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.
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.
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.
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
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.
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.
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.
