What is Refraction?
Refraction is the phenomenon that explains the change in direction of light waves (it also deals with sound , water ,and other waves)due to changes in speed; as it travels from a optically less dense medium to more dense medium or vice versa. Refraction is the reason of why we could see magnificent rainbows. And use useful instruments like the lenses, magnifying glass, prisms, and even our own eyes which works on the basis of refraction.
Wave speed and refraction:
Light would bend towards or away from the normal line (an imaginary line drawn at 90°angle) as it travels at an angle into a substance with a varying optical density commonly known as refractive index.
This bending or change of direction of the wave occurs due to its change of speed. For example, if you put a pencil in a clear glass of water ,you’d probably see the pencil as being distorted. As if the pencil has been cut into pieces if noticed horizontally away from the glass. And the bottom part of the pencil would seem closer than it is to the surface if observed vertically above the glass. It happens because the light travelling from air to water slows down, as it enters into a optically more dense medium. As a result of which the light continues to travel into water at different angle or direction.
Amount of refraction depends on two factors:
When light propagates from air to water the direction of the light seemingly undergoes a slight change. The amount of refraction depends on the speed changes and the angle of incident ray.
- Changes in speed: a substance will cause the emergent light to refract more or less depending on the speed changes it causes to light.
- Angle of incident ray: the light entering the medium at a greater angle will noticeably refract more. On the contrary, if the light enters the medium through the normal line (at 90° to the surface) it would certainly slow down but no change in direction could possibly occur.
(The normal line is an imaginary line drawn as a dotted line at 90° to the surface of the two substances as showed in the first diagram)
When light travels from a optically less dense medium to a more dense medium (such as air to water), the speed of the light wave decreases and refracts towards the normal. And if the light enters a medium that is optically less dense from a optically more dense medium (such as water to air) , the light bends away from the normal as the light wave leaving the denser medium speeds up.
Refractive index is the optical density of a medium. When waves meet a boundary between two materials, some of it is reflected and some are refracted. The transmitted wave changes speed as a result of which the direction might also change. The refractive index between two materials 1 and 2 is:
Speed of wave in material 1 V 1
1u 2 = =
Speed of wave in material 2. V 2
Sine of angle in material 1. Sin i
1 u 2 = =
Sine of angle in material 2. Sin r
Some of the refractive index of some common substances are given below:
So a higher refractive index would mean the speed of the light would decrease.And therefore, the change in direction would be greater as it enters the medium.
Refraction and other examples:
It is a phenomenon that applies to many of the things we use in our lives. The most common example could be our own eyes. Without refraction, it wouldn’t be possible for us to focus light on to our retina. Thus we wouldn’t see clearly or we could barely see anything. Some more examples where it applies, are discussed below:
Lenses: lenses are curved blocks either made of glass or plastic. There are two types of lenses:
- Convex lens
- Concave lens
A biconvex lens is the one that is used for magnifying glass; that’s thicker in the middle part than it is at the edges. The parallel rays of light enter the lens and then focused into a imaginary focal point which is the reason it’s called converging lens. Light rays entering a convex lens refracts inwards as it enters the converging lens and bends inwards again as it leaves. All these refractions cause the parallel light rays to disperse propagating directly away from the imaginary focal point.
A biconcave lens is thinner at the middle and thick at its edges and it’s also known as diverging lens. As light rays enter the lens it bends away outward and does the same again while it leaves .All these refractions cause the light rays to scatter, travelling away from the focal point.
Prismatic refraction: It was Sir Isaac Newton who experimented with a triangular glass block called prism and sunlight to find the nature of light. The result of which creates a spectrum of colors. When the white light enters the prism; each of the colors in the spectrum refracts at a slightly different angle cause of which all the different colors that constitutes the white light could be seen.
Newton’s experiment manifested the colors that make up the white light are actually all the same colors that a rainbow is made up of. The colors of the spectrum are red, orange, yellow, green, blue, indigo , violet. He also explained how none of the colors can be turned into other colors. But all of those seven colors could be recombined to give out white light once again.
Rainbows and refraction: How rainbows are formed? When the light shines through each water droplet, the different colors bends at slightly different angles due to their varying wavelengths. All of the colors reflect off the inside of the water droplets before refracting again as they leave.; the shorter the wavelength, the more they refract. So it would be more decipherable now of how rainbows are formed.
The dispersion of white light into seven different colors explain that light is made of waves. The slight difference between the refractive index of colors might be enough for shorter wavelengths of light to be refracted more. For example, if red light has a slightly higher refractive index than violet light ,it would refract at a slightly greater angle. As the violet light slows down even more than red light.
Sometimes, you might notice a secondary rainbow, which is the result of each ray of light reflecting twice inside of each droplet before it leaves the droplet. The consequence of the second reflection causes the colors on the secondary rainbow to be reversed. For instance, red is the first color on the primary rainbow, but the last on the secondary rainbow.