What causes the 'green flash' at sunset?

In one line

The rule

The green flash is an optical phenomenon that occurs because Earth's atmosphere bends (refracts) sunlight, splitting it into different colours, and briefly lets green light be seen just as the Sun sets or rises.

There is no single formula, but the effect depends on atmospheric refraction and dispersion (splitting of light into colours).

Let's understand it simply

Imagine you are watching the sunset at the beach. As the Sun is about to disappear, for a second, you might see a tiny green spot on top of the Sun. This is called the green flash. It happens because the air around Earth acts like a giant glass lens. Just like a prism splits sunlight into a rainbow, the atmosphere bends and spreads out the Sun's light. The green part gets bent just enough to be seen for a moment, right before the Sun goes down. It's like seeing a magic trick with colours in the sky!

How it works

1. As the Sun sets, its light passes through a thick layer of Earth's atmosphere.
2. The atmosphere bends (refracts) the sunlight, just like a glass of water bends a straw.
3. Sunlight is made of many colours. Each colour bends a little differently. Blue bends the most, then green, then yellow, then red.
4. Most blue light is scattered away (this is why the sky is blue), so we don't see it.
5. The green light is the next most bent. For a second, just before the Sun disappears, the green light is the last colour we can see at the top edge of the Sun.
6. This flash of green lasts only 1–2 seconds, and you need a clear, flat horizon (like the sea) to see it.

The story behind it

The green flash has been noticed by sailors and sky-watchers for centuries. It was first described in scientific detail in the 19th century. Jules Verne, a famous French writer, even wrote a novel called "The Green Ray" about people searching for this rare event. Scientists studied it to understand how Earth's atmosphere bends and splits light, helping us learn more about optics (the science of light).

Where you see it in real life

• At the beach in Goa: Watching the Sun set over the sea, you might spot the green flash if the sky is clear.
• On a ship: Sailors often see it because the horizon is flat and there is little pollution.
• On a mountain: High places with a clear view of the horizon can also show the green flash.
• During sunrise: The same effect can happen as the Sun rises, but you need to get up early!
• In movies or books: The green flash is sometimes shown as a magical or mysterious event.
• Rarely in cities: Pollution, buildings, and clouds usually block the view, making it hard to see.

Why it matters

• Shows how Earth's atmosphere bends and splits light, helping us understand optics.
• Helps scientists study the air and weather, since the flash changes with air conditions.
• Reminds us that nature has many hidden wonders if we look closely.
• Used in teaching about refraction, dispersion, and the colours of light in science classes.

Common mistakes

• Thinking the Sun itself turns green: Only the edge appears green for a moment, not the whole Sun.
• Believing it is always visible: You need a clear, flat horizon and clean air; it is rare in cities.
• Mixing it up with auroras (northern lights): The green flash is not related to auroras, which are caused by particles from space.
• Expecting a big, long flash: It is a small, quick spot—blink and you might miss it!

Remember it

The green flash is Earth's atmosphere acting like a prism, showing us a quick green goodbye from the Sun.