What is Light Refraction: Definition of Refraction and Laws of Refraction

As stated in the NCERT book:

“Light does not travel in the same direction in all media. It appears that when travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.” 

Refraction is the bending of light as it travels from one medium to another. This phenomenon occurs because light changes its speed as it travels from one medium to second medium. The side of the wave that enters another medium will travel at a different speed compared to the end of the wave that is still travelling in the first medium.

Here is a simulation that shows how light refraction causes image creation through lenses:

1. Change in Speed: In different substances, the speed of light changes. For example, light travels faster in air than in water and glass.  
2. Density: Any medium that is denser will have more particles that will interfere with the travel of light. Due to the larger number of particles, the light will slow down. A rarer medium has fewer particles, which makes the light travel faster.
3. Refractive Index: The phenomenon of refraction is also impacted by the refractive index. If the light travels from:

• • a low refractive index medium to the one which has a higher refractive index, the light will bend toward the normal, i.e. the light will slow down
  • high to low refractive index, it will bend away from the normal, i.e. the speed of light will increase

There are two fundamental laws of refraction, which are mentioned below:

1. Incident Ray, Refracted Ray and Normal: The first law of refraction states that the incident ray, the refracted ray and the normal all lie in the same plane. When light hits the boundary between 2 media, the incident ray (incoming light), the refracted ray and the normal (which is the imaginary line perpendicular to the surface) will all lie in the same plane.

2. Snell's Law: The second law describes how light bends, whereas mathematically it describes how much the light bends.

$$n_1\sin \left({\theta }_1\right)=n_2\sin \left({\theta }_2\right)$$ $n_1$ and $n_2$ denote the refractive index of the first and second medium, respectively,  ${\theta }_1$ is the angle of incidence and  ${\theta }_2$ is the angle of refraction.

Do remember that the speed of light is not the same in every medium. The speed of light is the maximum in vacuum since there are no particles that can interfere with light. However, it slows down in other media. Also, the speed of light changes with the refractive index of the material. The refractive index (n) measures how much the medium will slow down light as compared to a vacuum. $$\text{Refractive Index (}n\text{)}=\frac{\text{Speed of light in vacuum}}{\text{Speed of light in the medium}}$$

In short, the refractive index is inversely proportional to the speed of light. The higher the speed of light, the slower the speed of light will be.

The following table lists the speed of light in different media.

Let us understand the applications of refraction.

1. One of the applications of refraction is vision correction. Lenses utilize the concept of refraction for bending light rays so that lenses can focus correctly on the retina. It helps in correcting vision problems such as myopia, hyperopia and astigmatism.
2. Microscopes and telescopes are the instruments that use lenses for refracting light and magnifying tiny objects and distant celestial bodies.
3. Lenses in cameras will refract light to focus the light onto a sensor or film to create sharp images.
4. Refraction is also used for transmitting light and images within the body for minimally invasive surgeries.
5. Total internal reflection is used for optical fibres for transmitting data since light travels over long distances with minimal loss.
6. Refracting telescopes use lenses for focusing light from planets and stars. This is very helpful for astronomers.
7. Spectrometers use refraction of light into component colours to analyze the composition of a substance. 

Also known as the Fermat's principle, this is a fundamental concept in Optics that states that "light will follow the path that requires the least time when travelling between two distances". It is not necessary that the distance is also the shortest. This concept is expressed using concept of optical path length which is the product of distance travelled and refractive index of the medium.

Mathematically, the optical path will be: 

L=∫nds

Here, 

• L is the optical path length
• n is the refractive index
• ds is the infinitesimal segment of the path

When light bends, it will spend less time in a slower medium to optimize the total travel time. As you can see, whenever the light travels through a denser medium, its path becomes longer in distance but the time taken to travel will reduce.

This principle also applies to reflection since light reflects in a way that total time is minimized. This is why the angle of incidence is equal to the angle of reflection.

Source: NCERT Chapter 9, Figure 9.1

• Let us say that AB is a light ray which is falling on the surface PQ of the glass slab, $N_1$ E is the normal drawn perpendicular to the surface.
• We can see that AE is making the 'i' incident angle normal. Now ray AE is travelling from a rarer to denser medium, its speed will decrease and the ray will bend towards the normal.
• EF is the refracted ray that makes an angle of refraction with normal 'r'.
• The refracted ray travels from PQ to the SR surface. At the surface SR, we will draw another normal at point F since EF is the refracted ray incident on the SR surface.
• Let us now draw a normal ${\mathrm{N’}}_1$ ${\mathrm{N’}}_2$ at point F, we will say that the angle of incidence for this ray at surface SR is 'r'.
• As it is also evident that the path of light ray has changed. This change or shift is known as the lateral displacement of light.
• The emergent ray has shifted from the original path of the incident ray.