Tue., Mar. 7 notes

Tue., Mar. 7, 2006

The graded Experiment #2 reports were returned in class today. You are allowed to revise these reports. The revised reports will be due on Tue., Mar. 21. Please return the original report when you turn in a revised report.

Optional Assignment #3 was returned. You'll find answers online. Answers to the Surprise Optional Assignment are also available online.

A photocopied version of the Quiz #2 Study Guide was handed out in class.

A quick review of three of the main characteristics of the equinoxes. One of our goals today will be to see how these change on the summer and winter solstices.

Now imagine rotating the earth from its equinox orientation to the winter solstice orientation. You can see that rays will strike the earth perpendicularly at 23.5 degrees S latitude, that is where the sun will pass overhead at noon.

See p. 78b in the photocopied notes for a detailed discussion of each of the numbered points in this figure.

Pohnpei island in the Federated States of Micronesia is located at 7 degrees N latitude in the middle of the Pacific Ocean. The climate is warm moist and changes very little during the year. You can see at upper right the monthly average temperatures range from 80.0 F to 80.8 during the year, less than 1 F annual range (the annual range in Tucson is about 34 F (see pps 82b and 82c in the photocopied notes). Average annual rainfall in Kolonia, the largest town on Pohnpei, is close to 200 inches. Some parts of the island get close to 400 inches of rain per year.

Now we rotate the earth in the opposite direction from its equinox orientation to the summer solstice orientation. The sun will now be overhead at 23.5 N latitude at noon.

Refer to p. 80 in the photocopied notes for a discussion of the numbered points. Note in particular that the maximum amount of sunlight arriving at the ground during the day is found at 30 latitude, not at the equator or 23.5 degrees latitude.

Now we turn to a completely different topic, scattering of light. Quiz #2 won't have questions about any of this material.

A bright narrow beam of red laser light was shined across the front of the classroom. If you were to stand at the wall where the light struck and look back toward the laser (you shouldn't look at direct laser light) you'd be able to see the red light. Students in the classroom weren't able to see the light. This is illustrated in the top portion of the figure below.

Putting some chalk dust or cloud droplets (very small water droplets) in the laser beam caused some of the light to be scattered (redirected). Now students in the classroom were able to see some of the laser light. Everyone in the room that was able to see red light was looking back along a different ray of light.

What if you're standing outside looking up at the sky. If you look at the sun (just as you shouldn't look at direct laser light, you shouldn't look directly at the sun), you see bright white light. If you look away from the sun toward the sky you also see light. You are seeing sunlight that has been scattered by air molecules. Why is this scattered light blue?

Because they are very small, air molecules scatter shorted wavelengths of light more readily than the longer wavelengths of light.

If you look at the sun when it is high in the sky you see bright white light. If the sun is low in the sky it becomes yellow, orange, sometimes even red. The sunlight is not as bright as when it was high in the sky (though it is still too bright to safely look at).

If you look at the sky you see sunlight that has been scattered by air molecules. It has a blue color and is much less bright. It is safe to look at this blue scattered sunlight.

The drops of water in a cloud are larger than the wavelength of visible light. They scatter all the colors more or less equally. Thus if you shine white light on a cloud the light scattered by the cloud will also be white.

Here's another look at the same material.

The incident sunlight is white which means it is an equal mix of all the different colors of visible light.

When you look at the sun when it is high in the sky you see unscattered light shown in (1) above. Note it is the original equal mix of colors with a little bit of violet blue and green removed. This unscattered light is still bright and its color has shifted slightly to a yellowish white.

When you look at the scattered light you are seeing the small amount of violet, blue, and green that has been removed from the incoming sunlight. This scattered light is weaker and has a bluish tint.

Finally when you look at light coming from a sun that is low in the sky you see light that has had all of the shorter wavelength removed. Now the light coming from the sun has an orangish or reddish tint.