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How does a prism split light?

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Quick answer

A prism splits light because glass bends different colors by different amounts. When white light enters, each color slows and refracts at a slightly different angle. Exiting, they bend again, fanning out into a spectrum from red to violet, revealing the colors hidden in white light.

A prism splits white light because different colors bend by different amounts when they cross the glass. This separation happens twice, once entering and once leaving, spreading the colors into a rainbow called a spectrum.

Step by step

  1. White light, containing all visible wavelengths, travels through air and hits the prism surface.
  2. Upon entering the glass, light slows down and bends toward the normal, with shorter wavelengths bending more.
  3. The different colors now travel at slightly different angles inside the prism.
  4. As light exits the glass into air, it speeds up and bends away from the normal, increasing the separation.
  5. The colors emerge as a continuous band, with red deviated least and violet deviated most.

Key things to understand

  1. White light is a mixture of all visible colors, each with a different wavelength.
  2. When light enters a prism, it slows down and bends toward the normal line.
  3. Shorter wavelengths like blue bend more than longer wavelengths like red.
  4. The light bends again when it exits the prism, increasing the separation.
  5. The result is a fan of colors from red to violet, called a spectrum.

Labelled diagram

  1. White light

    A beam containing all visible wavelengths enters the prism.

  2. First refraction

    Light bends as it enters glass; shorter wavelengths bend more.

  3. Inside prism

    Colors travel at different angles, beginning to separate.

  4. Second refraction

    Light bends again exiting glass, increasing separation.

  5. Spectrum

    Colors fan out from red to violet on the screen.

How a prism separates white light

Real-world examples

  • A glass prism on a windowsill casts a rainbow on the wall when sunlight passes through it.
  • Raindrops act like tiny prisms, creating a rainbow when sunlight enters and reflects inside them.
  • Spectrometers in laboratories use prisms to separate light from stars or chemical samples for analysis.
  • Chandeliers and cut glass produce colored fringes at their edges due to prism-like refraction.

Common mistake

✗ Myth: A prism adds color to white light.

✓ Fact: A prism does not add color; it separates colors already present in white light. The colors are revealed because they bend by different amounts.

People also ask

Why does a prism split light into different colors?
A prism splits light because the glass has a different refractive index for each wavelength. Shorter wavelengths, like blue, slow down more and bend more than longer wavelengths, like red. This causes the colors to separate spatially, creating a spectrum.
What is the order of colors in a prism spectrum?
The order from least to most bent is red, orange, yellow, green, blue, indigo, violet. Red bends the least because it has the longest wavelength, while violet bends the most due to its shorter wavelength. This sequence is remembered as ROYGBIV.
Can white light be split without a prism?
Yes, any transparent material with non-parallel surfaces can split light, such as raindrops, diffraction gratings, or even a glass of water. Diffraction gratings use fine lines to separate colors by interference rather than refraction, often more precisely than prisms.
Why does a prism create a rainbow?
A prism separates white light into its component colors because each color refracts by a different angle. The rainbow appears because the colors spread out spatially, with red bending least and violet bending most.
What is dispersion of light?
Dispersion is the process where white light splits into its constituent colors due to different refractive indices for different wavelengths. It occurs in prisms, raindrops, and other transparent materials.
Can a prism split light into more than seven colors?
Yes, visible light contains a continuous range of colors, not just seven distinct ones. The seven-color naming is a convention; in reality, the spectrum is a smooth gradient from red to violet.
What happens if you use a second prism?
A second prism can recombine the separated colors back into white light if placed correctly. This shows that the colors were not created by the prism but were already present in the white light.

Part of the guide: Everyday Science: Light, Heat & Matter →