Chapter 6: Light and Color in Nature What is a mirage? How do rainbows form? Why is the sky blue and sunset red?

1. Chapter 6: Light and Color in Nature What is a mirage? How do rainbows form? Why is the sky blue and sunset red?

2. Light & Color in Nature • Refraction effects: • Mirages • Rainbows • Halos Interference effects: Oil films / soap bubbles Iridescence • Scattering effects: • Blue sky / sunsets

3. Atmospheric Refraction • Air density (refractive index) varies due to pressure or temperature. • Light rays refract (bend) toward the denser medium. This causes mirages or flattening of the sun at sunset time. Warmer air or low pressure Cooler air or high pressure

4. Flattening of the Sun The sun is visible before the sunrise and after the sunset making the days longer!

5. Inferior Mirage

6. Inferior Mirage (Contd.) Image appears below the object giving the illusion of water!

7. Superior Mirage

8. Superior Mirage (Contd.) Image appears above the object giving the illusion of taller image or image floating in the sky!

9. Rainbows: Refraction by Water Droplets Primary bow (brighter) Secondary bow (dimmer) Alexander’s dark band

10. Rainbows: Refraction by Water Droplets Primary Secondary • Formed by “class 3” rays. • Brighter with red on top. • Cannot be observed if sun is higher than 420 above the horizon. • Formed by “class 4” rays. • Less bright with blue on top. • Cannot be observed if sun is higher than 520 above the horizon. http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=61

11. Rainbows: Other Features Alexander’s Dark Band: No refraction is possible between 420 and 520 rainbows. Supernumerary Arcs: Pink & green bands below the primary bow. Produced by interference between sun rays.

12. Refraction Through Ice Crystals: Halos http://www.snowcrystals.com/ http://www.atoptics.co.uk/ Hexagonal (pencil shaped) ice crystals are formed in high cirro-stratus clouds. These ice crystals can refract light into two concentric circles (halos) depending on how they are oriented.

13. Refraction Through Ice Crystals: Halos Upper tangent arc 220 Halo 460 Halo Parhelia (sun dogs)

14. Interference Effects: Oil Films and Soap Bubbles • Thin transparent layer sandwiched between two transparent media. • Reflected waves can be in-phase or out-of-phase depending on the film thickness and its refractive index. • If white light is incident, some colors will interfere destructively. Thin film acts like a filter! http://micro.magnet.fsu.edu/primer/java/interference/soapbubbles/index.html

15. Scattering Effects Scattering of light occurs when a beam of light is broken into several smaller, less intense beams of light by gases, aerosols, or particulates in the atmosphere. Lunar sky looks black because there is no atmosphere to scatter light!

16. Rayleigh Scattering • Small particles of size 50 – 100 nm scatter light with an efficiency: • “Shorter” wavelengths are scattered more efficiently. At sunset or sunrise light travels a greater distance through the atmosphere, thereby allowing more of the blue light to be scattered. That’s why the sky appears blue and the setting sun appears red.

17. Scattering Effects (Contd.) Mie Scattering: Large particles (steam, clouds…) scatter all wavelengths equally well giving a milky white appearance. Clouds Glacial water

18. Trivia Why are the taxiway lights violet and runway lights amber? Why is red used to signal danger? Red suffers the least atmospheric scattering so it can be seen from far away.

19. Review Problems • In Rayleigh scattering, how much more efficiently is light of 420 nm scattered than light of 650 nm? • Suppose the earth’s atmosphere scatters 10% of the incident blue light, 5% of green and 1% of red light for every mile the light travels through it. 100 units each of R, G, and B lights are incident on it from the sun. Calculate how many units of each wavelength remain after the light has traveled 1 mile, 2 miles, 3 miles… through the atmosphere? 5.7 times After 1 mile: R=99, G=95, B=90 After 2 miles: R=98, G=90, B=81…