Meditating on Reflected Light

I was sitting in philosophy class bothered by the bright light coming in through a window to my right. I moved out of the sunlight as any normal human being would do, and noticed that the reflector (similar to the ones on bicycles) on a backpack across the room from me only seemed to reflect light when my eye was in the sunlight from the window. To better visualize this, I'll explain the layout in more detail than is probably necessary: I'm sitting at a table in the back of the room, facing the front. The window is on the wall to my right, and the sunlight is coming in towards the left side of the room, at a downward angle (I can cast a shadow of my hand on the table that I'm sitting at, in the small rectangle of light from the window). The small reflector is facing me, on a backpack at the front of the room. It is nowhere near any of the light coming in from the window.Even though we were discussing Descartes' Meditations, my mind was stuck on figuring out why the reflector only looked bright when my eye was in the sunlight shining in from the window. I probably looked crazy bobbing my head in and out of the light for the whole class, but it was in the name of physics so it's ok. Please help me figure out why this phenomenon occurs, or I will be head-bobbing for the rest of the semester. Thank you!
- Jason G. (age 19)
Ithaca, NY, USA

Here's a guess. Those reflectors are designed to reflect a significant fraction of light back the direction it came from. The reason is obvious- you want someone driving a car, which has lights, to be able to see you easily. So if you put your face in a light spot, from which light is going out all directions, including toward the reflector, you'd expect to see more light coming back from the reflector than if you put your face in a less lit spot.

I just tried the experiment with a bike reflector and sunlight streaming in the window. The reflector definitely has an extra-strong reflection back toward the light source. In fact, it's a little hard to type this because I got that reflection right in my eye. There wasn't enough reflection off my sun-lit face, however, to show up much when looking at the reflector. Perhaps in your case there was something much more reflective in the same lit-up region where you put your face. 

The way you can make a reflector that preferentially reflects straight back is by using little 3-sided regions meeting at right angles in a corner. Light bouncing off the 3 sides reverses directions along each of the three axes, so it ends up making a complete turn-around. 

By the way, you may be interested in this true anecdote about someone else who got distracted during class. 

Mike W.

(published on 02/18/2015)