Why Do CDs Reflect Light In Rainbow Colors?

Even though the compact disc may not have experienced the resurgence that vinyl records have seen in recent years, it remains an endearing and nostalgic form of physical media for many. Being so thin while containing so much information is fascinating in its own right, but perhaps an even more peculiar element of CDs, as well as DVDs and Blu-Rays, is their unique ability to reflect light as rainbow colors. It’s certainly a cool look, but have you ever wondered how it happens?

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To answer this requires little more than some basic science know-how. Light may largely appear white to the naked eye, but in actuality, it is typically made up of various colors, primarily red, blue, and green, and combinations of these shades. Whenever light comes in contact with your CD a phenomena known as interference takes place. This is where several light waves interact with one another, resulting in the amplitude of the waves being noticeably altered. This only occurs when light comes into contact with surfaces of a certain thickness, such as water droplets, bubbles, or a floating film of oil. 

The mirrored surface of a CD is surprisingly sophisticated in its construction, as you can imagine for something that has to carry a wealth of data. It produces particularly strong levels of interference that cause the waves to combine and the amplitudes to increase significantly. This results in the bold colors that you end up seeing, which can change depending on the angle from which you view the disc. 

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CDs may no longer be the industry disruptors they once were, but that doesn’t mean you can’t find use out of your old ones. Their unique manner of interacting with light opens the door to a wide array of activities and experiments. In mid-2021, the YouTube channel The Curious Minds STEAM Club shared a video showcasing a number of fun, easy projects involving CDs and similar discs that take advantage of their amazing interference capabilities. 

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The first experiment shows how to make creative rainbow patterns with CDs that require nothing more than a CD, a sheet of paper, some scissors, a pencil, and tape. The activity starts by tracing around your CD on a sheet of paper and cutting out the section. From there, you’ll fold the piece of paper in half numerous times until it divides into your desired number of sections. While still folded up, you can cut in the shape of your choosing so that once the paper is unfolded, each shape will be equal in size and space. Your paper is now ready to tape around your disc, which, when angled at the light correctly, will display a rainbow in the shape of your pattern.

Perhaps the simplest yet coolest example is shown at the end of the video. This involves a disc placed on a plate with a candle sitting upright in the disc’s center, held in by candle wax. The lit candle produces an especially strong ring of rainbow light on the CD that’s mesmerizing, to say the least. 

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