Index of Refraction

It was in 1621, when mankind was trying to unravel so many scientific truths that Willebrord van Roijen Snell discovered the concept of refraction. You may be aware that the light, in vacuum, travels at the speed of 299792.458 km/sec. But, what happens when the light enters different medium such as air, water or glass. Theory of refraction says that the light travels at different speeds in different mediums. It also explains that the light bends or changes its direction when it leaves one medium such as air and enters into another such as water. In 1637, another scientist René Descartes worked on the 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. The index of refraction of a substance is equal to the ratio of the velocity of light in a vacuum to its speed in that substance. Its value determines the extent to which light is refracted (bent) when entering or leaving the substance.

Theory of Index of Refraction
What enables a person to see clearly? It is the reflection of light by the objects. However, in case of some mediums such as water, glass and of course, air, a part of the light is reflected while the other part enters the medium. A lot depends on the density of the medium, because it causes the light to slow down and as well as bend. The concept of refraction becomes easy to understand if, in your mind, you form a picture of a line perpendicular (called Normal) to the point where the light is striking the medium, say water. The angle between the light and normal is called the angle of incidence. The part of light that enters the water from air and slows down, and continues its onward journey through the water. The water is dense than air which causes the light to bend toward 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 cause rainbows.

The Snell's Law
A light ray passing through a medium with a refractive index 'A', enters another with an index of 'B'. The angle of incidence is 'a' and angle 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 us angle of refraction i.e., sine(b) as: sine(b) = A*sine(a)/B or b = arcsin(A*sine(a)/B).

It is found that at Room temperature:

• Glass (acrylic) - 1.490 - 1.492 and that of crown glass - 1.50 - 1.54
• Air - 1.00027712
• Water - 1.33283

Rainbows
Do you know how a rainbow forms? It depends on certain conditions such as how high the sun is in the sky and size of the water molecules in the air that is between us and the sun. The sunlight enters the 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 it 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 a 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, the water molecules at certain height act as prisms.

Although the 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 and to make them appear closer. The concept is also used to improve the sparkle and appearance of the diamonds by cutting them at proper angle. Transmitting TV signals through the fiber optics is made possible with the help of the concept of refraction.