Optics

MST1

definition of light. How is vision possible?

light is electromagnetic radiation that we can see

vision is made possible by photons (packets of energy) being absorbed by photoreceptor cells of the eye

what kind of wave is light. Understand how to draw this

transverse electromagnetic wave (there is both a magnetic and an electric field). Light is part of the electromagnetic radiation spectrum that our eyes can detect!

understand the quantisation of energy

Light transfers energy in separate packets called photons, not as a continuous flow, and each photon’s energy depends on its frequency (E=hf)

how does energy of each quantum vary with wavelength?

• as wavelength increases, energy INCREASES!! (and frequency)

• inversely proportional!

shorter wavelength → higher energy

longer wavelength → lower energy

Example:

blue/UV light = shorter wavelength = higher energy

red light = longer wavelength = lower energy

range of spectra that can be produced by artificial light sources

Artificial light sources can produce very broad continuous spectra, uneven/spiky spectra, or very narrow single-wavelength spectra

examples:

• Tungsten bulb: broad spectrum, with a lot of energy in the infrared.

• Fluorescent lights / LEDs: can have uneven or spiky spectra, even if they still look white.

• Lasers: very narrow spectrum, close to a single wavelength.

Describe the propagation of light wave as a combination of electric and magnetic field components

time-varying electric fields (E) and magnetic fields (B) generate and re-enforce one another, underlying the propagation of electromagnetic radiation

• Magnetic flux changing with time —> change the electric field

• Electric flux changing with time —> change the magnetic field

they influence one another

Maxwell’s equations describe this relationship

How to calculate diopters?

100 divided by power to get the answer in cm

e.g. a 5D lens has a 20 cm focal length.

how to calculate frequency?

f = c/λ

• f = frequency

• c = speed of light = 3×10⁸ m/s

• λ = wavelength

3× 10⁸ is the speed of light in a vacuum, but this travels much shower through media

• frequency and wavelength are INVERTED! (as frequency increases, wavelength decreases)

what is the wavelength of the visible spectrum

About 380 to 700 nanometres (nm)

red = largest wavelength

blue = shortest wavelength

why do we usually refer to wavelength rather than frequency when speaking about light

because frequency extremely high

around 5 million-fold greater than FM radio (which we refer to in frequency)

660 nm light has a frequency of 5 × 10⁸ MHz

What is the relationship between wavelength (λ), frequency (Hz) and energy (in eV)

as wavelength increases, frequency and energy decrease

• frequency is proportional to the energy of each photon!

E = hf = hc/λ

what changes with refractive media: wavelength, speed, frequency and energy

• wavelength changes with refractive media

• speed changes with refractive media

• frequency does not change with refractive media

• energy stays constant in refractive media as well (because frequency defines energy)

what is the dual nature of light?

1. Light shows both wave-like and particle-like behaviour.

light travels like a wave and interacts with matter like a particle!

What experiments support light behaving as a wave?

1. Double-slit experiment

• in the double-slit experiment, light passing through 2 slits forms an interference pattern of bright and dark pands (like ripples on a pond), which is a wavelike property (particles would produce straight shadows)

2. Crossed polarising filters

• With crossed polarisers, the second polariser blocks the light because light has an orientation (polarisation), showing it behaves like a transverse wave. If they were particles, these would not be blocked!

What experiment supports light as a particle?

The photoelectric effect

• this experiment showed that electron ENERGY depended on wavelength (y), BUT not intensity of incident light

• if we increase the intensity of the light, more electrons will be emitted but the energy and wavelength will stay constant

• Blue or UV (higher f) light emits electrons (higher energy), but red did not

• This is not consistent with wave behaviour of light

if light was acting as a wave, increasing its intensity should increase the energy delivered to each photon

energy calculation. Can you calculate energy with c and λ?

E = hf

• h = Planck’s constant (6.626×10³⁴ Js)

• this calculation underlies the photoelectric effect!

E = hf = hc/λ

what are the approximate wavelength (nm) of colours of light

(red, orange, yellow, green, blue, violet)

Red: 640 nm

Orange: 590 nm

Yellow: 575 nm

Green: 510 nm

Blue: 470 nm

Violet: 400 nm

The sensitivity of our eyes to different wavelengths is described by what? What colours do our eyes have maximum sensitivity to?

• The photopic sensitivity curve (cone-mediated, bright light)

• We are most sensitive to colours in the middle of the visible spectrum (i.e. yellow-green)

Describe (and know how to draw) the photopic sensitivity curve and the scotopic sensitivity curve? What numbers do they peak at?

sensitivity to wavelengths during the day: photopic sensitivity curve (cone-mediated, bright light) - peaks (λ_max) ~ 555 nm

• L cones peak at ~565 nm and M cones at ~545 nm. S cones are not invited to the party…

sensitivity to wavelengths at night: scotopic sensitivity curve (rod-mediated, dim light) - peaks (λ_max) ~ 507 nm (just rods)

what does our peak photopic sensitivity coincide with?

Our peak photopic sensitivity falls near the peak of the solar emission spectrum.

Why are tungsten bulbs hot?

Because most of it’s energy goes into the IR part of the spectrum (very inefficient)! Want most in our visible light spectrum

What is white light? What does it mean that very different light spectra can all appear white?

Light can appear white if all 3 cone types are stimulated (even in an unbalanced manner!) - although not always the case, but you do not need to be very precise.

Different mixes of wavelengths can still give the visual system a “white” signal

What are the 3 main parts of understanding vision? (reductionism)

Vision can be broken into three main parts:

• the stimulus (light),

• the interpreter (the brain)

• the receptor (the eye)

How did eyes likely develop?

Eyes likely evolved step by step, from simple light-sensitive cells that detected light/dark, to pits that improved protection/reducing unwanted glare (limpet) then modified to allow directional sensitivity, and later to lens-containing image-forming eyes.

problem with the pit:

• risk of becoming blocked!

• Mutation —> adaptation: transparent membrane to cover pits

• Mutation —> adaptation: membranes that are thicker in the middle (this focuses LIGHT intensity) * mutation: encapsulate the lens

vertebrate vs compou