Can you see Light From the Side?

On my five-year-old son's behalf: Why is light invisible? To be clear, he explained that you see the sun if you look at it and you see sunlight on things [as reflected light] but you don't see light "sideways." I had no idea how to answer this. I would seriously appreciate an accurate way of explaining this. Thank you!
- Owen Hansen (age 41)
St. Paul, MN, USA

Hi Owen,

That's a brilliant observation for a five-year-old! Keep encouraging his natural inclination for physics... we're happy to help if he has more questions!

Our eyes can only see things when they scatter light into our eyes, where the light is detected and then an image is formed in the brain. We can see objects "sideways" because they scatter light in all directions. Light itself, however, doesn't scatter other light at all... two beams pass through each other without interacting at all. So, you can only see a light source by looking directly at it, and you can't see light sideways.

Unless! If the light is traveling through a medium that scatters light, then you can see the beam traveling along. For example, if you shine a laser through a glass of water or through glass or through a cloud of chalk dust or fog, then you'll see it just fine from the side. A really bright laser can even scatter so much while traveling through air that you can see it, especially at night.

So you can teach your son about how the eye sees, how light scatters, and so on.

David Schmid

p.s. We have other posts on this topic, like 

 

 

(published on 03/30/2014)