Welcome back! This week we’re experimenting with optics, as graduate student Naren Manjunath introduces us to demonstration L3-18, in which we focus light with a parabolic mirror. Check out his video below:

This demonstration uses one of the old overhead transparency projectors that focuses the light by a large parabolic mirror under the platform. These used to be standard issue in every classroom; but in the era of digital presentations, they only see use on special occasions.

The mirror focuses essentially all of the light striking it, both visible light and invisible (to our eyes) infrared light, to a single point. All of the energy carried by those light waves arrives at that point. When we place a piece of paper at that point, it absorbs that energy and heats up rapidly, bursting into flame.

Many old fashioned projectors like this would have a “heat filter” built in, a piece of glass treated to be transparent to visible light but reflect infrared light. This was to prevent exactly this from happening, since you don’t want your materials bursting into flame in the middle of class! Even some recent projectors have had similar filters, although modern light sources (such as LEDs) that produce light only in the visible wavelengths desired make them unnecessary.