You’ve probably heard folks talk about Einstein’s theory of speed of light and how it never changes. Most people overlook the fact that this is only true in a vacuum. However, because of the interactions of individual photons with the molecules in a substance, light flows more slowly through a more familiar medium like air. The slower the light travels, the more optically thick the medium is. So, what happens to light as it transitions from one medium to the next? The answer of this question is refraction of light.
What is refraction of light?
Refraction is the change in direction of a wave as it travels from one medium to another, as the result, its speed shift. Deep water waves travel faster than shallow water waves. When an ocean wave approaches a beach obliquely, the part of the wave that is farther away from the beach moves faster than the part that is closer to the beach, causing the wave to swing around until it is perpendicular to the coast. Sound waves travel faster in warm air than in cold air. At night, the air at the surface of a lake cools, and any sound that travels upward is refracted down by the warmer upper layers of air. As a result, sounds such as voices and music may travel far further across water at night than they can during the day.
Because of the difference in speed, the electromagnetic waves that make up light are refracted when they pass the boundary from one transparent medium to another. When a straight stick is partially submerged in water and viewed from an angle other than 90 degrees to the surface, it seems bent. This defines the refraction of light in water.
In moving from air to glass, a ray of light of one wavelength is refracted, by an amount that depends on its speed in air and glass, with the two speeds depending on the wavelength. A ray of sunshine is made up of various wavelengths that seem colorless when combined. The varying refractions of the various wavelengths spread them apart like a rainbow when they enter a glass prism. Thus the bending of light (also sound, water, and other waves) when it flows through one transparent substance into another is known as refraction of light.
Lenses, magnifying glasses, prisms, and rainbows are all possible because of this bending through refraction. Even our eyes are reliant on light bending. We wouldn’t be able to concentrate light onto our retina without refraction. The bending of a wave when it enters a medium with a different speed makes this phenomenon possible. When light flows through a fast medium into a slow (denser) medium, it bends toward the normal to the boundary between the two media. Snell’s Law quantifies the amount of bending, which is determined by the indices of refraction of the two media. Huygens ‘Principle can also be used to depict the bending.
What is index of refraction?
The speed of light in vacuum is divided by the speed of light in the medium to calculate the index of refraction. It is also named as index of refraction. It measures the bending of light ray when it enters another medium. If i is the angle of incidence of a ray in vacuum and r is the angle of refraction then refractive index (n) is defined as ratio of sine of the angle of incidence to the sine of the angle of refraction.
n = Sin i / Sin r
This is called Snell’s law. Index of refraction formula is also given as:
n = c / v
Here c is the speed of light in vacuum and v indicates the velocity of ray from certain medium. Refractive index is dimensionless quantity as it is the ratio of same terms. It is a property of a material that varies from material to material. The refractive index of air is 1. When x-rays (with refractive index greater than 1) enter a glass piece from air then, they will bend away from normal unlike light, which will bend towards normal. So the velocity of x-rays is greater in glass than empty space.
How refraction works?
Light gets refracted when it travels into a substance with different refractive index, at certain angle. The change in direction of light is due to the change in its speed. If light enters from air to water then it will slow down and will change its direction. The bending of light depends upon 2 factors: change in speed and angle of incident ray. If a substance causes light to slow down or speed up more then light will bent more. Secondly, if light is entering a medium at greater angle then amount of bending will be more noticeable. Now another point that when light will bend toward normal and when it will move away!
- If light enters a denser medium (higher refractive index) then it will slow down and will refract towards the normal line.
- If light enters a rarer medium from denser medium, then it will speed up and will move away from the normal line.
Refraction can also cause spectrum. For example, when white light passes through a prism, it splits into its 7 colors with red color of maximum wavelength and violet with minimum wavelength. Red light has a refractive index of 1.513 in glass. Violet light has a refractive value of 1.532. The shorter wavelengths of light are refracted more due to this minor variation.
How rainbow is made?
Rainbow pattern is caused due to refraction of light. When light enters a water droplet, gets reflected off inside and then escapes out of the tiny water droplet, rainbow is formed. Each ray of light reflects twice on the inside of each droplet before it leaves, resulting in the secondary rainbow that can occasionally be seen. The colors on the secondary rainbow are inverted as a result of the second reflection. The primary rainbow has red at the top, whereas the secondary rainbow has red at the bottom.
When the white light from sun hits the water droplets at certain angle then it causes rainbow. The angle at which light strikes the droplet is important because it decides whether the rainbow will appear or not. Usually rainbow is visible at dawn or late afternoon, because at this time sun is low in sky and provides an ideal angle for rainbow. During the interaction of light with water droplet, light is reflected back. The other portion of light enters the drop and slows down, because water is denser than air. The reduction of speed causes light to get bent (refraction). That’s why rainbow is always curved towards the normal line. When this portion of light will escape out the droplet, it will disperse into various wavelengths (dispersion) as during moving from denser to rarer medium, lights bends away from normal. At water to air interface, if angle of incidence will be greater than critical angle then total internal reflection will occur. In other case, there will be no rainbow.