1. Introduction: The Coin That Reappears
Place an empty steel bowl on a table. Put a one-rupee coin at the bottom. Now step back until the rim of the bowl just hides the coin from your eyes — you cannot see it anymore.
Stay exactly where you are. Ask someone to slowly pour water into the bowl without disturbing the coin.
Something strange happens. The coin reappears. You did not move. The coin did not move. But now you can see it again.
How? Light from the coin changed direction when it crossed from water into air. This bending followed a precise, predictable rule — and that rule is what this lesson is about.
Stop scrolling. Try it in your head before reading on. Why would adding water make the coin visible? What must the light be doing differently?