Why is the sky blue?

1. Why is the sky blue? Notes: April 26th 2011

2. Something to ponder…? • When you look at the sky at night it is black, with the stars and the moon forming points of light on that black background. So why is it that, during the day, the sky does not remain black with the sun acting as another point of light? Why does the daytime sky turn a bright blue and the stars disappear?

3. Light scattering • 1. Sun is an extremely bright source of light. • 2. We have an atmosphere. Small atoms of oxygen and nitrogen in the atmosphere effect sunlight. • Think of ringing bells!

4. Rayleigh scattering • Physical phenomenon that causes light to scatter when it passes through particles. • Tiny particles bend high frequency light. (oxygen, nitrogen)- blue mountain skies • Large particles bend low frequency light. (methane, sulfur)- sunsets in LA!

5. Blue sky… why not violet? • This scattering of the higher frequencies of light illuminates the skies with light on the BIV end of the visible spectrum. • Violet is scattered most easily, so why isn’t the sky violet?

6. Sensitivity • Our eyes are more sensitive to light with blue frequencies. • Three primary color cones in our eye: Blue, green, and red.

7. Why is the sun yellow? • The lower frequencies of sunlight (ROY) tend to reach our eyes unscattered as we look directly at the midday sun. Although sunlight is a combo of all colors, yellow is most intense.

8. Appearance of sun • Appearance of the sun changes with the time of day. • As the path which the light takes through the atmosphere increases ROYGBIV encounters more atmospheric particles, and scatters more light.

9. Sunset • During sunset hours, the light passing through our atmosphere to our eyes tends to be concentrated with red and orange frequencies. This is why sunsets have a reddish-orange hue.

10. What effects a sunset? • The effect of a red sunset becomes more pronounced if the atmosphere contains more and more sulfur aerosols (industrial pollutant)

11. Natures Beauty

12. Endless beauty

15. Blue haze • Some mountain ranges are famous for their blue hazes which result from aerosols from the vegetation reacting with ozone to form small particles (200 nm) which scatter blue light.

16. Tyndall Effect • John Tyndall (1859) discovered that when light passes through clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than red.

17. Critical Opalescence • People with blue eyes, really don’t have blue eyes. The fluid in our eyes scatters or reflects the blue light from our eyes. • In the cells of a blue-jay feather. • Blue gem stones.

18. Why is Mars sky red? • Images from Viking Mars landers (1977), and Pathfinder (1997) show a red sky from the Martian surface. This is due to a red iron rich dusts (magnetite) thrown up in dust storms, which absorbs blue wavelengths and reflects red.

19. Why are clouds white? • Clouds appear white because they consist of water droplets that are many sizes and scatter all wavelengths equally. • (Mie scattering)

20. Why is sea water greenish blue? • Water absorbs infrared waves (heats water) • Absorption of red light by water. • What is left is the complement of red, which is cyan. (Greenish blue)

21. Why are sea creatures red? • In deep sea water, since there is no red light coming in to be reflected, the lobsters and other sea creatures look black, and through evolution have avoided being eaten by their predators.

22. In Summary • The sky is blue because blue from sunlight is scattered in all directions by molecules in the atmosphere. • Sunsets are red because all the other color frequencies are filtered out. • Water looks greenish blue because water absorbs red light. • The color of things depend on what colors are reflected or absorbed by molecules.