When white light enters a glass prism, it slows down, bends, and separates into its component colors: red, orange, yellow, green, blue, and violet. This happens because white light is actually a mixture of many wavelengths traveling together, and the glass bends each wavelength by a slightly different amount. The result is the familiar rainbow-like band of color called a spectrum.
Why Light Bends at All
Light travels at different speeds in different materials. In empty space, it moves at roughly 300,000 kilometers per second. In glass, it slows down considerably. When a beam of light hits the surface of a prism at an angle, one side of the beam enters the glass and slows down before the other side does. This speed difference causes the entire beam to change direction, the same way a car pulls to one side when one wheel hits mud before the other. This bending is called refraction.
The amount of bending depends on the angle at which the light hits the surface and on how much the glass slows it down. A material’s ability to slow and bend light is measured by its refractive index. Air has a refractive index close to 1. Common glass sits around 1.5, meaning light travels about 1.5 times slower in glass than in a vacuum.
Why Different Colors Separate
Here’s the key detail: glass doesn’t slow all wavelengths of light by the same amount. Shorter wavelengths (violet and blue light) slow down more than longer wavelengths (red and orange light). Because the glass bends light more when it slows it more, violet light gets bent the most and red light gets bent the least. Every color in between lands somewhere in the middle, fanning out in order.
This wavelength-dependent bending is called dispersion. It’s the reason a prism produces a full spectrum rather than just bending white light as a single beam. The visible spectrum spans roughly 400 to 700 nanometers in wavelength. Violet sits at the short end (400 to 440 nm), then blue (440 to 480 nm), green (480 to 560 nm), yellow (560 to 590 nm), orange (590 to 630 nm), and red (630 to 700 nm). You may have learned the mnemonic “ROY G. BIV,” though modern optics generally treats indigo as part of the blue-violet range rather than a distinct band.
The Prism’s Shape Matters
A prism’s triangular cross-section is what makes the color separation visible. Light refracts twice: once when it enters the prism and again when it exits. At both surfaces, shorter wavelengths bend more than longer ones, so the two refractions compound the spreading effect. If you sent light through a flat pane of glass (like a window), it would bend going in and bend back by the same amount going out, so the colors would recombine and you’d see white light again. The angled faces of a prism ensure the two bends work together instead of canceling out.
The angle at the prism’s tip, called the apex angle, controls how dramatically the light spreads. A prism with a steeper apex angle produces a wider spectrum. The total amount a beam gets deflected from its original path depends on this apex angle, the angle at which the light enters, and the refractive index of the glass.
How the Glass Itself Changes the Result
Not all glass produces the same spectrum. Two broad categories matter here: crown glass and flint glass. Crown glass has a lower refractive index and lower dispersion, meaning it bends light less and separates colors less dramatically. Flint glass, which historically contained lead, has a higher refractive index and higher dispersion, producing a wider, more spread-out spectrum from the same beam of white light.
This difference is more than a curiosity. Camera lenses, telescopes, and binoculars combine crown and flint glass elements to cancel out unwanted color fringing. A positive lens made of crown glass paired with a negative lens made of flint glass brings different wavelengths back to roughly the same focus point, producing a sharper image. Isaac Newton himself recognized that color separation in lenses degraded telescope images, though it took later opticians to develop the compound lens solution.
Newton’s Experiment That Started It All
In 1672, Newton published a letter in the Philosophical Transactions of the Royal Society that overturned existing theories of light and color. Before Newton, most scientists believed that glass somehow modified or colored white light as it passed through. Newton showed the opposite: white light is, in his words, “a heterogeneous mixture of differently refrangible rays.” Color is an intrinsic property of each wavelength, not something created by the glass.
His critical proof was elegant. He used a second prism to isolate a single color from the spectrum, then passed that color through a third prism. The single-color beam bent but didn’t split further or change color, demonstrating that the glass wasn’t adding anything. The color was already there, bundled invisibly inside the white light all along.
When a Prism Acts as a Mirror
Prisms don’t always create rainbows. Under the right conditions, light hitting the inside surface of a prism at a steep enough angle doesn’t pass through at all. Instead, it bounces back entirely, a phenomenon called total internal reflection. This happens when light tries to move from glass (a denser medium) into air (a less dense medium) at an angle greater than a specific threshold called the critical angle.
For a standard glass prism in air, total internal reflection occurs when the glass has a refractive index above about 1.414. Many common optical glasses exceed this value, which means a right-angle prism can redirect a beam by 90 or 180 degrees with virtually no light loss. Binoculars use this principle: pairs of prisms fold the light path to make the instrument compact while flipping the image right-side up. Unlike mirrors, which can lose a few percent of light to absorption, a prism reflecting through total internal reflection bounces back essentially 100% of the light.
Prisms in Science and Technology
The same color-separating ability that makes prisms visually striking also makes them useful instruments. In spectroscopy, prisms split incoming light into its wavelength components so that a detector (often a camera sensor) can record which wavelengths are present and how intense each one is. This is how scientists identify the chemical composition of distant stars, detect pollutants in water, and analyze biological samples.
Diffraction gratings, which use finely spaced grooves instead of glass to separate light, have largely replaced prisms in everyday spectrometers because they’ve become cheaper and more versatile. But prisms still hold an edge in situations where gratings perform poorly, particularly in the ultraviolet and infrared ranges where gratings absorb too much light. Prisms are also critical in ultrafast laser science, where they compress laser pulses that last only femtoseconds (quadrillionths of a second) by precisely controlling how different wavelengths travel through the glass. Without prism-based pulse compression, many cutting-edge experiments in physics and chemistry wouldn’t achieve the time resolution they need.