Rainbows

Hello!

This is an advanced topic so if you don't know much math, I would recommend to read it later.

Let's suppose it is raining outside for an entire night. That means that you can't do any astronomy at all, which might be a bit sad (for me at least). However, during the morning, you look out your window and see a beautiful rainbow!

Rainbows are a beautiful example of the natural phenomena that occur on our planet. It is one of the few natural phenomena where the entire visible spectrum is in display. Today, we will look at how they form and the spectrum of light in more detail.

Most of the light of the sun is actually white light, despite the yellowish hue we expect. The white light is actually a mix of all of the visible spectrum plus some extra EM waves that are too long or short for us to see. When they normally hit Earth, they usually bounce off the surface or get trapped by the Earth. But when sunlight passes through a raindrop, it essentially acts as a prism, "diffracting" the white light into the spectrum. The reason for why this happens is a bit complex.

Essentially, light with different frequencies (and polarizations) acts different from each other while in a medium due to the interaction of EM waves with the atoms of a medium. This may hinder the speed of light depending on its energy. Due to this, the light bends differently and it is broken into the spectrum that we all see in the sky.

Here's something you can try.

Take a laser with a specific color (red, green etc.) and make the light beam pass through a glass pane. Based on that, it is possible to find out the speed of light in the glass by measuring the angle of incidence and angle of refraction and plugging the angle values in a equation known as "Snell's Law".

Snell's Law states:

\(\frac{\sin \theta_1}{\sin \theta_2} = \frac{m_1}{m_2}\)

where \(\theta_1\) is the angle of incidence, \(\theta_2\) is the angle of refraction, \(m_1\) is the medium where the light originates and \(m_2\) is the medium where the light passes through. By knowing the speed of light in air, you can estimate the speed of light in the glass pane.

Repeat it with different wavelength of lights and note the results down. What do you see? Do you see a pattern? Tell me in the comments!

Thanks for reading!