Light

1. The speed of light.

Light travels at 3 x 10⁸ m/s, or 186,000 miles per second. How far will light from the sun travel in 20 seconds?

How long does it take light to reach the earth, which is 93,000,000 miles from the sun?

What is a light year? Express a light year in ordinary units, like miles, seconds, etc.

2. Light waves: frequency and wavelength.

The speed of light is equal to the wavelength times the frequency: v = lf. Calculate the frequency of microwaves of wavelength 1 cm; of red light of wavelength 600 nm.

3. Shadows

Draw the relative positions of the sun, earth and moon for a solar eclipse (see the text).

Use shadows to determine the height of a tall building…or a tree. Find the length (L) of the shadow of the building, then find the length “l” of the shadow of a stick (a ruler) of known height (h). The ratio of the height of the building (H) to the height of the ruler (h) is the same as the ratio of the lengths of the shadows (L/ l). See the figure. You’ll have to go outside to do this, since that is where the shadows (and the building) are. If there aren’t any shadows today, skip this.

H

h

L l

4. The eye.

Look over the section in Hewitt on the structure and function of the eye. Then do the “experiments” to

a. Find your blind spot. See figure 26.15 (answer the question)

To do this carefully, it’s a good idea to focus on a fixed object in the room so you aren’t tempted to move your eyes as your friend walks into view.

b. Check your peripheral vision: see the first two paragraphs of page 507. Record your observations.

5. Arrange the following waves in order of shorter to longer wavelengths: radar, blue light, green light, x-rays, WUAL radio waves. Do ultrasonic waves belong in this list?