October 25, 2021

I, Science

The science magazine of Imperial College

It's the physical processes of refraction, reflection and constructive interference that create the explosion of colour in this giant bubble ...


Framed against the Dubai skyline, it’s the physical processes of refraction, reflection and constructive interference that create the explosion of colour in this giant bubble.

Scientists once believed that we emit light beams from our eyes, allowing us to see. However, in the 11th Century, the Arabian scholar Abu Ali al-Hassan Ibn Al-Haytham used experiments to disprove this emission theory and show that light rays actually enter our eyes. His many experiments on light often involved refraction and reflection.

Similar to how we see the colours in rainbows, we perceive colour on the surface of bubbles through the reflection and refraction of light waves. White light is made up of a spectrum of colours that each have a different wavelength. Just like ocean waves, light waves have peaks and valleys (crests and troughs). Red light has the longest wavelength and so the largest distance between each consecutive trough. A bubble consists of an inner bubble wall and an outer bubble wall that are a few micrometers apart. When a light ray hits the outer surface some light is reflected, while the rest is transmitted to the bottom layer where it is then also reflected.

The light rays that are transmitted through the bubble are refracted slightly. Although light rays travel in straight lines, when they enter a denser medium, in this case the soapy bubble film, their speed slows down and so they change direction. This, in combination with constructive interference produces the intensified colours we see in the photograph.

It was in the early 19th century that the English polymath Thomas Young first established the role of interference. Light reflected from the inner surface of the bubble will have travelled a few micrometers further than light reflected at the outer surface, and thus the two separate reflections will be out of step. These two reflections will interfere with each other. If the light waves meet crest-to-trough the colours will cancel each other out and we will see no colour – known as ‘destructive interference’. However, if the troughs of each reflected wave coincide they combine to cause an intensified colour effect – known as ‘constructive interference’.

The bubble’s structural formation creates the perfect medium for the processes of constructive interference, reflection and refraction to interact and produce such vivid colours. Ibn al-Haytham’s early work on the theories of refraction and reflection and Thomas Young’s explanation of interference were both paramount to our understanding of how colours form in bubbles.


IMAGE: Natascha Mehrabi