The Physics Behind Polarisation: How do Polarising Filters Work?

If you are into photography, you may have heard of something called a polarising filter. This filter can be placed over your camera lens, and when you are taking photographs of objects with reflection (such as objects near windows), observe that the reflections disappear! Why is this so? In this article, we will explore the phenomenon known as polarisation, which is what causes the reflection in windows to seemingly disappear when you are taking photographs with the filter. 

How does light get polarised?

To better understand how light gets polarised, we first need to understand that visible light is an electromagnetic wave. As such, it is also a transverse wave. A transverse wave is a wave whose oscillating direction is perpendicular to its propagating direction. To better understand what is a transverse wave, imagine you are holding a rope that is tied to a pole. Then, move your arm up and down. The rope is actually acting as a transverse wave. Notice how the rope’s “bumps” are moving towards the pole. The movement towards the pole is known as the propagating direction, while your arm moving up and down is known as the oscillating direction. This understanding of transverse waves shall be useful in understanding how polarisation occurs! 

Of course, light is not just a singular wave going from its source to an object. In the same propagating direction, there are many different light waves that are oscillating in many different directions. This forms what we shall know as unpolarised light (Science Facts, 2020). 

An image polarisation in action (BYJU’s, 2024)

(A polariser is what causes unpolarised light to become polarised. But why?)

Polarisation comes in three types: linear, circular, and elliptical. We would focus more on linear polarisation as it is the simplest to understand. 

Linear polarisation refers to the case where the oscillating direction of light is limited to a single plane parallel to the direction of propagation of the light waves (BYJU’s, 2024). In such cases, the polariser is generally made of tiny slits in a specific direction. Referencing the image above, think of the vertical lines as tiny slits, and only light waves oscillating in the vertical direction would be able to pass through the polariser. 

When the unpolarised light passes through the polariser, light waves that are oscillating in other directions that are not in the direction of the slits of the polariser will get blocked, while light waves oscillating in that aforementioned direction will go through, thus polarised light comes out on the other side of the polariser. 

Conclusion 

In conclusion, polarising filters use a principle known as polarisation to “remove” the reflections you see in reflective surfaces such as windows. Polarising filters will block out some components of light that are oscillating in a certain direction, which then would appear to disappear when seen through your camera lens, when in reality, it is just that the light waves are not getting reflected into your lens!

Works Cited 

BYJU'S (2024). Polarization of Light - Definition, Types, Methods, & Applications. [online] BYJUS. Available at: https://byjus.com/physics/polarization-of-light/  [Accessed 3 Jul. 2025].

Science Facts. (2020). Polarization of Light: Definition, Diagram, and Applications. [online] Available at: https://www.sciencefacts.net/polarization-of-light.html [Accessed 1 Jul. 2025].