Same White Light, Totally Different Inside! Uncovering the Secrets of Sunlight and Fluorescent Lamps with a Diffraction Grating

I’m science trainer Ken Kuwako. Every day is an experiment.

Today, I’d like to share an experiment where we observed white light using a diffraction grating. Sunlight and fluorescent light both look “white” to our eyes, but when you break them down into their components, they turn out to be completely different kinds of light. Let’s take a peek at those hidden differences with a diffraction grating (and remember—never look directly at the Sun!).

Check out the video below.

What Exactly Is a Diffraction Grating?

Here’s what a diffraction grating looks like. The viewing window contains the grating itself, and this particular one has 5,000 lines packed into every single centimeter. You can even buy them online quite easily.

Simple Spectroscope Kit MJ (Set of 8) – Kenis

Five thousand lines per centimeter means each groove is spaced only 0.002 mm apart—that’s just 2 micrometers. When light hits these incredibly tiny grooves, each wavelength bends at a slightly different angle, causing the colors to spread apart. It’s exactly the same reason the back of a CD or DVD shimmers with rainbow colors.

This particular spectroscope was purchased from Kenis. Light enters through the slit on the right side, passes through the diffraction grating attached to the viewing window, and the separated colors are projected to the left.

Breaking Apart Fluorescent Light — Strange “Lines” Appear

First, let’s look at fluorescent light. The band of light separates into blue, cyan, green, orange, and red from right to left.

What’s fascinating is that the light doesn’t appear continuous. Instead, it shows up as distinct bright lines.

This happens because gases such as mercury inside the fluorescent lamp emit only specific wavelengths of light. Each type of gas produces its own unique set of wavelengths, known as an “emission spectrum.”

In fact, astronomers use this exact property to figure out what distant stars are made of. By splitting starlight into spectra, humanity can identify the ingredients of stars we could never physically visit. Simply by analyzing light, we can uncover the chemistry of the universe.

What Happens When You Split Sunlight?

Next, we examined sunlight. Once again, be extremely careful and never look directly at the Sun.

Here’s the result. The colors smoothly transition from blue on the left to red on the right, just like a rainbow.

Unlike the sharp “lines” seen in fluorescent light, every color blends continuously into the next. This is called a “continuous spectrum.”

In other words, sunlight contains light of every visible wavelength.

The reason we see rainbows after rain is that tiny water droplets in the air act like miniature prisms, separating sunlight into different colors. The continuous spectrum seen through the diffraction grating is essentially the same phenomenon happening right before your eyes.

If you’ve studied diffraction formulas in school, try comparing them with what you observed here. Why does blue appear closer to the center while red spreads farther outward? The key lies in the relationship between wavelength and diffraction angle.

Blue light has a shorter wavelength, while red light has a longer one. Start from that idea, and the answer may reveal itself naturally.

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