Mirage and looming are optical illusions which are a result of refraction of light.
A swimming pool always looks shallower than it really is because the light coming from the bottom of the pool bends at the surface due to refraction of light.
Formation of a rainbow is an example of refraction as the sun rays bend through the raindrops resulting in the rainbow.
When white light passes through a prism it is split into its component colours – red, orange, yellow, green, blue and violet due to refraction of light.
Applications of Refraction of Light
Refraction has many applications in optics and technology. A few of the prominent applications are listed below:
A lens uses refraction to form an image of an object for various purposes, such as magnification.
Spectacles worn by people with defective vision use the principle of refraction.
Refraction is used in peepholes of house doors, cameras, movie projectors and telescopes.
The refractive index, also called the index of refraction, describes how fast light travels through the material.
The refractive Index is dimensionless. For a given material, the refractive index is the ratio between the speed of light in a vacuum (c) and the speed of light in the medium (v). If the refractive index for a medium is represented by n, then it is given by the following formula:
n=c/v=cv
Based on the refractive index of the medium, the light ray changes its direction, or it bends at the junction separating the two media. If the light ray travels from a medium to another of a higher refractive index, it bends towards the normal, else it bends away from the normal.
A light ray refracts whenever it travels at an angle into a medium of different refractive index. This change in speed results in a change in direction. As an example, consider air travelling into water. The speed of light decreases as it continues to travel at a different angle.
Refraction of light in glass is shown in the figure above. When light travels from air into glass, the light slows down and changes direction slightly. When light travels from a less dense substance to a denser substance, the refracted light bends more towards the normal line. If the light wave approaches the boundary in a direction that is perpendicular to it, the light ray doesn’t refract in spite of the change in speed.
Refraction is the bending of a wave when it passes from one medium to another. The bending is caused due to the differences in density between the two substances.
Defining Refraction
“Refraction is the change in the direction of a wave passing from one medium to another.”
Refraction of light is one of the most commonly observed phenomena, but other waves like sound waves and water waves also experience refraction. Refraction makes it possible for us to have optical instruments such as magnifying glasses, lenses and prisms. It is also because of the refraction of light that we are able to focus light on our retina.
Why do stars twinkle? Did you know that the twinkling effect of stars is due to atmospheric refraction? The starlight undergoes several refractions while reaching the Earth. This atmospheric refraction occurs in a medium of gradually changing refractive index.
A rainbow is caused because each colour refracts at slightly different angles as it enters, reflects off the inside and then leaves each tiny drop of rain.
Rainbow
A rainbow is formed when light enters each water droplet, and the different colours bend (refract) at slightly different angles. They reflect off the inside of the drop before refracting again as they leave. The shorter wavelengths refract more.
A rainbow is easy to create using a spray bottle and the sunshine. The centre of the circle of the rainbow will always be the shadow of your head on the ground.
The secondary rainbow that can sometimes be seen is caused by each ray of light reflecting twice on the inside of each droplet before it leaves. This second reflection causes the colours on the secondary rainbow to be reversed. Red is at the top for the primary rainbow, but in the secondary rainbow, red is at the bottom.
Isaac Newton performed a famous experiment using a triangular block of glass called a prism. He used sunlight shining in through his window to create a spectrum of colours on the opposite side of his room.
This experiment showed that white light is actually made of all the colours of the rainbow. These seven colours are remembered by the acronym ROY G BIV – red, orange, yellow, green, blue, indigo and violet.
Prism
When white light shines through a prism, each colour refracts at a slightly different angle. Violet light refracts slightly more than red light. A prism can be used to show the seven colours of the spectrum that make up white light.
Newton showed that each of these colours cannot be turned into other colours. He also showed that they can be recombined to make white light again.
The explanation for the colours separating out is that the light is made of waves. Red light has a longer wavelength than violet light. The refractive index for red light in glass is slightly different than for violet light. Violet light slows down even more than red light, so it is refracted at a slightly greater angle.
The refractive index of red light in glass is 1.513. The refractive index of violet light is 1.532. This slight difference is enough for the shorter wavelengths of light to be refracted more.
Light refracts whenever it travels at an angle into a substance with a different refractive index (optical density).
This change of direction is caused by a change in speed. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction.
How much does light bend?
Refraction of light in water
When light travels from air into water, it slows down, causing it to change direction slightly. This change of direction is called refraction. When light enters a more dense substance (higher refractive index), it ‘bends’ more towards the normal line.
The amount of bending depends on two things:
Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more.
Angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.
Refractive index of some transparent substances
Substance
Refractive index
Speed of light in substance (x 1,000,000 m/s)
Angle of refraction if incident ray enters substance at 20º
Air
1.00
300
20
Water
1.33
226
14.9
Glass
1.5
200
13.2
Diamond
2.4
125
8.2
All angles are measured from an imaginary line drawn at 90° to the surface of the two substances This line is drawn as a dotted line and is called the normal.
If light enters any substance with a higher refractive index (such as from air into glass) it slows down. The light bends towards the normal line.
If light travels enters into a substance with a lower refractive index (such as from water into air) it speeds up. The light bends away from the normal line.
A higher refractive index shows that light will slow down and change direction more as it enters the substance.
Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another.
This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.
The principle of total internal reflection is the basis for fiber optic light transmission that makes possible medical procedures such as endoscopy, telephone voice transmissions encoded as light pulses, and devices such as fiber optic illuminators that are widely used in microscopy and other tasks requiring precision lighting effects. The prisms employed in binoculars and in single-lens reflex cameras also utilize total internal reflection to direct images through several 90-degree angles and into the user’s eye. In the case of fiber optic transmission, light entering one end of the fiber is reflected internally numerous times from the wall of the fiber as it zigzags toward the other end, with none of the light escaping through the thin fiber walls. This method of “piping” light can be maintained for long distances and with numerous turns along the path of the fiber.
Total internal reflection is only possible under certain conditions. The light is required to travel in a medium that has relatively high refractive index, and this value must be higher than that of the surrounding medium. Water, glass, and many plastics are therefore suitable for use when they are surrounded by air. If the materials are chosen appropriately, reflections of the light inside the fiber or light pipe will occur at a shallow angle to the inner surface (see Figure 7), and all light will be totally contained within the pipe until it exits at the far end. At the entrance to the optic fiber, however, the light must strike the end at a high incidence angle in order to travel across the boundary and into the fiber.
The principles of reflection are exploited to great benefit in many optical instruments and devices, and this often includes the application of various mechanisms to reduce reflections from surfaces that take part in image formation. The concept behind antireflection technology is to control the light used in an optical device in such a manner that the light rays reflect from surfaces where it is intended and beneficial, and do not reflect away from surfaces where this would have a deleterious effect on the image being observed. One of the most significant advances made in modern lens design, whether for microscopes, cameras, or other optical devices, is the improvement in antireflection coating technology.
