# The Magnifying Glass vs. Water Baggie Effects

Sure, it’s basic science: It is well known that a magnifying glass can start a fire. And recently on FB, it has been shown that a clear sandwich bag filled with water can also start a fire. My problem with this is that it dawned on me the light leaving the bag is going from a medium of higher density to lower density. As such, the law of refraction say the light should be diverging, not converging. So…wait a minute, the same is true of the magnifying glass, too, right? Light is leaving a denser medium to a less dense medium.

So, I have two sets of facts that are paradoxical. Yet, both have been shown to start a fire. What gives? There must be something more to the story I am missing here.

The surface is curved.

ETA look up “convex lens”

The bag forms a sort of convex, and therefore convergent, lens. It will become clear if you draw a diagram and trace a few rays of light through the lens.

Or simply note that the bag has a shape similar to the magnifying glass.

It’s not just the change of medium that causes convergence or divergence. It’s the curvature. If you have a flat surface (like an ordinary window), there won’t be any convergence or divergence at all.

An ordinary lens (one made of a slower material than its surroundings) will converge if it’s convex (bulging out in the middle), or diverge if it’s concave (thicker on the edges, and thin in the middle).

Well, there will be dispersion in a window pane. This causes blurring and “rainbow” effects. But usually it’s not noticeable for the thickness of typical glass panes and the angles people are look through them at.

Since the OP is under the impression that dispersion is a major effect in small lenses compared to the focusing effects, dispersion should not be dismissed entirely. It’s there, just not a big deal unless you are interested in higher quality imaging. (And “quality” isn’t a real concern if you’re using a baggie as a lens.)