A rainbow appears when sunlight interacts with many tiny raindrops in the air. Each drop bends, reflects, and spreads white sunlight into its visible colors. This makes the sky show a curved band of red, orange, yellow, green, blue, indigo, and violet.
Rainbows matter because they reveal how light travels and how water droplets can act like small prisms.
Key Facts
- White sunlight is a mixture of visible colors with different wavelengths.
- Refraction bends light when it enters or leaves a raindrop because its speed changes.
- Dispersion separates white light because different colors bend by different amounts.
- A primary rainbow forms after refraction, one internal reflection, and refraction out of the drop.
- The primary rainbow is seen about 42 degrees from the antisolar point, with red on the outside.
- Index of refraction relation: n = c / v, where n is refractive index, c is light speed in vacuum, and v is light speed in a material.
Vocabulary
- Refraction
- Refraction is the bending of light as it passes from one material into another where its speed is different.
- Dispersion
- Dispersion is the separation of white light into colors because each wavelength bends by a different amount.
- Internal reflection
- Internal reflection is the bouncing of light off the inside surface of a raindrop before it exits.
- Wavelength
- Wavelength is the distance between matching points on a wave and helps determine the color of visible light.
- Antisolar point
- The antisolar point is the point in the sky directly opposite the Sun from the observer's viewpoint.
Common Mistakes to Avoid
- Thinking the rainbow is located at one fixed place in the sky. A rainbow is a viewing geometry, so each observer sees light from different raindrops at the correct angle.
- Drawing rainbow colors in the wrong order. In a primary rainbow, red is on the outside of the arc and violet is on the inside because violet bends more than red.
- Assuming raindrops add color to sunlight. Raindrops do not create new colors, they separate the colors already present in white sunlight.
- Placing the Sun in front of the observer when viewing a rainbow. For a primary rainbow, the Sun must be behind the observer and raindrops must be in front.
Practice Questions
- 1 A student observes a primary rainbow at an angle of about 42 degrees from the antisolar point. If the antisolar point is 10 degrees above the horizon, about how high above the horizon is the top of the rainbow arc?
- 2 Light travels through water at about 2.25 x 10^8 m/s. Using c = 3.00 x 10^8 m/s and n = c / v, calculate the refractive index of water.
- 3 Explain why a rainbow is usually visible only when the Sun is behind the observer and rain is falling in front of the observer.