Chapter 15: Light, Color and Atmospheric Optics

1. Chapter 15: Light, Color and Atmospheric Optics • White and colors • White clouds and scattered light • Blue skies and hazy days • Red suns and blue moons • Twinkling, twilight, and the green flash • The mirage: seeing is not believing • Halos, sundogs, and sun pillars • Rainbows • Coronas and cloud iridescence

2. White and Colors • Review: • Nearly half of radiation from sun is visible light (what wavelengths are longest?) • Four possible fates of light as it enters atmosphere (absorbed, reflected, scattered, transmitted) • So what wavelengths is all white light? Black light? • We see using rods and cones

3. White Clouds and Scattered Light • Reflection – occurs when sunlight bounces off a surface at same angle • Scattering – occurs when sunlight is deflected in all directions • Why is the bottom of thunderstorms dark?

4. Blue Skies and Hazy Days • Selective scattering – ability of molecules to scatter one wavelength better than others • The larger the particle, the more they scatter all wavelengths. Haze is white because of this • Crepuscular rays – scattering of sunlight by dust and haze to produce white bands of light

5. Red Suns and Blue Moons • Sun can appear red because it travels through more atmosphere near sunset than at midday • Blue moon and suns can happen if the particle is the same size as visible light (scattered red more) • Volcanoes can cause red suns for many days

6. Fig. 15-8, p. 420

7. Fig. 15-9, p. 420

8. The Mirage: Seeing Is Not Believing • Refraction – bending of light due to density changes • Light that travels from less to more dense loses speed, and vice versa • Lights twinkle due to refraction • Twilight occurs due to refraction

9. The Mirage: Seeing Is Not Believing • Mirage – an image that appears displaced from its true position • Caused by light moving through different air densities • Inferior mirage – images that appear lower and inverted from original • Superior mirage – images that appear higher than original

10. Inferior Mirage • Warm surface, decreasing temperatures aloft • The proper conditions for an inferior mirage are commonly found over a hot road surface during summer, or over an unfrozen lake on a very cold day.

11. Stepped Art Fig. 15-16, p. 424

12. Superior Mirage • Cold surface, warmer temperature aloft • The proper conditions for a superior mirage are commonlyfound over water and over snow-covered surfaces.

13. Stepped Art Fig. 15-17, p. 425

14. Fata Morgana • Complex vertical temperature profile • According to legend, King Arthur’s half-sister, Fata Morgana in Italian, was a fairy enchantress trained by Merlin the Magician. She lived in a crystal palace beneath the water and could build fantastic castles from thin air.

15. Halos • Halo – Ring of light encircling sun or moon • Ice crystals refract light to create halo • Dispersion – breaking up of light into different wavelengths (prism)

16. Sundogs • Created by hexagonal ice crystals oriented horizontally • Ice crystals act like a prism • Sundogs are commonly seen when cirrostratus cloudsare in the sky.

17. Sun Pillars • Caused by reflection of light off ice crystals • Sun pillars can be seen shortly after sunrise orshortly before sunset on very cold days.

18. Sun Pillars

19. Rainbows • Water droplet phenomenon • When sunlight enters rain drop, it refracts and disperses • Red refracts less than violet • This light is reflected off backside of raindrop at critical angle • Each wavelength is reflected at different angle to our eyes (dispersed) • It takes many different raindrops to produced a rainbow (each raindrop gives a different wavelength) • Must face the falling rain with sun at your back • The primary rainbow, the brightest of the rainbows

20. Fig. 15-26, p. 429

21. Fig. 15-28, p. 430

22. The Secondary Rainbow • Two internal reflections (secondary rainbow) • A tertiary rainbow also exists, but it is too faint tobe seen with the human eye.

23. Coronas and Cloud Iridescence • Diffraction – bending of light around objects • Corona – Bright ring of light around the moon or sun • Blue appears on inside of ring, red on outside

24. The Corona • Need a cloud that has droplets of similar size • New clouds are the best corona producers

25. Cloud Iridescence • Iridescence - bright areas produced by diffraction • Associated with thin clouds within 20 degrees of sun

26. The Glory • Sun must be to your back, cloud in front of you • It is very common to see a glory from the windowof an airplane. Make sure you are sitting on theside opposite the sun.