Posts Tagged ‘Phase’

Bubbles and Interference

This week, you shall be entertained with the physics of soap bubbles! Soap-bubble rainbows are caused by thin-film interference. When a light ray strikes the film of of soap bubble, some of the light reflects off of the film’s top surface (Ray 1). Other rays (Ray 2), however, pass into the film, reflect off of the bottom surface of the bubble, and passes back through the film.


Ray 1 changes phase 180° as it reflects (which only happens when light reflects off a material with a higher index of refraction than the medium it is traveling in), and Ray 2 does not change phase as it reflects but it travels an extra distance through the film. This results in a path length difference of about 2x the thickness of the film.

So you end up with the two different rays being either in phase with each other (constructive interference), completely out of phase (destructive interference), or somewhere in between. The path length difference at a certain point on a soap bubble might cause destructive interference for blue light, but constructive interference for red light. You’ll end up seeing red at that spot.


When you wash your hands today, take a moment to bask in the loveliness of soap bubbles! ^ ^


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