# Tag Archives: water # Double Rainbow

Can you see the double rainbow?

Ever noticed how the order of the colours changes for the second rainbow (red – yellow – green – blue – violet…)

Typically rainbows are formed when light is refracted (bent) through a raindrop.

A double rainbow forms when light is refracted twice in a raindrop, and occurs commonly when the sunlight is low in the sky. See here for more.

# The invisible rod

Challenge: How can you make a quartz rod invisible with some water, sugar and a beaker?

If we take something that is typically transparent (i.e. the quartz rod) you can normally see it quite clearly when placed in a liquid, by the way in which light is bent as it passes through. This ‘bending’ of light – refraction – can be described by Snell’s law: $n_{1}sin(\theta_{1})=n_{2}sin(\theta_{2})$

where ‘n’ is the refractive index of the material.

So you might imagine that if we can change the rod, or the liquid itself, so that light entering from behind the beaker does not refract further on entering the quartz rod, we can effectively make the quartz rod invisible. To do this we want to match up the refractive indices ( $n_{1}, n_2$).

With water, as you add more and more sugar the refractive index increases, until finally it approaches that of quartz ~1.46. 