# Lecture 2 - Light, Sources of Light

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## What is Light?

#### Particles and forces

1. gravity, mass, graviton (hypothetical) holds together planets up to galaxies
2. electromagnetic force, electro-magnetic charge, photon holds together things with charge: atoms, molecules, solids and liquids, biology, computers
3. weak nuclear force, isospin, V and W bosons
4. strong nuclear force, colour, gluons holds together baryons: protons, neutrons

Electromagnetic radiation (the photon field) is the carrier of the electromagnetic force. Like all the fundamental fields it can be present at a wide range of energies and wavelengths.

• Cosmic background (left over from the big bang): 1000 km ~ 2.2 x 10^(-52) joules per photon
• Cosmic rays: 5 x 10^(-13) m ~ 7 x 10^(-32) joules per photon

#### Light

A particular energy range of electronic radiation, the range to which human photoreceptors are sensitive, is called light. Its energy range is

• blue: 300 nm = 3 x 10^(-7) m ~ 7 x 10^(-39) joules per photon
• red: 700 nm = 7 x 10^(-7) m ~ 2 x 10^(-39) joules per photon

For comparison, at room temperature (~290 K) the peak of the photon distribution is

• 10 km ~ 10^(-50) joules per photon

A few interesting things to consider about light.

• the atmosphere is transparent to light
• photoreceptors are to see things far away
• `far away' means `too far away to touch'
• eyes are to image things far away
• water is (almost always) much murkier than the atmosphere, so fish eyes are made differently
• insect eyes are very small, so they also are made quite differently
• the energy range of light is the range in which biological processes are active.

#### Particles and Waves

• Light exhibits interference (diffraction) so it must be a wave.
• Light appears in discrete units of energy (photoelectric effect) so it must be a particle.

#### Properties of Light

• energy
• frequency
• wavelength:
• velocity (really direction):
• polarization (perpendicular to velocity):
• spectral power distribution: .

The spectral power distribution ignores the phase and polarization of the light. According to the current state of the art, visual properties of light, inter alia its perceived colour, depend only on the spectral power distribution of the light.

#### How is Light Created?

• Black Body Radiation colour temperature
• thermal motion of charged particles and antiparticles
• tungsten filament light bulbs, the sun, flames
• Changes of state in charged components of atoms and molecules
• Light emitting diodes (LEDs), reflected light, scattered light, fluorescent lights
• As a side product of the strong and weak nuclear forces
• It's not recommended that you use one of these as a light source at home.

## Light and Biology

In the current era life on earth is inseparable from light. Why?

• entropy continuously increases
• energy needs to be continuously injected into any system with order
• injected energy is light.

High energy, within limit, photons are preferred to low energy ones because its hard to collect several photons that add up to the exact energy that, say, an electornic transition requires.

A side point.

• If the earth is to avoid heating up the amount of energy that arrives must equal the amount of energy that leaves.
• Light arrives at short wavelengths (high energy photons), and leaves at long wavelengths ( mostly low energy photons, or heat)

#### Biology without Light

This seems to be where life started

• The earth was created of constituents full of nuclear energy.
• Because of this the deeper you go the hotter it gets.
• Where there is heat there are chemical reactions.
• They store energy chemical energy in the form of molecules
• Light is created by the chemistry that powers organization
• Simple photoreception evolved so that some organisms could organize themselves by stealing free energy from light emitting organisms

#### Biology with Light

Light is the primary source of energy

• light is captured by a molecule that has two states
• the `receptor' store the energy for a short time by going into a (usually) unstable higher energy state
• an enzyme accepts the energy to build a stable high energy molecule:
• ATP or a sugar or a fat
• from which the energy can be extracted when it is needed

#### Implications for Vision

Why does life need vision?

1. To capture the energy of photons
2. To perceive things at a distan
• First, to get out of the shade
• Second, to perceive organisms whose energy can be stolen
• Third, to perceive organisms trying to steal our energy
• ...
• Finally, to create visual culture, without which life is not worth living.

Where did vision come from?

• in principle not much different than photosynthesis
• appears in very primitive bacteria
• conserved by evolution

What was the first vision?

• bacterial purple motility follows stimulation
• What the Frog's Eye Tells the Frog's Brain.
• Jump when the light gets dimmer over a large area jump away from the dimness This uses the ability to see temporal gradients in illumination.
• Shoot out your tongue exactly in the direction of a dark spot moving fast and nearby. This uses the ability of the eyes to image: predictive control may also use motion detection.

## Where Does Colour Come From

How does bacterial purple work?

1. a molecule accepts a photon of a specific wavelength (energy)
• changes configuration
• energy difference between configurations must be (almost) the energy of the photon
2. new configuration enters into a chemical reaction
• blah, blah, etc, etc.

Photoreceptors in the human eye work exactly the same way,

• except that there are several photoreceptors,
• each of which has molecules that react at different wavelengths