Week 12 - Atmospheric Optics

What are the different processes that lead to us observing visually spectacular optical phenomena?

• Shortwave solar radiation reaches through Earth’s atmosphere, waves in the visible light spectrum may be subject to physical processes including absorption, reflection, refraction, scattering and transmission

• The visible light (white light) spectrum is made up of all the colours of the rainbow

• When light hits an object (our eyes cannot detect), some wavelengths are absorbed/refracted and some are reflected

• Our eyes receive light that are reflected and the rods and cones react to the different wavelengths

What is absorption?

A photo of light is absorbed by a particle or water droplet

What is reflection?

Light bounces off a surface

What is refraction?

• The change in the path of light waves caused by a change in density between two media (different refractive indexes)

• Angle of refraction depends on the difference in density across the boundary and the angle at which the light enters a new medium (creates an optical illusion that an object is shorter than it really is)

What is scattering?

• Absorption and re-emission at the same wavelength, entering and leaving the object (light bounces off an object or surface in a variety of directions)

• A wave property of light

What is transmission?

Light entering an object but not leaving

What is Rayleigh Scattering?

• If the scattering particles are much smaller than the wavelength of light

• Equal preference to be scattered in any direction

• Interactions between light and the particles leads to re-emission

What is Mie Scattering?

• If the scattering particles are about the same size, or larger than, the wavelength of light

• Preference to be scattered in the same direction (forwards) rather than absorption

Why do we perceive clouds to be white because of Mie Scattering?

• Scattering involves no transfer of energy or properties such as temperature

• Clouds are made up of many water droplets which means that light is scattered in all directions

• White light is the result of all visible wavelengths striking the same cones in our eyes with the same intensity

Why does the sky appear blue because of Rayleigh scattering?

• More effect at shorter wavelengths (blue and violet)

• As light from the sun reaches Earth’s atmosphere, blue light is scattered strongly while other colours pass straight through (scattered less effectively)

• The sun appears yellow because all the spectrum reaches your eyes

• If you look away from the sun, your eyes receive only the blue light is scattered

Why does the sunrise/sunset appear red because of Rayleigh scattering?

• Light from the sun has to pass through a ‘thicker’ layer of atmosphere to reach our eyes

• Since the sun is low on the horizon, the light is also passing through the denser part of the atmosphere, closer to the ground, home to more particulates and pollutants

• Blue and green light is even more strongly scattered (away from us), leaving only the longer wavelengths in the red part of the spectrum to reach our eyes

• Red light tends to be scattered less effectively but there are more opportunities for stronger scattering when the sun is low in the sky (more chance to interact with other particles in the atmosphere)

• Same process for smoky skies (depends on the composition of clouds and particles in the lower atmosphere)

How does scattering cause blue haze?

Vegetation releases very small particles (emission of particular chemicals) which interact with ozone to scatter at shorter wavelengths towards the blue part of the spectrum

How does scattering cause crepuscular rays?

• Usually seen near sunrise of sunset, beams of yellow light stream through gaps in clouds where the cloud itself is strongly scattering light otherwise

• Thick cloud blocking light from reaching our eyes and scattering

How does refraction affect the amount of daylight we receive on Earth?

• The density of the atmosphere changes with height and this causes light to refract

• Refraction causes the sun to rise earlier and set later than if light travelled in an exactly straight line

• We receive around 12 ours and an extra 8 minutes of daylight on the Equinox

• Also makes the sun appear higher in the sky than it really is

How does refraction cause us to perceive the twinkling of the stars?

• Causes changes in density between different layers of the atmosphere

• The effect is greater when stars are near the horizon and their light has to travel through a thicker layer of atmosphere with a different density to reach our eyes

Why do stars twinkle but not planets?

• Planets are much closer to the light source

• The light still scatters and twinkles but we do not see it twinkling because the scattering is within the width of the disk

• Stars are so far away that the light source is a point source

• The scattering appears to go ‘outside’ the original shape of the point source, causing ‘twinkle’ (distortion into a slightly wider path)

How does refraction cause mirages?

• Appears to displace distant objects on the ground or in the sky

• An inferior mirage appears to shift an object down and a superior mirage appears to shift an object up

How does a temperature inversion cause the nature of a superior mirage?

• When the air temperature is colder closer to the surface than above, the change in density can lead to the refraction of light

• The nature of a mirage can change rapidly with subtle changes in air temperature in the near-surface layers

How does refraction cause atmospheric halos?

• Rings of light that appear around the sun due to the refraction of light by ice crystals, often seen with cirrostratus clouds

• Ice crystals refract light preferentially at angles of 22° and 46°

How does refraction cause sun dogs?

• Bright spots of light that appear on one or both sides of the sun, commonly as a part of a 22° halo

• Caused by refraction and scattering of light by horizontally-oriented plate-shaped hexagonal ice crystals

How does reflection cause sun pillars?

Seen usually at sunrise or sunset and caused by reflection of light by tiny ice crystals

How do both refraction and reflection cause rainbows?

• When sunlight enters a raindrop, it refracts at the boundary (requires lots of raindrops to fall through the sky to see a full rainbow)

• The different wavelengths that make up white light refract at slightly different angles, so the colours spread out and separate

• The light reflects within the raindrop, then refracts once more as it exits the raindrop, spreading and separating out the colours as they are refracted at different angles on their way out of the raindrop

  • Red light - 42°

  • Violet light - 40°

How can we use the different angles of refraction of different wavelengths of light to predict the location of rainbows?

• They always appear at 42° elevation from the ground

• We generally see them when the sun is low in the sky and sunlight is coming in almost horizontally

• Rainbows are always present but whether we perceive them or not depends on the intensity of the refraction of light (which is why they are considered to be commonly occurring optical phenomena)

• When the 42° angle between the sun and raindrops is likely to coincide with a ground-based observer (especially from points of elevation e.g. high-rise buildings in early mornings)

How do double rainbows form?

• When light is reflected an additional time within raindrops

• The secondary bow appears outside the primary bow, around 10° higher elevation in the sky

• The second reflection also reverses the order of the colours and reduces the intensity of the light

• A secondary rainbow has violet at the top and red at the bottom, appearing less bright than the primary rainbow (always present even if it not always visible, depends on the intensity of light)

How does refraction and reflection within water droplets cause fogbows?

• Fog droplets are much smaller than raindrops

• Less refraction and reflection

• Less intense colours - appears to be a ‘white rainbow’

How are moonbows (lunar rainbows) formed?

• Rainbow produced by moonlight

• Form in the same way as rainbows but are not as bright

• The low light can make it difficult for our eyes to distinguish the colours

What are the key conditions needed for rainbows which can use to predict them?

• Partly cloudy skies with a mix of showers and sunshine

• The sun low in the sky - early morning or late afternoon

• We need to be facing away from the sun and looking towards the showers so that the light reflected in the raindrops can reach your eyes for the rainbow to be visible to you