Definition of Light Refraction
Refraction occurs naturally due to the variations in the pathways of light and has got its own rules known as the laws of refraction of light. When light changes its way from medium to another, light deflects from its original way, with certain degrees. This deflection is known as refraction of light. We are familiar with refraction in our daily activities. To illustrate it, we can dip our hand in a swimming pool, keeping the rest of the body parts outside water.. We see it absurdly bending, which is due to the light behavior of refraction. Two things are required for the bending of light viz denser medium and rarer medium.ย
If the path carrying light has loosely bound molecules, light finds its way clear and can travel faster. This path is called the rarer medium. Conversely, if the light path contains densely packed molecules compared to the first medium then the light-path is somewhat interrupted. Thus, the medium is called denser medium. For example, air molecules are loosely packed and are always a rarer medium while water molecules, in comparison with air, are more dense and act as denser medium. One thing to note is that light can also travel from denser to rarer medium.
When light coming from a rarer medium turns its path towards a denser medium, it deviates towards the normal. Similarly, when it follows the reverse path, it deviates far from the normal. The normal is the virtual perpendicular boundary between two refractive media. Refraction is the natural property of light which builds the foundation for optics. Understanding it helps to gather knowledge on the simple concept of vision to complex phenomena of telecommunication and wireless networks.
First Law of Refraction: Coplanar Rays and Normal
A process of refraction gives rise to three geometric lines called the incident ray, refracted ray and the normal. Incident ray is the initial ray before meeting another medium while refracted ray is thus formed ray after the light enters another medium with which refraction happens. The first law of refraction is based on the relationship between these three geometrical lines. It states that, โThe incident ray, the refracted ray, and the normal to the surface at the point of incidence all lie in the same plane.โ Thus they are called the coplanar rays.
Thus, the bending of light is predetermined and stays with its planar geometry. For example, if we draw the incident and refracted ray along with the normal of a laser light incident on a clear glass slab at some angle we will see all three lines lie on the same two-dimensional surface. They do not scatter in three-dimensional curves.
Second Law of Refraction (Snellโs Law)
Snellโs law gives the relationship between the angle of incidence and angle of refraction and also the refractive indices of two media. It is given as:
ฮผ1โsin i=ฮผ2โsin r
Or, sin i/โsin r = ฮผ2โ/ฮผ1 = ฮผ [Equation 1]
Where:
- ฮผ1 and ฮผ2โ are the refractive indices first and second media respectively,
- i is the angle of incidence,
- r is the angle of refraction.
Thus on taking the ratio of sine of two angles i.e. angle of incidence and angle of refraction, we get a constant value called the refractive index. This refractive index tells us how much the light will be curved by a particular medium.
Snell’s Law is taken into account while preparing lenses, cameras, and fiber optics. It tracks the path of light, measures the angle of deflection and also helps in calculating the unknown refractive index of certain media.
Deriving Snellโs Law from Wavefront Geometry
Realizing light as a wave, Snellโs law has also been derived from the concept of wavefront. A wavefront is essentially a line connecting all the points vibrating in phase.
When light waves travel at an angle from one medium to another, the whole wavefront gets bent. It may get slowed down or speed up depending upon the direction of light. This causes refraction of the wavefront.
We can show mathematically that every point on a wavefront acts as a source of secondary wavelets. Thus, applying this principle on the light wavefronts which is also called the Huygens’ Principle,
Source: https://i.sstatic.net/1eBae.png
Let two parallel rays A and B be coming from medium 1 and then entering to medium 2. After hitting the interface point L of the two media, the rays again move in the same parallel manner along directions U and V respectively.
Suppose, at time t=0, light ray A hits L at point C and B hits at point D. B travels with the same speed as that of A (let, v1=c/n1) and arrives at D in t seconds. Ray A again travels through the medium 2 at a speed v2=c/n2 and reaches the point Q.
Now geometrically, looking at the parallel lines in the figure above, we can derive our formula. Let C and D are x distance apart.
xsinโก(ฮธi) = PD
= v1t
= c/n1t
Again, xsinโก(ฮธr) = CQ
= v2t= c/n2t
Thus, n1sin(ฮธi) = c/x t [Equation i]
n2sin(ฮธr) = c/x t [Equation ii]
Rearranging [i] and [ii] we will get our Snellโs law as, n2/n1 = sin(ฮธi)/sin(ฮธr). Thus, we can show the wave nature of light from Snellโs law too.
Refractive Index: Meaning and Typical Values
The refractive index is the indication of refraction. It is a dimensionless quantity which displays the travelling capacity of light in a material compared to vacuum. It is given as:
ฮผ = c/v [Equation 2]โ
Where:
- c is the speed of light in vacuum (~3.0 ร 10โธ m/s),
- v is the speed of light in other media.
A greater refractive index value means that light is travelling very slower in that material medium and is greater deflected when entering that medium.
Some useful refractive index values are listed below.
- Vacuum: 1.000
- Air: ~1.0003
- Water: 1.33
- Glass: 1.5 โ 1.9 (depending on type)
- Diamond: 2.42
Refractive index is used to calculate the angle of deviation of the path of light which is employed in designing optical instruments like lenses, microscopes, telescopes etc. It also determines optical density which means the density of a particular light medium.
Factors Affecting Refraction: Wavelength and Medium Density
The two major things that affect the refraction of light are:
Wavelength of Light: Refraction varies with wavelength. Shorter wavelengths of light are deflected with more angles than longer wavelengths of light. This is the main cause for the dispersion of light when incident on a prism.
Optical Density of the Medium: Greater density of a medium means greater hurdles for light and hence its speed slows down, giving more deviations and vice-versa.
Other minor things influencing refraction are:
- Temperature: Greater temperature can decrease the density and hence light would travel faster with a small angle of refraction. Inverse will be with the less temperature.
- Impurities: Impurities in the media like water or glass can slow down the speed of light.
- Pressure: For gases, increasing pressure increases density and thus changes the refraction pattern.
Critical Angle and Total Internal Reflection
In the case when light passes from denser to rarer medium, if the deviation of light is so much that it gets refracted along the margin between two media and the angle thus formed is known as the critical angle.
If ฮผ1 and ฮผ2 are the refractive indices of corresponding denser and rarer medium then the critical angle ฮธcโ is given by:
sinฮธcโ= ฮผ2โ/ฮผ1 [Here ฮผ1 > ฮผ2] [Equation 3]
Again if the angle of incidence exceeds this critical angle or say the deviation is very large such that the light bounces back to its initial medium, the Total Internal Reflection (TIR) occurs.
This principle is widely employed in modern technologie like optical fibres, binoculars etc. Not only artificial but the natural phenomena of twinkling of stars and diamonds, formation of Mirage etc. are the results of TIR.
Refraction Through Prisms and Dispersion of Light
A prism is a transparent object with polished, flat surfaces that is able to refract light. A monochromatic or a white light bends as it becomes incident on a prism. Dispersion results from every color’s varying wavelength which gives different speed for each of them.
Red light, having longest wavelength, deviates least while Violet light, having smallest wavelength, deviates the most. Earth acting as a prism for the monochromatic light coming from the sun, we are able to see beautiful colors.
Dispersion through prisms helps in:
- Understanding the composition of light,
- Designing spectrometers,
- Studying atomic and molecular absorption/emission spectra.
Isaac Newton first was the first person to use the word spectrum after his dispersion demonstration using a prism in the 17th century.
Applications in Lenses, Microscopes, and Telescopes
Every optical instrument is created by the sole use of laws of refraction.
- Lenses: Convex lenses converge light rays, and concave lenses diverge them. This is crucial in correcting vision (glasses). It is also used in cameras for focusing light and creating beautiful pictures of objects.
- Microscopes: It uses various lenses to magnify the image of tiny objects. Refraction is done in a controlled manner to bend light in the desired way which gives high resolution and clarity.
- Telescopes: Refracting telescopes use convex lenses to collect and focus light from distant celestial objects. Galileoโs early telescope was based on a simple refraction pattern.
Refraction in Nature: Rainbows, Halos and Mirages
Refraction is responsible for many beautiful natural phenomena:
- Rainbows: They are created when sunlight is reflected and then refracted and finally dispersed by the rain drops. We can compare raindrops to the transparent, refracting prism.
- Halos: The upper atmosphere can contain ice crystals which refract light from the sun or moon and beautiful Halos are seen around them.
- Mirages: Due to the density variation in the layers of air, the light will enter and pass out accordingly through them. Therefore an illusion is created on the surface of hot rods or the deserts. This illusion is known as Mirage.
Measuring Refractive Index with a Refractometer
Refractometer is used for the accurate measurement of refractive index. It works by making a light incident on the sample and measuring the angle of refraction.
Types of refractometers are:
- Handheld: Portable and mostly used in food and agriculture.
- Digital: Provides more precise readings with automatic calculations.
- Abbe Refractometer: Used in laboratories for liquids, offering high precision.
Refractometers help in:
- Identifying materials,
- Testing purity of substances (e.g., sugar content in juices),
- Quality control in industries.
Refractometry is useful in both research and industry because it provides information about how a material is made by determining the deflection of light passing through it.
Common Misconceptions About the Laws of Refraction
Although being simple, some misconceptions still exist about refraction. Some of them are noted below:
- Refraction only happens with water โ Refraction occurs in all transparent or translucent media like glass, oil, ice, plastic etc.
- Refraction and Reflectionโ For people having poor knowledge on optics, refraction and reflection may sound the same. Mirror and lenses are two different objects working on the principle of reflection and refraction respectively. Both phenomena have their own laws for transmission.
- Speed of light remains same everywhere โ Speed of light decreases or increases based on the media it takes. Here the path represents the medium. Light always chooses a path which takes less time to reach the destination.
Proper knowledge in optics is required to supply quality education in physics and to perform scientific and technological activities.
Conclusion
Optics explains the interesting phenomena of light transmission. The phases like critical angle and Total Internal Reflection are the major discoveries of laws of refraction as almost all optical devices are discovered implying these phases.
Refraction is not a new technology but an already existing natural scenario. Its recognition in physics had led to a promotion in physics. From magnifying images of micro particles to minimizing the images of distant objects, this law is applied. Any difficulty in human eyesight is also cleared using correction lenses, relying on laws of refraction. Refractive index has become an important measurement tool for quality control, chemical industries, microbiology and medical science. Thus, the comprehension of this simple phenomena will upgrade optics, engineering and physics education.
References
Razek, M. H. A. (2020). Refraction of light and its applications. Ain Shams Eng. J.
Beeson, S., & Mayer, J. W. (2008). The refraction of light. In Patterns of Light: Chasing the Spectrum from Aristotle to LEDs (pp. 33-47). New York, NY: Springer New York.
Kaur, K., & Gurnani, B. (2023). Refraction of Light. In StatPearls [Internet]. StatPearls Publishing.
Osaigbovo, F. (2022). Light and the laws of reflection and refraction as they impact on photography. Yฤฑldฤฑz Journal of Art and Design, 9(1), 49-59.
Learning, L. (2021). The Law of Refraction. Fundamentals of Heat, Light & Sound.
Andrews, S. S. (2023). Refraction. In Light and Waves: A Conceptual Exploration of Physics (pp. 211-245). Cham: Springer International Publishing.
https://byjus.com/physics/refraction-of-light/
Refraction
