How can you see Light in Mid-air?

How can you see the spotlight's light in mid-air?
- Shreyas Nagesh (age 12)
Bangalore, Karnataka, India

Hi Shreyas,

Good question! You can only see light if it hits you directly in the eye. For example, if someone shines a beam onto your eye, you see it directly. This can be dangerous, so don't try it with a bright source!

So how can we see light "in mid-air"? If the beam isn't pointed towards your face, you wouldn't expect to see it. However, there are lots of little particles in the air: nitrogen, oxygen, dust, water vapor, etc. All of these particles can scatter light, or reflect in a different direction. This scattering is usually pretty weak, so you don't always see it. If there is a lot of dust in the air (like if you clap too chalk erasers together) or if you have a super-bright source (like a spotlight), then you can see this scattered light quite clearly, because some of it bounces off the particles in the air and straight into your eyes!

By the way, this principle ("You can only see light if it hits you directly in the eye") is often useful when you work in an optics laboratory. If you see a fiber optic cable being lit up by the laser you send through it, then you know that it is scattering light, which is being lost from your experiment and into your eyes. A good, non-broken fiber, won't scatter much light, so you'll barely be able to see the light traveling through it.

Hope that helps!

David Schmid

 

(published on 11/04/2013)