Tale of two dots

January 31, 2022

You may have heard that light always travels in a straight line. But can you prove it?

Parts

Black cardboard paper.
Two differently colored LEDs
Two coin cells (e.g. CR 2450)
Paper tape
Scale or any other straight object.

Steps

Make a hole in the black cardboard paper

Stick the LEDs to either end of the scale/stick

Place the LEDs in front of the paper

You will need a dark room for this. You will also need to adjust the position of the LEDs till you get the dots on the other side of the paper (just move them back and forth a bit till you get a sharp crisp image of two dots)

See the dots are flipped

Two important things are observed.

Even though there is one hole, you can still see both dots.
The dots are flipped.

How does it work?

If we draw the experiment setup on a piece of paper…the answer becomes apparent.

Points to ponder

Light not only travels straight, it travels in all directions from the source. In this experiment, most of the rays of light are blocked by the opaque paper that does not let light through. The hole makes it easier to observe light by letting in just one “beam” of light in from both sources.
The yellow light and the red light just passed each other without affecting each other at all. This is quite amazing - photon’s don’t get distracted at all! To me, the mind-blowing part of this experiment is that the yellow light, as it crosses the red light through that tiny hole is not affected at all. The just go through like each other did not exist. That’s exactly why we can see distinct object like a chair, table, or a pen lying on it. The light reflected from these objects do not interfere with other light coming from other objects. How cool is that.
What happens when you vary the distance of the LEDs from the black cardboard? What happens if you move the black cardboard away from the wall?