Light

Once Upon a Time…

• People have been trying to understand light for as long a people have been able to ask the question “what is light?”

• Understanding light is incredibly important to astronomy!

The Nature of Light

• The basic nature of light was an enigma for centuries. The problem: light sometimes acted as particles and other times as waves.

• The debate continued for the next 200 years with each side supporting their theories with experimental “proof.”– Newton – Corpuscular theory of Light– Huygens – Wave theory of Light

Waves

Wave = A traveling disturbance that transmits energy with no net movement of matter.

Light as a Wave

• Light waves are characterized by a wavelength l and a frequency f.

f = c/l• f and l are related through

l

Wave Motion

Transverse = vibrations are perpendicular to wave motion.

Longitudinal = vibrations are parallel to wave motion.

• Waves motion can be transverse or longitudinal

Waves• All waves involve oscillation or vibration of

some kind (even with no material transport).

• The medium is the substance through which the wave moves (water, air, rope, etc.).

Boundary Interactions

Reflection

Refraction

Refraction

Refraction of Light

• As white light (sunlight) travels through a prism, each color refracts slightly differently creating the spread out rainbow we are used to seeing.

Change of Speed

• The speed of a wave can change with a change in medium.

– Sound travels more quickly in iron than in air.

– Light travels more quickly in air than in glass.

Light and EM?

• In the mid- to late- 1800s, some bright men noticed the similarity between EM studies and light as a wave studies. (Maxwell, Faraday, Hertz)

• Maxwell calculated the speed of light and it was successfully compared to experimental values.

What about a medium?The search for the ether!

The Michelson-Morley experiment of 1887.

If an ether wind was blowing, the 2 half pulses of light should arrive at the telescope at different times.

Won Nobel Prize in 1907! First

American physicist to do so!

No Ether? Then what?

• The Michelson-Morley result played a huge role in the revolution that led to Einstein’s interpretation of light in his special theory of relativity in 1905.

“... the introduction of a light-ether will prove to be superfluous…”

- Albert Einstein

Electromagnetic WavesElectromagnetic waves: Oscillating electric and magnetic fields. Changing electric field creates magnetic field, and vice versa

New Thoughts on Light

• Now light is

An electromagnetic disturbance with wave properties that was self-sustaining (able to carry energy through a perfect vacuum).

• But there were still problems!

Light as a Particle

• Light as a wave can’t explain vision, photography, the photoelectric effect, etc.

• We needed light as a particle.• Enter Max Planck

Won Nobel Prize in 1918!

Max Planck In 1900, Max Planck was working on the

problem of how the radiation an object emits is related to its temperature.

He came up with a formula that agreed very closely with experimental data, but the formula only made sense if he assumed that the energy of a vibrating molecule was quantized--that is, it could only take on certain values.

It was an extremely radical idea to suggest that energy could only come in discrete lumps, even if the lumps were very small.

Photons

Is it a wave or a particle??

The amount of Energy a photon carried was based on its wave properties.

E = hf

Photons!

Based on Planck's work, Einstein proposed that light also delivers its energy in chunks; light would then consist of little particles, or quanta, called photons.

E = hf

The symbol indicates its dual nature.

Characteristics of LightOur modern concept of light says that both Newton

and Huygens were correct!

There are 3 fundamental elements of this concept:

Duality – Electromagnetic radiation act either as a stream of particles or as a pure energy carried in a self-sustaining wave, depending upon circumstances.

Complementarity – The wave and particle properties of light are mutually exclusive.

Convertability – The distinction between energy, light, and matter disappeared.

Electromagnetic Spectrum

Light and Matter Interactions

What happens when light and matter interact?

Easiest possibility = reflection (why is an

orange orange?)

Interaction of light with matter means the creation (emission) or annihilation (absorption) of photons by atoms and molecules.

Consider 2 cases:

• Emission of photons from isolated atoms in a gas. Characterized by discrete photon frequencies that are uniquely diagnostic of the emitting atom.

• Emission of photons from solid matter. Characterized by a continuous distribution of photon frequencies that depends only on the temperature of the emitting system, not its composition.

Planck Won

Nobel Prize in 1918!

Bohr Won Nobel

Prize in 1922!

Energy Levels• Electrons in an atom

exist in well-defined energy levels characterized by precise values of energy (they are quantized).

• When matter is at rest (0 K) all electrons are in their lowest level (ground state).

Electron Orbits• Electron orbits in the electron cloud are

restricted to very specific radii and energies.

r1, E1

r2, E2

r3, E3

• These characteristic electron energies are different for each individual element.

Absorption

When a ray of light comes in from an outside source and hits an atom, it may only be absorbed if it is the correct energy to move the electron up to one of the possible energy levels.

If a very high energy photon comes in (maybe UV or X-ray), then if the photon was absorbed the electron would be forced to hold a great deal of energy. Is this much allowed?

Absorption

The electron may only have certain amounts of energy. So it may only move to the energy levels for the specific element.

If a lower energy photon comes in (maybe sky blue), then if the photon was absorbed the electron would be forced to hold a the energy of that photon. Is this much allowed?

Absorption

This IS allowed and so it happens! The photon is absorbed (ceases to exist) and the electron moves up to another energy level.

However, electrons hate to be above the ground state and will try to get rid of extra energy. One way of doing this is to emit light.

Emission

Then the electron would drop back down to a lower energy level.

The energy emitted would be exactly equal to the energy it had previously absorbed. This would mean a new sky blue photon would come out.

Absorption

If a lower energy (fire engine red) photon came in and was absorbed, the electron would take this much energy. Is this much allowed? Will it happen?

Emission

It could then also be emitted. A new fire engine red photon would come into existence.

Absorption

If another red photon comes in…

And then before it can be emitted a green photon comes in….

Emission

Then you could get…

Or…

Absorption and Emission

So, the photons absorbed are the same as those emitted.

Absorption

So if a continuous spectrum of light hit a cloud of these atoms, what you’d see on the other side would be…

Absorption Spectrum

Emission

If you could only see the light given off by a cloud of these atoms, you would see…

Emission Spectrum

Kirchoff’s Laws

Kirchoff’s Laws

Planck Curves

Planck Curves

Planck Curves

Doppler Shift

Blue Shift (to higher frequencies)

Red Shift (to lower frequencies)