Why the Sky Is Blue (and Sunsets Red): Teaching Rayleigh Scattering with Simple Demos

 

Why the Sky Is Blue (and Sunsets Red): Teaching Rayleigh Scattering with Simple Demos

It’s one of the most common questions in physics—and one of the most beautiful. Why is the sky blue during the day but red at sunset? The answer lies in Rayleigh scattering, and it’s easy to demonstrate in the classroom with a few simple materials.

The Science

Rayleigh scattering occurs because the molecules in the atmosphere scatter shorter wavelengths of light (blue and violet) more than longer ones (red and orange). During the day, the Sun’s light passes through a shorter section of the atmosphere, so more blue light is scattered across the sky. At sunrise and sunset, sunlight travels through more air, scattering the blues away and leaving the reds and oranges.

Classroom Demonstrations

You can recreate this effect using everyday materials:

• A transparent tank of water with a few drops of milk or a small amount of washing-up liquid.

• Shine a white light through the tank.

• Observe from the side and then from the far end of the tank.

Students will see that the light appearing through the liquid looks bluish from the side (scattered light) and reddish from the far end (transmitted light). It’s a simple, safe, and memorable way to visualise how the atmosphere filters sunlight.

Why It Matters

This demonstration connects theory to direct observation. It’s not just explaining a phenomenon—it’s showing it in action. Students gain an intuitive understanding of how light interacts with matter, reinforcing concepts of wavelength, scattering, and colour perception.

The Takeaway

Rayleigh scattering transforms a simple beam of white light into one of the most familiar sights in nature. With a lamp, some water, and a drop of milk, you can bring the physics of the sky straight into the classroom.