Question: can air make a shadow

Mia West answered on 21 Mar 2023:

Hi JomiO that’s a really good question!

I don’t know how much you know about light already so I’ll give you two explanations: one less & one more technical. If you don’t understand the more technical version, don’t worry! It’s really complicated stuff.

I guess we should start by defining what a shadow is (so we’re on the same page!): a shadow is, according to the dictionary, `the dark figure cast upon a surface by a body intercepting the rays from a source of light’.

Less Technical:

Light gets stopped when it’s absorbed by atoms. When most of the light from the source gets absorbed by a bunch of atoms (an object) in the way of the source, we get a shadow. A block of wood, for example, will absorb most of the light shone at it and casts a shadow.

However, air is made up of gases such as oxygen, nitrogen & carbon dioxide. Gases are extremely light and not very dense (gases are about 1000 times less dense than solids). Therefore, when you shine light through the air, not much light gets absorbed by the gas & it passes through basically unaffected.

Air does stop some light though. For example dust particles in the air are pretty big & will absorb some light.

More Technical:

You might have heard in school that you can describe light as either a wave or as being made up of tiny particles called `photons’. We call this `wave particle duality’.

So if we think about light as being made up of photons, we can think of a shadow being made by an object when it `absorbs’ or `reflects’ the photons.

It turns out that an individual atom or a very simple compound will only absorb or reflect a photon if it has a very specific energy. Or alternatively, if we think of light as a wave, when the light has very specific wavelength.

So air, which is mostly made up of simple compounds such as oxygen (O2 – as they come in a pair), nitrogen (N2) and carbon dioxide (CO2) can only absorb photons of very specific wavelengths, so most wavelengths of light pass straight through.

On the other hand, if we have a much more complex object made of multiple compounds, such as wood, the object can absorb a much greater range of wavelengths. So of course wood casts a shadow when we shine visible light at it. But some objects, glass is a great example, are transparent to visible wavelengths of light and so they don’t cast a shadow.

So all in all, visible light will (for the most part) simply pass through air because it’s made of such simple atoms which won’t absorb most photons which travel through it. Though of course dust particles in air will absorb light so air will stop being quite so transparent!

Furthermore, air is approximately 1000 less dense than solid objects. So there’s much less `stuff’ which might absorb light when it passes through.

So to conclude then, air does stop the path of some light, but overall it’s pretty transparent and most people would say it doesn’t cast a shadow!

Hope that makes sense! Let me know if you have more questions.