Can Two Glass Plates and Tape Create a Rainbow? We Built a DIY Device to See Light Interference in Action
I’m Kuwako Ken, your science trainer. Every day is an experiment.
Just by stacking two pieces of glass together, shimmering rainbow stripes suddenly appear. It feels almost like magic—but this beautiful phenomenon can actually be recreated with everyday materials. All you need are two microscope slide glasses and a strip of transparent tape. In this experiment, we explored the mysterious behavior of light known as “wedge-shaped interference.”
What Is Wedge Interference? A Phenomenon Caused by Light Acting Like a Wave
First, let’s look at why these colorful stripes appear.
Light behaves not only like particles, but also like waves. When a tiny layer of air exists between two glass plates, light reflects off both the top and bottom surfaces of that air gap. These two reflected waves overlap with each other. When wave peaks line up, the light becomes brighter. When peaks and valleys overlap, they cancel each other out and become darker. This is called interference.
The air layer between the two glasses isn’t perfectly uniform—it gradually changes thickness like a wedge. Because the thickness changes from place to place, different colors strengthen or weaken at different points, producing those beautiful rainbow stripes.
In fact, this is exactly the same principle behind the rainbow colors on soap bubbles or the oily patterns floating on puddles after rain. The same hidden physics is already all around us in daily life.
How to Build the Device — You Only Need Four Simple Materials
For this experiment, we used two large microscope slide glasses. The setup is incredibly simple. Place a strip of transparent tape on one edge of one glass plate to create a tiny gap, then place the second glass plate on top. Finally, press the glasses together firmly using clips or your fingers.
A single layer of tape is about 0.05 millimeters thick—around 50 micrometers. That’s roughly half the thickness of a human hair. This incredibly tiny gap becomes the stage where light waves interfere with each other.
For comparison, the wavelength of visible light is more than a thousand times smaller still. Inside this microscopic air layer, light is performing an astonishingly delicate dance.
What You’ll See — Rainbow Stripes Appear Near Your Finger
When you press the glass more tightly with your finger and shine sunlight or another bright light onto it, faint vertical stripes begin to appear. Take a look at the video below.
Can you spot the rainbow-colored stripes appearing near the finger?
The important point is that the stripes appear near the area where the two glass plates are closest together. As the gap between the glasses changes, the conditions for interference also change, which alters the spacing between the stripes.
In fact, by carefully measuring the stripe spacing, scientists can even calculate the exact angle of the air wedge itself. That’s how precise this phenomenon is.
The stripes can be surprisingly difficult to see at first, so try tilting your viewing angle slightly. Changing the angle of incoming light often makes the pattern much clearer.
This Same Principle Is Used in High-Precision Technology
Wedge interference isn’t just a classroom curiosity—it’s also used in modern industry. In the manufacturing of lenses and semiconductors, engineers inspect surfaces by analyzing patterns of interference fringes that reveal microscopic unevenness.
If the stripe pattern becomes distorted, it indicates flaws or warping on the surface. Using light waves as an ultra-sensitive ruler, scientists can measure structures on the scale of nanometers—just one millionth of a millimeter.
That means this simple experiment, made from two pieces of glass and a strip of tape, lets you experience the exact same physics behind cutting-edge optical measurement technology.
Try it near a sunny window someday. It’s a rare chance to actually see—with your own eyes—that light truly behaves like a wave.
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