How To Understand The Universe

Transcription

1 Chapter 4 Light and Color, and Atmospheric Optics Useful links: c_optical.html Processes affecting propagation of electromagnetic radiation (light) what can happen to visible light as it passes through the atmosphere? Reflection Scattering Transmission Refraction Diffraction 1

2 Why do objects have color? Objects appear to have color since they are able to selectively absorb and reflect certain wavelengths of visible light... The sun emits white light - the sum of all wavelengths (colors) of visible light together A White Object An object will appear white when it does not absorb any wavelength of visible light... it is all scattered hence, the object will appear white Reflection of Light light can simply be reflected the incident angle (θ 1 ) will equal the reflected angle (θ 2 ) 2

3 Scattering Scattering: Light can be scattered...redirected in many directions Mie Scattering large particles in the atmosphere are able to scatter all wavelengths of white light equally when all wavelengths of white light are scattered equally, then Mie scattering is occurring Rayleigh scattering Smaller particles (including air molecules) tend to scatter the shorter (violet, green, and blue) wavelengths of white light more effectively than the longer (orange, yellow, and red) wavelengths. Note that Rayleigh scattering involves much smaller scattering particles (compared with wavelength) than Mie scattering 3

4 White Clouds VS Dark Clouds Cloud droplets or ice crystals scattering all wavelength equally. Cloud limited sun light passing it (attenuation). Rayleigh Scattering blue sky It's because of Rayleigh scattering - the selective scattering of the shorter wavelengths of visible light (violet and blue) by atmospheric gases. 4

5 The Blue Ridge Mountains in Virginia. The blue haze is caused by the scattering of blue light by extremely small particles smaller than the wavelengths of visible light. Notice that the scattered blue light causes the most distant mountains to become almost indistinguishable from the sky. Crepuscular Rays Bright lightbeams due to mie scattering by haze and dust seen most often near sunrise and sunset 5

6 Red Sun Created by Rayleigh scattering At mid day, only a bit of the short wavelengths of visible light are scattered since the radiation is passing through a small distance in the atmosphere. At sunset, however, the radiation must pass through a much thicker layer of the atmosphere When the sun is at an angle of 4 from horizontal, the atmosphere appears to be 12 times thicker than at midday, hence, much more blue light, and some green light are scattered Orange/red sunsets in a dirty atmosphere Clear atmosphere: bright, yellow-orange sunsets When pollution is present, the atmosphere contains more particles such as aerosols having larger diameters than the atmospheric gases hence, more of the intermediate wavelengths of visible light such as yellow and green are scattered in addition to the blue light what largely remains is red light..., hence the sun appears red. 6

7 Refraction Refraction is the bending of light as it passes from one medium to another with different densities. Amount of refraction depends upon: density of the materials angle at which the light enters the material wavelength - causes colors of white light to separate when passing through a prism. light will bend away from the normal when passing into a less dense medium light will bend toward the normal when passing into a more dense medium air water Due to the bending of starlight by the atmosphere, stars not directly overhead appear to be higher than they really are. 7

8 Due to refraction: The sun will appear to rise sooner (by approximately two minutes) than it actually does The sun will appear to set a bit later (by about two minutes) than it actually does 8

9 The Green Flash Caused by refraction: Short wavelengths (blue and green) refract more than longer wavelengths (red). Hence, blue and green light should appear along the top of the sun. But, blue light is scattered away by Raleigh scattering, leaving green... -> the green flash The green light is usually to faint to see. but is sometimes apparent momentarily, for a second or so. Twilight Sunlight can be seen well after the sun sets due to both scattering and refraction. Twilight - is the name given to the time after sunset (and immediately before sunrise) when the sky remains illuminated. amount of twilight varies... it adds about 30 minutes before sunrise and after sunset in mid latitudes length of time increases with increasing latitude 9

10 Bright red sky over California produced by the sulfur-rich particles from the volcano Mt. Pinatubo during September, The photo was taken about an hour after sunset. Inferior Mirages Mirage - when an object appears to be displaced from its true position created as light passes through layers of air having different densities Formed when the air near the ground is very warm compared to the air just above it. puddles of water on the highway - what are you actually seeing? water/oasis in the desert 10

11 The road in the photo appears wet because blue skylight is bending up into the camera as the light passes through air of different densities Superior Mirage Produced when you have cold air near the ground and warmer air above it. --> Produces the "Fata Morgana" - see focus box on pg

12 The Fata Morgana mirage over water. The mirage is the result of refraction light from small islands and ships is bent in such a way as to make them appear to rise vertically above the water. Halos, Sundogs, and Sun Pillars Halos: a ring of light encircling and extending outward from the sun or moon A 22 halo around the sun, produced by the refraction of sunlight through ice crystals. 12

13 The formation of a 22 and a 46 halo with column-type ice crystals Created as sunlight is refracted as it passes through ice crystals. Indicates cirriform clouds (high, thin clouds composed of ice crystals) two types: 22 halo - refraction of light through randomly suspended columntype ice crystals (< 20 μm). 46 halo - refraction of light through hexagonal columntype ice crystals μm). Tangent Arcs a bright arc of light at the top of the 22 halo formed by refraction of light passing through horizontallyoriented pencil-shaped hexagonal ice crystals 13

14 Refraction and dispersion of light through a glass prism Sun Dogs Created at white light is refracted through ice crystals in the form of hexagonal plates. This is similar to the dispersion of white light as it passes through a prism. 14

15 The bright areas on each side of the sun are sundogs A brilliant red sun pillar produced by the reflection of sunlight off ice crystals. 15

16 Ice crystal phenomena Theory Observations What creates rainbows? Rainbows are generated through refraction and reflection of light in small rain drops. sun must be behind you rain drops must be ahead of you the angle between your line-of-sight and the sunlight will be You are here! 16

17 Sunlight internally reflected and dispersed by a raindrop (a) The light ray is internally reflected only when it strikes the backside of the drop at an angle greater than the critical angle for water (48 for water). (b) Refraction of the light as it enters the drop causes the point of reflection (on the back of the drop) to be different for each color. Hence, the colors are separated from each other when the light emerges from the raindrop. Many drops are involved in producing the primary rainbow! 17

18 Two internal reflections are responsible for the weaker, secondary rainbow. Notice that the eye sees violet light from the upper drop and red light from the lower drop. Double Rainbow 18

19 The formation of a circumzenithal arc The short-lived circumzenithal arc can only form when the sun is lower than 32 above the horizon. Diffraction Occurs when light is bent as it passes around objects that have approximately the same size as the wavelength of the light itself The "bent" waves will interact with each other producing an interference pattern constructive interference - when a ridge (trough) from one wave is superimposed on a ridge (trough) from another - end up with a large amplitude destructive interference - with the ridge of one wave is superimposed on the trough of another - they cancel out producing no amplitude. Demo 19

20 Corona: a bright ring of light the corona The corona around the moon results from the diffraction of light by tiny liquid cloud droplets of uniform size sometimes appears white, with alternating rings of light and dark other times, the rings of light have color. This occurs when the cloud droplets have a uniform size. Corona around the sun. This type of corona, called Bishop s ring, is the result of diffraction of sunlight by tiny volcanic particles emitted from the volcano El Chichón in

21 When the drops in a cloud have many different sizes, the corona can become distorted Areas within 20 of the sun where the corona has become distorted producing patches of color is called iridescence Cloud iridescence The series of rings surrounding the shadow of the aircraft is called the glory. For the glory and the brocken bow to occur, the sun must be on your back, so that sunlight can be returned to your eye from the water droplets. Diffraction of light coming from the edges of the droplets produces the ring of light we see as the glory and the brocken bow. Eye 21

22 The Heiligenschein is the ring of light around the shadow of the observer s head when facing the dew on the grass with your back to the sun on a clear morning. The Heiligenschein forms when sunlight, which falls on nearly spherical dew drops, is focused and reflected back toward the sun along nearly the same path that is took originally. 22