special

Relativity.• One of several proposals from this paper is that:

MASS ENERGY AND ENERGY MASS.

2mcE

Equation

End of 6.1.1 - PRACTICE