The sky appears blue due to Rayleigh Scattering, a phenomenon where short-wavelength light (blue and violet) is scattered more than longer-wavelength light (red and yellow) by the tiny gas molecules in Earth's atmosphere. While violet light scatters the most, our eyes are more sensitive to blue, making the sky appear blue. At sunrise and sunset, the sky turns red or orange because sunlight passes through more atmosphere, scattering blue light out of view and leaving behind the longer wavelengths

Understanding Light and Its Properties

To understand why the sky appears blue, we need to first understand light. The light we see from the Sun, known as white light, is actually made up of different colours. These colours can be seen in a rainbow or when light passes through a prism.

The visible spectrum of light consists of:
🔴 Red
🟠 Orange
🟡 Yellow
🟢 Green
🔵 Blue
🟣 Indigo
💜 Violet

Each of these colours has a different wavelength:

• Red light has the longest wavelength (around 700 nm).
• Blue and violet light have much shorter wavelengths (around 400–450 nm).

These wavelengths determine how light interacts with particles in the Earth’s atmosphere.

Fundamentals of Light: Explaining the Nature of Light Waves

Light is a transverse, electromagnetic wave that is seen by the typical human eye. An experiment on diffraction and interference first illustrated the wave nature of light. Light can travel through a vacuum. A light wave is said to be transverse because all its constituent parts oscillate along paths that are perpendicular to the direction of its propagation. There are two main types of waves - 

• Electromagnetic waves 

When a wave is caused due to vibration in electric and magnetic fields and does not need any medium to travel, then it is called an electromagnetic wave.

• Mechanical waves

The oscillation of matter that is responsible for transferring energy through a medium is called a mechanical wave. There are two types of mechanical waves - 

• Transverse wave - eg. light

• Longitudinal wave - eg. sound

The Role of Earth’s Atmosphere

The Earth’s atmosphere is made up of gases like nitrogen (78%) and oxygen (21%), along with small amounts of argon, carbon dioxide, and water vapour. When sunlight enters our atmosphere, it doesn’t pass straight through—it collides with these gas molecules.

These collisions cause the light to scatter in different directions. However, not all colours scatter equally—shorter wavelengths (blue and violet) scatter much more than longer wavelengths (red and yellow). This brings us to a key scientific concept:

Rayleigh Scattering: The Science Behind the Blue Sky

In 1871, Lord Rayleigh, a British Physicist, published two papers on the colour and polarisation of skylight to evaluate Tyndall’s effect in water droplets to assess volumes of small particles and refractive indices. He noticed the scattering of light by the particles present in the atmosphere. Hence, the phenomenon is known as Rayleigh Scattering. As per the Rayleigh Scattering Law, the amount of scattering of the light is inversely proportional to the fourth power of the wavelength.

It was observed that the shorter wavelengths scatter more light. Hence, blue length, which has a shorter wavelength than red light, scatters more. 

Basics of Light and Colour Perception

Isaac Newton was the first to observe that colour isn’t inherent to objects, rather, the object’s surface reflects some colours and absorbs all the others. The human eye only perceives the reflected colours. The human eye and brain translate light into colour. Our eyes have light receptors that transmit messages to the brain, producing the familiar sensations of colour. Light travels into the retina, which has millions of light-receptive cells called rods and cones. The combined response to different signals for each colour makes it possible to distinguish millions of colours.

 Colour Perception: How We See Different Colours in the Sky

Sunlight is composed of visible colours that are categorised as - blue, green, red, yellow, orange, violet and indigo. The mixture of all these colours is called white light. When white light strikes a white object it appears white to use as it reflects all colours equally. When it strikes a coloured object, a coloured light is reflected. Black colour equally distributes all light, hence we only see black when light is reflected on it. When light is moving through the atmosphere, some of the waves are scattered, which gives the sky its colour.

What is Rayleigh Scattering?

Rayleigh is elastic scattering from small [articles such as atoms and molecules. This results in uniformly scattered radiation in all directions. The Rayleigh scattering wavelength is dependent on shorter wavelengths in the atmosphere to be more scattered. The Rayleigh scattering from molecules gives the sky its blue colour. When the blue light from the sun strikes the upper atmosphere it is scattered approximately 10 times more than red light. Hence, the blue light is scattered into the human eye while the red light goes largely unscattered and back out into space.

Factors Influencing Sky Color

The scattering of molecules affects the colour of light coming from the sky. The light’s wavelength and size of the particle determine the colour. For instance, short-wavelength blue and violet are much more scattered by molecules in the air than other colours of the spectrum. Since the human eyes cannot see the colour violet very well, the sky appears blue. 

The effects of Rayleigh scattering can be seen at sunrise and sunset too. During these periods, the sunlight passes through more air than at any time during the day, when the sky is higher in the sky. Since red has the longest wavelength of any visible light, the is red when it is on the horizon.

Applications and Implications of Rayleigh Scattering in Science

Rayleigh scattering affects the polarisation of light waves. When light is scattered at a certain angle, it becomes partially polarised because the electric field oscillates in a specific plane. Rayleigh scattering impacts the transmission of radio waves which can cause signal loss and interference. Its relevance is observed in wireless communication systems such as mobile networks and satellite signals. Other areas where the effects of Rayleigh scattering can be observed are - 

• Astronomy

• Medical imaging

• Meteorology

• Climate change

• UV radiation

• Optical phenomena in gems and minerals

• Photography

• Atmospheric research

• Relation to Sunsets on Other Planets

• Lighter skin and hair colour

The application of Rayleigh scattering can be found in many science fields. It helps in shaping our perception of colours and their occurrence in the world. CV Raman further researched the light scattering phenomena to uncover other facets of the physical world. He built his study and research upon the works of Lord Rayleigh in this field.

Why Does the Sky Change Colour?

The sky is not always the same shade of blue—it changes depending on the time of day, weather, and atmospheric conditions.

1. Sunrise and Sunset: Why Is the Sky Red or Orange?

During sunrise and sunset, the Sun is lower in the sky, meaning its light must pass through more atmosphere before reaching our eyes. As a result:

• Blue and violet light are scattered out of our line of sight because they have shorter wavelengths.
• Only longer wavelengths like red, orange, and yellow remain, which is why the sky appears red or orange.

2. Cloudy vs. Clear Skies: Why Do Clouds Appear White?

Clouds consist of larger water droplets, which scatter all colours equally instead of favouring blue. This is called Mie Scattering, and it makes clouds appear white or grey, depending on their thickness and the amount of light passing through them.

3. Pollution and Dust Effects: Why Does the Sky Look Hazy?

In areas with high pollution or dust, the sky often looks pale or whitish. This happens because pollution particles cause more scattering of all wavelengths, reducing the intensity of blue light.

Why Does the Sky Look Different on Other Planets?

Not all planets have blue skies! The colour of a planet’s sky depends on its atmosphere’s composition and how it scatters light.

Mars: The sky appears reddish-orange due to fine dust particles in the atmosphere, which scatter red light instead of blue.
Venus: A thick carbon dioxide atmosphere with sulfuric acid clouds gives it a yellowish-brown sky.
Saturn’s Moon Titan: Its dense, nitrogen-rich atmosphere scatters sunlight in a way that makes the sky look hazy orange.

This shows that Earth’s unique atmosphere and the way it scatters light are responsible for our beautiful blue sky.

It all comes down to Rayleigh Scattering—the way shorter-wavelength light (blue) is scattered in all directions by gas molecules in the atmosphere. While blue and violet light scatter the most, our eyes perceive the sky as blue because we are more sensitive to blue light.

Next time you look up at the sky, remember the fascinating science behind its colour! From blue daytime skies to fiery sunsets, light scattering plays a crucial role in shaping our daily view of the sky.