What is the Reddish Colour of Sun at Sunrise/Sunset?

S4-SA1-0358

Grade Level:

Class 7

Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics

Definition

What is it?

The reddish colour of the Sun at sunrise and sunset is due to a phenomenon called Rayleigh scattering. This happens because sunlight travels a longer distance through the Earth's atmosphere when the Sun is low in the sky, causing most of the blue and violet light to scatter away, leaving more red and orange light to reach our eyes.

Simple Example

Quick Example

Imagine you are trying to see your friend across a crowded school playground. If there are many students (like air particles) between you and your friend, it's harder to see them clearly. Similarly, when sunlight travels through more air, the 'crowd' of air particles scatters away some colours, like blue, making the Sun appear reddish.

Worked Example

Step-by-Step

Let's understand how light travels through the atmosphere:

1. **Sunlight has many colours:** Sunlight is made up of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
---2. **Atmosphere is like a filter:** Earth's atmosphere contains tiny dust particles and air molecules.
---3. **Blue light scatters most:** These tiny particles scatter shorter wavelength colours (like blue and violet) much more effectively than longer wavelength colours (like red and orange).
---4. **Sun high in the sky (midday):** When the Sun is directly overhead, sunlight travels a shorter path through the atmosphere. Some blue light is scattered, making the sky blue, but enough of all colours reach us to make the Sun appear white or yellowish.
---5. **Sun low in the sky (sunrise/sunset):** When the Sun is near the horizon, sunlight has to travel a much longer path through the atmosphere.
---6. **More blue light scatters away:** Over this longer path, almost all the blue and violet light gets scattered away in different directions by the atmosphere.
---7. **Red/Orange light reaches our eyes:** The red and orange light, which scatters the least, travels mostly straight through the atmosphere and reaches our eyes. This is why the Sun appears reddish.

**Answer:** The longer path of sunlight through the atmosphere at sunrise/sunset causes more blue light to scatter away, allowing mainly red and orange light to reach us.

Why It Matters

Understanding light scattering is crucial in fields like Space Technology for designing optical instruments and remote sensing. It's also vital in Climate Change studies to understand how aerosols affect Earth's radiation balance. Careers in atmospheric science or optics rely heavily on this knowledge.

Common Mistakes

MISTAKE: Thinking the Sun itself changes colour at sunrise/sunset. | CORRECTION: The Sun's actual colour doesn't change; it's how its light interacts with Earth's atmosphere that makes it appear reddish.

MISTAKE: Believing dust particles are the only reason for the reddish colour. | CORRECTION: While dust does contribute, the primary reason is the scattering by tiny air molecules (nitrogen and oxygen) themselves, which is explained by Rayleigh scattering.

MISTAKE: Confusing the blue sky with the reddish sunset as different phenomena. | CORRECTION: Both the blue colour of the sky during the day and the reddish colour of the Sun at sunrise/sunset are caused by the same principle of Rayleigh scattering, just under different conditions of light path length.

Practice Questions

Try It Yourself

QUESTION: Why does the sky usually appear blue during the day? | ANSWER: The sky appears blue because blue light from the Sun is scattered more by the Earth's atmosphere than other colours.

QUESTION: If the Earth had no atmosphere, what colour would the Sun appear at all times? | ANSWER: The Sun would appear white at all times, and the sky would be black, because there would be no atmosphere to scatter light.

QUESTION: Imagine you are on a planet with a very thin atmosphere, much thinner than Earth's. How would the colours of sunrise and sunset compare to Earth's? | ANSWER: The sunrise and sunset colours would be less intense or perhaps less reddish, because with a thinner atmosphere, less blue light would be scattered away, meaning more of the original colours would reach the observer.

MCQ

Quick Quiz

What is the main reason for the reddish appearance of the Sun during sunrise and sunset?

The Sun emits more red light at those times.

Sunlight travels a longer path through the atmosphere, scattering away blue light.

The Earth's rotation changes the Sun's colour.

Clouds reflect red light more efficiently.

The Correct Answer Is:

Option B is correct because at sunrise/sunset, sunlight travels through more atmosphere, scattering out shorter wavelength blue light and letting longer wavelength red/orange light pass through. The Sun's emission and Earth's rotation do not change the Sun's colour.

Real World Connection

In the Real World

This concept is used by meteorologists at ISRO to predict visibility and atmospheric conditions, which is crucial for satellite launches and weather forecasting. It also helps photographers capture stunning sunset pictures by understanding how light behaves. Even traffic signals use specific colours like red for 'stop' because red light scatters less and is visible from a longer distance.

Key Vocabulary

Key Terms

Rayleigh scattering: The scattering of light by particles much smaller than the wavelength of the light.| Atmosphere: The layer of gases surrounding Earth.| Wavelength: The distance between two consecutive crests or troughs of a wave; different colours have different wavelengths.| Scatter: To spread or deflect in different directions.

What's Next

What to Learn Next

Now that you understand how light interacts with the atmosphere, you can explore concepts like 'Why is the sky blue?' or 'What causes rainbows?'. These build on your knowledge of light scattering and refraction, helping you see the world around you with new eyes!