Bubble Rainbow Rings science experiment | Fizzics Education

Bubble Rainbow Rings

Bubble Rainbow Rings

Follow FizzicsEd 150 Science Experiments:

Detergent

Water

Table Salt

Plastic bowl

Straw

Bright torch

 

Materials needed for the bubble interference experiment showing a torch, a bowl, a cup of water, salt, a straw and detergent
1 Pouring detergent into water that is in a plastic cup

Mix up some bubble mix (water plus detergent plus a sprinkle of salt).

2 Placing bubble mix along the rim of a bowl

Coat the underneath of a plastic bowl with bubble mix, ensuring a coating across the whole surface.

3 Blowing a bubble onto a upturned bowl using a straw

Use the straw to blow a bubble on the surface of the bowl.

4 Light shining through a bubble that is sitting on an upturned bowl

Shine your torch on the bubble from the side.

5 Interference patterns in a bubble forming rainbow rings

You should see perfect interference rings, especially when you’re looking from below the bubble.

What is Happening?

White light is made up of many colours. These different colours have different wavelengths, beginning with red at 700nm through to violet at 380nm. We can see the many colours of light on bubbles as the bubble surface can break up the white light into the visible colours of the rainbow.

Since light has wave properties, it will experience interference (the addition of waves). This interference is like that seen with water waves & ripples that you see when you drop a rock into a pond.

We get interference from a thin film like a bubble, because the light waves which bounce off the front and back surfaces of the bubble wall add constructively or destructively.

  • Constructive interference occurs when two identical waves moving in sync meet (they are in phase). The heights of the two waves (the amplitudes) then add together to produce an even taller wave that has the amplitude equal to the sum of the two smaller wave amplitudes. Think of this like building waves on top of each other! You’ll see light coloured rings at these points.Two waves combining to form a larger wave
  • Destructive interference happens when two identical waves are moving completely out of sync with each other (when one wave moves upwards the other is moving downwards, thereby being 180 degrees out of phase). The two opposite but identical waves then cancel each other out. Think of this like the trough of one wave being filled by the crest of the other. Here is where you will see darker rings.
    Two waves out of sync meeting and forming a line

Whether you get destructive or constructive interference depends on the colour of the light and the thickness of the bubble.

The bubble’s thickness is not uniform, that is why you see fringes or bands of different colours, and dark bands where the light interferes destructively. Over time, the thickness of the bubble changes, leading to a changing and swirling of the colours!

Going Further

For more details about how wave interference works, check out this explanation from PhET

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