Ever wondered why a coin placed in a cup made of glass looks bigger than its actual size? This effect is caused by a phenomenon of light known as the refraction of light. Like the reflection of light occurs daily to provide us vision, refraction also occurs daily. From the twinkling of distant stars to the concept behind magnifying glass, everything is caused by the refraction of light.
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“Refraction of the self-image reveals the shades encumbered inside a person, just like the refraction of light gives 7 shades.”
Refraction Versus Reflection
It is noticed that scholars get confused when asked the difference between reflection and refraction. Since they sound so much the same, it is possible to mix things up. Look at the table between reflection and reflection to grasp the difference between the two.
Do at Home: Take a beaker (glass cup) and fill it with water. Place a pencil/pen inside the beaker and observe that the pencil seems broken when dipped in a beaker. This is the outcome of the refraction of light.
Many such instances related to refraction might be seen in our surroundings. Let us now learn the definition and Physics laws related to the refraction of light.
Refraction: Definition and Laws
When light travels, it can pass from all transparent media. It is reflected from opaque or shiny surfaces. When the light goes from air to water, its speed changes. This is because different materials have different compositions. Now when a light ray passes from one medium to another it bends due to the change of speed in different mediums. This bending of light is termed refraction. For understanding refraction, first, understand the following terms:
Denser Medium: A medium is said to be denser if its refractive index is greater than the refractive index of the medium from where the light is coming.
Lighter Medium: A lighter medium has its refractive index less than the refractive index of the medium from where the light is traveling.
Refractive Index: It is defined as the ratio of the speed of light in a vacuum to the speed of light in any other medium like water, glass, etc. This is formulated as i = c/v. (where c = velocity of light in space and ‘v’ = velocity of light in other mediums) For example, the refractive index of air is approximately 1. This is the deciding factor for the speed of light in any medium.
From the figure, you can see how the incident ray from air or space bends in a transparent substance. This is the depiction of the refraction of light.
Statement 1: The incident ray, refracted ray, and the normal from the point of incidence all are coplanar. This means that they all lie on the same plane.
Statement 2: The ratio of the incidence angle (see angle ‘i’ in the figure) and the angle of refraction (see angle ‘r’ in the figure) is always a constant term.
Sin iSin r= k
The constant ‘k’ depends on the wavelength of the incidence ray and the 2 materials from which the ray is passing.
Curiously Enough: The second statement from the reflection laws is also known as Snell’s Law.
When a light beam is an incident on the interface of two substances, its frequency remains constant. Light, on the other hand, travels at varying velocities in different materials. As a result, the wavelength (λ) of light at the interface varies. This, in turn, causes a shift in direction, or refraction.
There are primarily two types of refraction in nature, they are as follows:
Refraction from lighter (rarer) to denser medium: This type of refraction occurs when the light travels from a substance having a less refractive index to a medium having a larger refractive index. In such cases, the light ray is bent towards the normal.
Refraction from denser to lighter (rarer) medium: In this type of refraction the light ray passes from a substance having a high refractive index to a medium having a less refractive index. In this refraction, the ray of light is bent away from the normal.
Effects of Refraction
Refraction results in several effects like the enlargement of objects when viewed from lenses, the shifting of the real depth to an apparent depth, atmospheric refraction, and much more. Let us study them one by one:
A normal shift occurs when an object is submerged in a liquid with a refractive index greater than the air.
- The object inside the liquid transmits light waves that reach our eyes. These light waves move straight until they interface between the liquid and air.
- On passing from the interface, they experience refraction and are bent away from the normal (because the light ray is going from a denser medium to a rarer medium).
- The refracted rays reach our eyes. When the path of the light rays is traced back, all the light rays meet at a point that is above the actual object.
- Thus we see the apparent position of the object instead of the real position. This is termed the normal shift.
The same effect is seen in breaking a pencil when placed in a beaker. We see the apparent image of the pencil, which is above the actual pencil, giving the vision that the pencil is broken.
There are two cases of normal shift:
Case 1: Observer in rarer medium, an object in a denser medium
When the light rays travel from denser to rarer medium the image of the object appears above the actual object. This implies that the real depth is greater than the apparent depth of the object.
Case 2: Observer in a denser medium, an object in rarer medium
In this case, the image of the objects is seen below the actual object. The real depth is less than the apparent depth of the object.
Another very important effect of refraction is atmospheric refraction. This is caused by the bending of light from different layers of the atmosphere. Listed below are some important phenomena which occur due to atmospheric refraction.
1) Objects behind a fire seems in a flickering motion
An object kept behind a fire, or hot surface seems to flicker when viewed through the air. Above the hot surface, the air gets lighter and rises. Cool air resides in the top space. The refractive index of cool air is greater than that of hot air. Because of the fluctuating refractive index of the medium, light rays seem to come from several directions. The apparent position fluctuates as a result.
2) Stars and planets on the horizon looks higher than usual
Because of atmospheric refraction, stars seem slightly higher than their true position when seen from closer to the horizon. The refractive index of the Earth’s atmosphere grows as we move from the sky to Earth. As a result, light from a star near the horizon bends towards the normal as it moves from a rarer to a denser medium resulting in the star seeming higher than the actual position.
3) Twinkling of Stars
The twinkling of stars is the result of two effects of nature. The first effect is the intensity that reaches from the star to our eyes. If the amount of light from the star is less, we see less brightness. On the other hand, if the amount of light is more, we see brightness in the star.
The second effect is atmospheric refraction. The light from the stars bent multiple times before reaching our eyes. Due to changing atmospheric indices, we see the star’s apparent position, which creates the twinkling effect.
Refraction From a Prism
Refraction through a prism is a very important concept related to the refraction of light. A prism is a pyramidal-shaped glass that is encumbered by three refracting surfaces. The light on entering a prism is refracted toward the normal. It travels inside the prism until it escapes out through refraction again. This time the ray is refracted away from the normal. See the figure below:
i is the angle of incidence
r1 is the refracting angle when light travels into the prism
r2 is the refracting angle when the light escapes from the prism
e is known as the emergent angle
δ is termed as the angle of deviation
A is defined as the angle of the prism
- When refraction happens in a prism, it is observed that A = r1 + r2 and i + e = δ + A.
- The refractive index of surface AC = sin isin r1 and the refractive index of surface AB = sin r1sin e.
- The scattering of light into its 7 constituent colours is a result of refraction from the prism. This was first recognised by Sir Issac Newton.
The formation of rainbows is the practical application of refraction through prisms. The water droplets after a rain act like prisms and refract the sunlight to form rainbows.