The Origin of Rayleigh Scattering-Why the Sky Appears Blue

1. What Is Rayleigh Scattering?

Rayleigh scattering is a physical phenomenon that describes how light interacts with very small particles.

It was first explained in the 19th century by British physicist John William Strutt (Lord Rayleigh). His work showed that when light passes

through a medium containing particles much smaller than the wavelength of light—such as gas molecules in the atmosphere—the light is

scattered rather than traveling in a straight line.

This type of scattering is especially important in understanding how sunlight behaves in Earth’s atmosphere.

2. How Sunlight Interacts with the Atmosphere

Sunlight reaching Earth appears white, but it is actually a mixture of many different colors, each with a different wavelength.

When sunlight enters the atmosphere, it encounters countless tiny gas molecules, primarily nitrogen and oxygen. These molecules are far smaller than visible light wavelengths, which makes Rayleigh scattering the dominant effect.

A key characteristic of Rayleigh scattering is that shorter wavelengths scatter much more strongly than longer wavelengths.

In simple terms:

• Blue light has a short wavelength

• Red light has a longer wavelength

• Short wavelengths are scattered far more efficiently

3. Why the Sky Looks Blue

Because blue light is scattered strongly in all directions, it spreads across the entire sky.

No matter where you look during the daytime, scattered blue light is reaching your eyes from every direction.

Longer wavelengths, such as red and orange light, pass through the atmosphere more directly and are less scattered.

As a result:

• The sky appears blue during the day

• The sun itself appears slightly yellowish rather than pure white

4. Why the Sky Is Not Purple

Although violet light has an even shorter wavelength than blue light, the sky does not appear violet for several reasons:

Together, these factors make blue the dominant color we perceive.

• Sunlight contains less violet light than blue light
• Human vision is less sensitive to violet wavelengths
• Some violet light is absorbed in the upper atmosphere

5. Why Sunsets and Sunrises Appear Red

When the sun is low on the horizon, sunlight travels through a much longer path in the atmosphere.

During this long journey:

• Most blue and green light is scattered away
• Red and orange wavelengths remain and reach the observer

This is why sunsets and sunrises often display warm red, orange, and golden tones.