# 100 Nano-Stories: Why is Silica Aerogel Blue?

## Episode #04: Optical Properties!

4 min readJan 2, 2021

# Preface

Welcome back to another article, my space enthusiasts! If you are confused with what I am talking about, I can help you catch up to speed with my previous article!

Carlos, I know you talked about Aerogels and Airloys in your previous series, but why does the Silica Aerogel always have to be blue? Why can’t Silica Aerogel be brown, pink, yellow, etc.?

Ah, you seek the Optical Properties of Classic Silica Aerogel! I will get to that in a minute, but have you looked out your window yet? It’s a new year, so look up to the blue sky!

Wait, Carlos, isn’t Silica Aerogel also blue? Is this another of your tricks?

# Rayleigh Scattering

Silica Aerogel is a real phantom. Put a piece of silica aerogel in light, and it looks like it has vanished into thin air, but in reality, the optical properties make silica aerogel have a yellow-ish coloration. But when you place the silica aerogel in a room with not a lot of light, the aerogel suddenly “reappears” with a blue-ish coloration.

But why?

Two words, reader: Rayleigh Scattering.

Uh, Carlos…

Also, what does the sky have to do with it, now that you talked about it?

Good question, reader. When light from the cosmos enters the Earth’s atmosphere, it will confront the molecules that are abundant in the atmosphere (in the air, there is mostly nitrogen and oxygen). When this occurs, the light will bounce from one molecule to another, sort of like a bouncy ball hitting two walls. This type of bouncing of light being redirected is called scattering.

Okay… but wouldn’t that mean that all light will be scattered, and everything turns white?

# Rayleigh Scattering Equation

This is when Rayleigh Scattering comes into play, reader. This is how to describe Rayleigh Scattering:

Carlos, I don’t like math! I don’t get it!

Okay, then, I’ll explain the mathematical formula in English.

The wavelength, or Rayleigh Scattering, is inversely proportional to 1 divided by the wavelength raised to the 4th power. This means that if the wavelength of a certain ray of light is short, then the wavelength of light will scatter more than a ray of light where the wavelength is larger.

However, the nanopores of the silica aerogel network are smaller than the wavelength of visible light! (FYI, visible light ranges from 750 nanometers to 380 nanometers!)

Wait, Carlos! What does HEV Blue Light mean?

That means that High Energy Visible (Blue) Light has the shortest wavelength of all the visible light spectrum. Because the nanopores of silica aerogel are between 1–100 nanometers, the wavelength of blue light will be scattered more than yellow or red light because HEV Light has the smallest wavelengths.

Wait, Carlos, but I see purple in the visible light spectrum… so why is it not purple?

Some aerogels are purple, but that has nothing to do with the light spectrum… sort of. There is violet light passing through, but if you notice the visible light spectrum, there isn’t a lot of violet light. Some people can’t even see violet light! Crazy, right?

The other reason why these aerogels look purple has to do with how they are made of. These aerogels are made not from classic silica aerogels, but from Lanthanide Oxide Aerogels, which use an element called Neodymium!

Let me guess, Carlos… is this for another article? 🤔

We’ll see about that! 😉

For now, Happy New Year! 🎉

# Vocabulary 📓

optics- the branch of physics that studies the behavior and properties of light

heterogeneities - diverse ingredients in aerogel

inhomogeneities - not in uniform, the aerogel does not have the same properties at every point

spatial arrangement - the property possessed by an array of things that have space between them

monolithic - When something is monolithic it’s big and made of one thing

scattering - Light rays are being redirected in different directions on passing through particles/atoms/molecules that are comparable to the wavelength of the light

# cjarquin0005@gmail.com

© 2021 by Carlos Manuel Jarquin Sanchez. All Rights Reserved.