Essay questions on light scattering

by Quirino Sugon Jr.

1.  Why is the sky blue?  

The atmosphere around the Earth is largely made up of two colorless gases: oxygen and nitrogen. Red and blue light reacts very different from each other to oxygen. Because the wavelength of blue light is roughly the size of an atom of oxygen, blue light interacts with the oxygen and is scattered by it, while red light, with its longer wavelength, goes right pass the oxygen atoms. If the Earth had no atmosphere, the sun’s light would travel directly from the Sun in a straight line towards our eyes and we would see the Sun as a very bright star in sea of blackness. But because the Sun’s blue light is scattered by the oxygen in the atmosphere, blue light from the Sun enters our eyes from all sorts of different angles and we see the entire sky as blue. The atmosphere scatters violet light even more effectively, but our eyes are more sensitive to blue. Wherever we look towards the sky, some light is bouncing off an oxygen atom and entering our eyes, making the sky appear to be blue. (Skywatch)

2.  Why are sunrises and sunsets red?

Read the same article in Question 1.

3.  Why is the sea blue?

Visible white light is made up of a spectrum of all the colors-red, orange, yellow, green, blue, indigo, and violet. When we look at an object and see it as blue, we are seeing the blue light of the spectrum reflected from the object. All other colors are absorbed and cannot be seen. In the case of the sea, red light is absorbed as soon as it breaks through the water’s surface. And by a depth of about 25 feet virtually all the red light discernible to the human eye is gone; a bright red air tank on a diver, for example, would seem a dull dark brown. At a depth of 75 feet a yellow air tank looks more greenish blue, because the discernable yellow light has been absorbed by the water. The still shorter rays of light are almost all absorbed by 100 feet. All that remains are the shortest rays: blue, indigo, and violet. Below 100 feet or so, all light appears a monochromatic blue. So, when the sea is pure and clear, as often is the case in the open ocean, the least-absorbed shade of the spectrum blue, is reflected to our eyes. (marine-surveyor)

4.  Why is the sea sometimes bluish green or brown or even red?

Read the same reference in Question 3.

5.  How do rainbows form?

Read the Wikipedia article on rainbow.  What causes the dispersion of light?  What must be the angle formed by the sun, the water droplet, and the eye of the beholder? What is the Alexander’s band?  What does Newton have to do with rainbows? Why do you think rainbows formed after the Great Flood of Noah?