Dispersion is the spreading of light into its component colors because different wavelengths travel differently through a material. A prism makes this effect easy to see by bending white light and separating it into a visible spectrum from red to violet. This matters because it explains rainbows, lens color fringing, and how scientists identify materials from the light they emit or absorb.
Prisms show that white light is not a single color, but a mixture of many wavelengths.
Key Facts
- Index of refraction: n = c / v, where c is light speed in vacuum and v is light speed in the material.
- Snell's law: n1 sin(theta1) = n2 sin(theta2).
- In normal dispersion, shorter wavelengths have larger refractive indices, so violet bends more than red.
- White light separates because each wavelength follows a slightly different path through the prism.
- Visible light wavelengths are about 700 nm for red to 400 nm for violet.
- A rainbow forms when sunlight is refracted, internally reflected, and dispersed inside water droplets.
Vocabulary
- Dispersion
- Dispersion is the separation of light into different colors because the index of refraction depends on wavelength.
- Prism
- A prism is a transparent optical object with flat faces that refracts light and can separate white light into a spectrum.
- Index of refraction
- The index of refraction is a number that tells how much light slows down in a material compared with its speed in vacuum.
- Wavelength
- Wavelength is the distance between matching points on a wave, such as crest to crest, and it determines the color of visible light.
- Refraction
- Refraction is the bending of light when it crosses into a different medium and changes speed.
Common Mistakes to Avoid
- Thinking the prism creates colors, which is wrong because the colors are already present in white light and the prism separates them.
- Assuming all colors bend by the same amount, which is wrong because the index of refraction varies with wavelength.
- Forgetting to measure angles from the normal line, which is wrong because Snell's law uses angles measured from the perpendicular to the surface.
- Saying red bends more than violet in ordinary glass, which is wrong because violet usually has a larger refractive index and bends more strongly.
Practice Questions
- 1 Light enters glass from air at an incident angle of 40.0 degrees. If the glass has n = 1.50 and air has n = 1.00, use Snell's law to find the refracted angle in the glass.
- 2 A beam contains red light with n = 1.514 and violet light with n = 1.532 in a prism. If both colors strike the glass from air at 30.0 degrees, find the refracted angle for each color and identify which bends more toward the normal.
- 3 Explain why sunlight passing through raindrops can form a rainbow, and why the colors appear in a consistent order.