Science 101

## Q: What Color Is the Sun?

A: Remember when you were in elementary school and you were drawing a picture with the Sun in the sky? What color crayon would you use for the Sun? It was yellow, right? But is the Sun really yellow? Let’s find out!

The first thing I should say is that you should never look directly at the Sun (unless it’s right on the horizon when rising or setting). The Sun’s radiation—both visible and invisible forms of light—is so intense that it can damage your eyes. So, simply looking up at the Sun to see what color it is isn’t an option. But, if we’re clever, we can still figure out what color it is.

In the previous Science 101 column on the “scientific method” (in quotation marks because there really isn’t one), I also discussed why objects appear to have one color rather than another. We saw that an object will appear to be whatever color of light it reflects. If you shine white light (composed of all colors) on an object, and only red light is reflected, then the object is red; if only green light is reflected, the object is green; and so on.

Beware, however, if you have light that’s not white, i.e., doesn’t contain all colors. Let’s say you have a red lightbulb. If you let that red light shine down onto a white piece of paper (see Figure 1), what color does the paper appear? (Red) Can we explain that? Sure! Anything that’s white, like your paper, reflects all colors. The paper will reflect the red wavelengths coming from your lightbulb, and because that’s the only color that light is emitting, that’s the only color the paper will reflect. Your eyes detect the red light coming from the paper, and so the paper looks red. If you had instead used a green lightbulb, the paper would appear green. So, without ever looking directly at the lightbulb, we can tell what color it is by looking at what color the paper appears. For the white paper to actually appear white, we would need a white source of light. Let students try this activity with whatever colored lightbulbs you (or they) can find.

Let’s take the white paper outside on a sunny day and see what color it appears when illuminated by the Sun. Not surprisingly, it appears white (see Figure 2); so, we could conclude that the Sun is white. Are there other phenomena that show the Sun is white? Yes! The water droplets in clouds and the ice crystals in snow are very efficient at scattering all colors of light. Like the white paper, if you shine colored light on some snow, it will appear to have the color of the light. The fact that snow is white and clouds are white is another way to tell that the Sun is white. If the Sun were yellow, clouds would look yellow and snow would look yellow (even if no dogs were around).

Normally one should never look at the Sun through a telescope. Even without a telescope, the Sun is blindingly bright (unless it is behind some clouds or just rising or setting), and it would be many times brighter when viewed through a telescope. Without proper protection, glancing at the Sun through a telescope can cause blindness very quickly. However, with proper filters, one can safely look at the Sun through a telescope. Sometimes those filters are colored. If you search for images of the Sun, many of the photographs you’ll find were taken through a filter of a particular color. Often a special red filter is used, which lets through only a particular color of red light emitted by the hydrogen atoms in the Sun. You can see a link to such a photo in the Online Resources. So, in those photos, the Sun looks red. But that redness is just from the filter, not because the Sun is red. It’s as though you put red cellophane over the front of a flashlight; the light would look red due to that red filter.

We can look at the Sun directly and see its color. Many times, I have shown visitors the Sun through a telescope. The filter I use (to make it safe to look at the Sun) is a “neutral” filter, which blocks all colors of light equally, so it doesn’t change the apparent color of the Sun. When people first look at the Sun through the telescope, the most common remark I hear is, “I didn’t expect it to be white!” There’s a link to a photo of the Sun taken through a neutral filter in the Online Resources.

The fact that the Sun is white is why we can see so many different colors in the outdoor world. If sunlight were purely green, then everything outside would look green or dark. We can see the redness of a cardinal and the blueness of a blue jay because sunlight contains red and blue light. The same is true for all the other colors. And all those colors combined make the light from the Sun look white.

Other clues that the Sun is white:

• When a lightbulb engineer wants to design a bulb with light similar to sunlight to provide natural-looking illumination, she designs a white bulb, not a yellow bulb.
• What color is the Moon when it’s high in the sky? (white) Where does the Moon’s light come from? (reflected light from the Sun)
• The fact that you see all the colors present in a rainbow—which is sunlight split into its component colors by water droplets—and no colors are missing, is direct evidence that sunlight is white.

The Sun really looks white only when it’s high in the sky. That’s because of the way the air scatters light. The molecules in the air scatter shorter wavelengths of light more than longer wavelengths of light. So blue light—which has shorter wavelengths—gets scattered a lot more than red light—which has longer wavelengths. (That’s why the sky is blue.) As the Sun starts to set, its light passes through more air on the way to our eyes. In Figure 3, you can see the longer path that the light takes through the atmosphere when the Sun is rising or setting. And the more air the light passes through, the more the shorter, blue wavelengths of light are scattered off in other directions, leaving the longer wavelengths—yellow, orange, and red—to reach your eyes. That’s why the rising or setting Sun often looks yellow or orange in color. For the same reason, the Moon can also look orange when it’s very low in the sky, just rising or setting. If you want to see how the Sun looks without the interference of Earth’s atmosphere, follow the link in the Online Resources for “The Sun as seen from space.”

So then, why did we use a yellow crayon for the Sun? Maybe because if you used the proper color—a white crayon—it wouldn’t show up very well on the white paper. Yellow provides the needed contrast. Fun fact: While American children color the Sun yellow, Japanese children color it red. And the Japanese flag has a red disk on it, which represents the Sun. One last bit of advice: Never look at the Sun through a colander. You’ll strain your eyes.

Never stop learning.

#### Online Resources

The Sun photographed through a (red) hydrogen filter

Matt Bobrowsky is the lead author of the NSTA Press book series, Phenomenon-Based Learning: Using Physical Science Gadgets & Gizmos. You can let him know if there’s a science concept that you would like to hear more about. Contact him at: DrMatt@msb-science.com