The Physics Of Refraction

Have you ever seen the rainbow effect that glass produces as the sun shines upon them? Or wondered how spectacles help in allowing us to see more clearly? Well, these are examples of a phenomenon known as refraction. In this article, we will explore more about what refraction is and some terminologies that are often used when talking about refraction, specifically in the context of light. 

What is refraction? 

Refraction is a phenomenon that all waves have. According to Britannica Editors (2025), refraction refers to the “change in direction of a wave passing from one medium to another”, where medium refers to the environment that the wave travels in, such as air and glass. This change in direction would be caused by the change in speed that the wave travels in the new medium. 

Let us look at an image that illustrates this phenomenon: 

Image (BYJU’S, 2019) of refraction in action

Light is a type of wave known as electromagnetic wave, so we can use light as an example to illustrate how refraction works. Now, imagine light travelling from air to a material like glass. As we have learnt earlier, when light hits the surface of the glass at an angle, the light will change the “direction” it is travelling while in glass. This is because the light ray slows down as it travels in glass, thus resulting in the change in direction (as we can see from the above image where the light ray does not travel in a completely straight line from point A to point B). 

So, why do waves change their speed when travelling in different mediums? In the case of electromagnetic waves or sound waves, this is because of density. As such, if the medium is less dense (like air), waves would be able to travel relatively faster than if they are travelling in more dense materials (like glass). 

However, there is a difference for mechanical waves such as water waves. Water waves travel by the oscillation of the water particles itself, unlike electromagnetic waves that can travel in a vacuum, or sound waves that require the use of a medium to travel. When water waves come from the deep ocean to the beach (where the water would be shallower), water waves would actually slow down despite shallow waters being less dense. This is because of the change in the depth of water (The Physics Classroom, 2023). As the depth of water decreases, the speed of water will fall as well. Conversely, as the depth of water increases, the speed of water will increase! Therefore, refraction occurs in water waves by the changes to the depth of water that the water waves are travelling. 

Some Common Terminologies…

After understanding the mechanics of refraction, we can now look deeper into some terms that are often used when talking about refraction! The following terms below would be more towards the context of light. 

Medium & Refractive Index

As seen earlier, we learnt that a medium is the environment that waves travel in. Specifically in light, the density mentioned above could also be known as optical density. In an optically dense material, light travels slower. Optical density is not necessarily mass density, as optical density refers to the amount of light that the medium allows to pass through, whereas mass density refers to the amount of mass per unit volume of an object (This vs. That, 2023).

How optical density is quantified is by using an index known as the refractive index. The higher the refractive index, the more optically dense the medium is, causing the light ray to change its direction more drastically. To find the refractive index of a medium such as glass, the following formula would be used:

where: 

• n is the refractive index 

• c is the speed of light in a vacuum 

• v is the speed of light in the medium 

By this logic, the refractive index of a vacuum would be approximately 1! 

Below is a table of the refractive indices of common mediums such as glass, water and air: 

Image (Ashish, 2018) of the refractive indices of common mediums 

Angle of Incidence, Angle of Refraction, Normal 

Image (Gallegos, 2023) showing how light rays can get refracted as it travels from water to air

We would need terms to describe the angles that the light ray makes inside the material in order to conduct further study. As seen from the image above, there are angles being drawn out where the light ray meets the boundary between the two mediums. These angles are important in order to collect data for further analysis on the concept of refraction. 

Referencing the image above, the angle 1 refers to the angle of incidence, as it is the angle that the incident ray (the incoming light ray that reaches the boundary of two mediums) makes. The angle 2 refers to the angle of refraction, as it is the angle that the refracted ray (the outgoing light ray that has passed through the boundary) makes. These angles are measured with reference to a dotted line, known as the normal. The normal refers to an ‘invisible’ line that is perpendicular to the boundary that the light ray is hitting. 

From here, these would lead to concepts such as: 

• Critical Angle: This refers to the largest angle of incidence such that the angle of refraction is at 90°, which means the refracted ray would be travelling along the surface of the boundary.

• Total Internal Reflection: This is when the angle of incidence is greater than the critical angle. When this occurs, the light ray will reflect back into the medium.  

Snell’s Law 

With the knowledge of refractive index and angles of incidence and refraction, we can now take a look at one of the laws of refraction: Snell’s law. This law of refraction would show the relationship between the angles of incidence, refraction and refractive index. This will allow scientists to know how much a light bends, and allows designers and engineers to apply the concept of refraction into objects such as fibre optics and spectacles. 

Mathematically, Snell’s law can be expressed as the following equation: 

where:

• n1 is the refractive index of the medium that the light ray was travelling from 

• i is the angle of incidence 

• n2 is the refractive index of the medium that the light ray is entering into 

• r is the angle of refraction 

Conclusion 

In conclusion, refraction is a phenomenon that affects waves such as light, where waves will slow down or speed up depending on what medium they are travelling in. Refraction is affected by the depth of water in the case of water waves, and density of a medium in the case of electromagnetic waves and sound waves. Snell’s law is an important law that informs us of how much light bends as it travels from one medium to another. With such discoveries and knowledge, we are able to enjoy various inventions that make use of this phenomenon. 

Works Cited

Ashish (2018). What Is The Index Of Refraction (Refractive Index)? [online] Science ABC. Available at: https://www.scienceabc.com/pure-sciences/what-index-of-refraction-defintion-examples-water-air-glass.html [Accessed 22 Jan. 2026].

BYJU'S (2019). What is Refraction of Light? - Definition, Refractive Index, Applications. [online] BYJUS. Available at: https://byjus.com/physics/refraction-of-light/ [Accessed 11 Jan. 2026].

Gallegos, J. and Stokkermans, T.J. (2023). Refractive Index. [online] PubMed. Available at: https://www.ncbi.nlm.nih.gov/books/NBK592413/ [Accessed 22 Jan. 2026].

The Physics Classroom (2023). Reflection, Refraction, and Diffraction. [online] Physics Classroom. Available at: https://www.physicsclassroom.com/class/waves/Lesson-3/Reflection,-Refraction,-and-Diffraction [Accessed 14 Jan. 2026].

This vs. That. (2023). Mass Density vs. Optical Density - What’s the Difference? | This vs. That. [online] Available at: https://thisvsthat.io/mass-density-vs-optical-density [Accessed 20 Jan. 2026].