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.
Understanding How Rainbows Appear
A rainbow is not an object sitting at one place in the sky. It is a viewing pattern made by sunlight, drops, and your eye. For a particular colour to reach you, a drop must be in a very specific direction from your head.
Millions of drops are present, but only a small set sends the right rays toward you. This is why the Sun must be behind you while the rain is in front of you.
If you move, the useful set of drops changes. Another person nearby sees a rainbow made by different drops.
The familiar arc comes from the geometry of those selected directions. Light reaching your eye forms a cone around the point directly opposite the Sun. The ground usually blocks the lower part of that cone, so the visible part looks like an arc.
From an aeroplane or a high mountain, a person can sometimes see a complete circle because there is no horizon hiding its bottom. The position of the Sun matters greatly. When the Sun is more than about 42 degrees above the horizon, most of the primary rainbow lies below the horizon and cannot be seen.
Some rain showers produce a second, wider arc outside the first one. This secondary rainbow follows a longer route inside each drop, with two bounces instead of one. Extra bouncing loses more light, so this arc is usually dimmer.
Its colour order is reversed from the primary arc. The sky between the two arcs can look noticeably darker. Fewer rays are sent into that region, creating a feature called Alexander's band.
Very faint extra pastel bands may appear just inside the main rainbow. These form because light behaves as a wave. Rays leaving slightly different parts of a drop can strengthen or weaken each other, making narrow bright and dark bands.
Rainbows are easiest to spot when the Sun is low, rain is falling nearby, and the sky opposite the Sun is dark enough for the colours to stand out. A bright white cloud behind the rain can hide them. Polarised sunglasses can change the brightness because rainbow light is partly polarised during its path through the drops.
Never look directly at the Sun while searching. When learning this topic, draw the Sun, your eye, and one raindrop before adding many drops.
Track one ray at a time. Pay attention to which way the ray bends at each surface, then connect that path to the angle at which the ray finally reaches the observer.
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.