What is Refractive Index?

Refractive index or index of refraction is the ratio of speed of light in vacuum to its speed in a particular substance under consideration. The concept was invented in the year 1621, and is used in fiber optics, diamond industry, in manufacturing telescopes, etc. Here is more on this...
It was in 1621, when mankind was trying to unravel so many scientific truths, that Willebrord van Roijen Snell discovered the concept of refraction. Light travels at the speed of 299,792.458 km/sec in vacuum. But, what happens when it enters different medium such as air, water or glass? The theory of refraction says that light travels at different speeds in different mediums. It also explains that it bends or changes its direction when it leaves one medium such as air and enters another medium such as water. In 1637, another scientist, René Descartes, worked on this theory and came up with equations that forms the Snell's law.
The Definition of Refraction
The index of refraction or the refractive index is a measure of the extent to which a substance slows down light waves passing through it. It is equal to the ratio of velocity of light in a vacuum to its speed in that substance. Its value determines the extent to which the light is refracted (bent) when entering or leaving the substance.
Theory of Refractive Index
Reflection of light by objects enables a person to see them. However, in case of mediums such as water, glass, and air, part of the light is reflected and the other enters the medium. A lot depends on the density of the medium, because it causes the light to slow down and bend. A line perpendicular to the point where the light is striking the medium, e.g. water, is termed as 'Normal'. The angle between the light and this 'normal' is called angle of incidence.
The part of light that enters the water from air and slows down, continues its journey onwards through the water. Water is denser than air, which causes the light to bend towards the normal. Now, the angle between the normal and the light is called the angle of refraction. If the light were to exit the water and enter the air, then the light bends away from the normal, which under certain conditions causes a rainbow.
Snell's Law
A ray of light passing through medium with refractive index 'A', enters another medium with an index 'B'. The angle of incidence is 'a' and that of refraction is 'b'. With this information in hand, we can express the Snell's formula as:
A*sine(a) = B*sine(b).

This gives angle of refraction (Refractive index), i.e., sine(b) as:

sine(b) = A*sine(a)/B or b = arcsin(A*sine(a)/B).

Refractive indices at room temperature for some mediums:
• Glass (acrylic) 1.490-1.492 and crown glass1.50-1.54
• Air 1.00027712
• Water 1.33283
Rainbows
The process of a rainbow formation depends on certain conditions such as position of the sun in the sky, and the size of water molecules in the air that is between the viewer and the sun. Sunlight enters water molecules, (i.e., from vacuum to air then into the water), changes its direction, and exits the water molecules (i.e., from water to air), and again changes its direction before it reaches our eyes. The light is composed of different colors representing different wavelengths.
Angle of refraction of light also depends on the wavelength of light passing through the medium. Because of this, bending of the colors in light is not uniform. The light waves that are part of visible light spectrum bend and emerge out of the water molecules separately, giving us a half circle of distinct rainbow colors. In this case, water molecules at certain height act as prisms.
Although light has dual nature (wave as well as particle), in the field of optics it is treated as a wave. The concept of refraction is used to make periscopes, binoculars, and telescopes. In these instruments, the refractive index of the glass is used to collect the light reflected by different objects to make them appear closer. It is also used to improve the sparkle and appearance of the diamonds by cutting them at a proper angle. Transmitting TV signals through fiber optics is made possible with the help of the concept of refraction.