# Unpacking Light

Use a quantitative spectroscope to get a glimpse of light composition no matter where you are. This project takes no time at all and is completely portable!

What you’ll need:

1. Quantitative Spectroscope (PH100QA)
2. A notebook to record your observations!

Background:

You probably know that light comes in many colors. Of course. And I’m sure you know that those colors can be mixed. Right. And if you start with the primary colors, you can create any other color. Check. But what if I told you that was not the whole story? What if I told you that there are colors that can’t be made from the primary colors? What if I told you that there is a continuous range of fundamental colors? What if I told you that there are colors your TV can’t make – colors your eyes can’t sense – and that all colors are created by atoms and molecules vibrating and emitting radiation! Would that blow your mind?

It’s true! From the sun to LED lights, to the output of a laser, to fluorescent lights, to halogen bulbs, to your TV, to street-lamps, to fire itself. All of this light comes from atoms and molecules releasing energy in the form of light. That energy comes in wonderful combinations, but always in combinations of fundamental colors.

We can unpack the light we see into fundamental colors using a spectroscope. The spectroscope has a diffraction grating which splits the light into its fundamental colors, from red to purple. Each type of light will contain different colors.

Try it yourself!

1.  Point the spectroscope at a source of light. Then look through the view-hole, not at the source of the light but off to the side (either-side is okay). WARNING! – DON’T LOOK AT THE SUN, JUST LOOK TO THE SIDE!

a. You should see a rainbow, or maybe parts of a rainbow. The rainbow may have gaps in it, or it may even have only specific colored lines. This tells you which fundamental colors make up the light that you’re examining.

b. Look at a bunch of different sources of light. How are they similar? How are they different? Which is the most full? Which is the most empty?

c. All of the colors you see through the spectroscope are fundamental. These are colors that cannot be created by your TV. Even though the colors seem like standard colors (red, orange, yellow, etc.), none of these colors are combinations of any other color. That is what makes them different from your TV.

The following picture (Fig. 1) shows a colorful curvy shape with a triangle inside. The colors that you see in your spectroscope are all on the very edge of the curvy shape. The triangle shows the colors that can be made on a typical TV. The curve inside the triangle shows the colors made by hot objects (like the sun or stars) of different temperatures.

Figure 1. Image Credit: Spigget

2.  Pick a source that has a very empty rainbow. You can try to determine the wavelength of the light components in the rainbow. Here is how you can do that:

a. A diffraction grating of this type will diffract the spectrum to both the right and the left, so you should see the spectrum on both sides. The one on the right should be close some lines (the scale), but you may have to adjust how you're holding the spectrometer to get it to line up just right.

b. Make sure that the light source is visible through the slit (this is crucial) and then, without moving the spectrometer at all, try to peer to the right and the left.

c. Now look to see where the rainbow is relative to the lines inside the spectroscope. For example 400 nm light would make a line which is 3.8 cm from the slit. So you should see the blue part around 4 lines from the slit. 700 nm light, or red light, would make a line which is 6.7 cm from the slit, or about 7 cm from the slit.

d. Record the number on the scale which corresponds to the brightest part of the spectrum; this is y . Then we use the following equation to determine the wavelength of light,

wavelength = (105.3 nm/cm) x y

e. Bonus: If you analyze sunlight, and you’re very very careful, you will see a nice bright rainbow, but you might notice that there are very narrow gaps in the spectrum. See if you can determine the wavelengths of these gaps. Why might these gaps be there?