A rainbow is a natural optical and atmospheric phenomenon that occurs when sunlight interacts with water droplets in the air — from rain, mist, or even waterfalls. Its formation involves three main processes: refraction, internal reflection, and dispersion of light. Below is a detailed, step-by-step explanation suitable for a blog article.
1) The Science Behind a Rainbow
Imagine a tiny, nearly spherical raindrop suspended in the air. When sunlight enters it, three key processes happen:
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Refraction (bending of light)
When sunlight travels from air (refractive index ~1.00) into water (~1.33), it bends according to Snell’s Law.- Shorter wavelengths (violet/blue) bend more sharply than longer wavelengths (red), causing dispersion—the separation of colors.
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Internal reflection
Inside the droplet, some of the light reflects off the inner surface.- For a primary rainbow, this happens once.
- For a secondary rainbow, it happens twice.
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Refraction out of the droplet
As light exits the droplet, it bends again. At this point, sunlight has been split into distinct colors and redirected at specific angles relative to the observer’s line of sight.
Because millions of droplets reflect light at the same critical angles, you see an arc of colors — the rainbow.
2) Why Is a Rainbow Curved?
The rainbow forms a circle around the antisolar point (the spot in the sky directly opposite the Sun).
- Primary rainbow: light exits droplets at an angle of ~42° for red and ~40° for violet from the antisolar point.
- Secondary rainbow: appears outside the primary arc, at angles between 51°–54°.
From the ground, the horizon blocks the lower half of the circle, so we usually see only a curved arc.
Tip: The lower the Sun is in the sky, the higher and larger the rainbow appears. From airplanes or mountaintops, you can sometimes see a full circular rainbow.
3) Order of Colors & Types of Rainbows
Primary rainbow (most common)
- Produced by one internal reflection.
- Color order (outer to inner): Red → Orange → Yellow → Green → Blue → Indigo → Violet.
- Red is always on the outer edge; violet is on the inner edge.
Secondary rainbow
- Produced by two internal reflections inside the droplet.
- Fainter and has reversed color order: Violet on the outside, red on the inside.
- The area between the primary and secondary rainbows looks darker — this is called Alexander’s band.
4) Why Does the Rainbow "Follow" You?
A rainbow isn’t a physical object — it’s an optical effect.
If you move, the set of droplets that reflect light toward your eyes changes, but the angle stays constant. That’s why it seems like you can never reach the end of a rainbow.
5) Effect of Raindrop Size
- Larger droplets (heavy rain) → brighter colors, sharper edges.
- Smaller droplets (light drizzle) → softer, blended colors.
- Very tiny droplets can cause extra faint bands just inside the main rainbow, called supernumerary rainbows — caused by light wave interference.
Special case: Fogbow (white rainbow)
- In very fine mist or fog, diffraction dominates and dispersion is weak, producing a white or pale-colored arc.
6) Conditions Needed to See a Rainbow
To spot a rainbow, four main conditions must be met:
- A light source — usually the Sun (or Moon → “moonbow”).
- Water droplets — from rain, mist, spray, or waterfalls.
- Sun behind you — the Sun must be low in the sky, ideally in the early morning or late afternoon.
- Dark background — a darker sky behind the rain enhances visibility.
7) Common Misconceptions & Related Phenomena
- “The rainbow touches the ground” → False. A rainbow forms at a specific viewing angle, not at a fixed physical location.
- Haloes & sundogs → These are caused by ice crystals in high clouds, not raindrops.
- Upside-down rainbows → These are usually circumzenithal arcs, formed by sunlight refracting through ice crystals, not true rainbows.
8) Why Rainbows Look Brighter After a Storm
After a storm:
- The eastern sky is often dark due to leftover rain clouds.
- The Sun in the west is low and shines brightly.
- This creates strong contrast between the illuminated raindrops and the dark background, producing vivid, striking rainbows.
9) Photography Tips for Capturing Rainbows
- Polarizing filters: A rainbow’s light is naturally polarized, so rotating a polarizer can enhance or reduce its brightness.
- Exposure settings: Slight underexposure makes the rainbow colors pop.
- Composition: Include foreground elements (trees, mountains, lakes) for scale and a more dramatic photo.
10) Quick Summary
- Process: Sunlight → refraction in → internal reflection → refraction out → dispersion into colors.
- Angles: Primary rainbow at ~42°, secondary at ~51°–54°.
- Colors: Primary — red outermost, violet innermost; Secondary — reversed order.
- Viewing tips: Sun behind you, rain in front, best when Sun is low.
- Special cases: Supernumerary bands, fogbows, full-circle rainbows.
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