Light — Reflection & Refraction

Priya is twelve years old and obsessed with getting the perfect selfie. Not with a phone — with a metal spoon from her kitchen, because she'd read somewhere that curved surfaces make funny reflections.

She holds the back of the spoon up (the convex side). Her face stares back at her — smaller, upright, fitting the whole room behind her. She tilts the spoon. The image tilts but stays small and right-way-up.

Then she flips it. The hollow side of the spoon — the concave side — faces her now.

She's upside down.

She moves it closer to her face. Suddenly she's right-way-up again, but huge. She's taken a step back and she's upside down again. She moves it slowly and watches herself flip.

She doesn't know it yet, but she's just discovered the difference between an image formed inside the focal length and one formed outside it.

A year later, Priya's younger brother Rahul drops a pencil into a glass of water and immediately panics.

Priya looks at the glass. The pencil looks bent at the water's surface, as if someone snapped it at the point it entered the water.

Rahul pulls it out. The pencil is perfectly fine.

Priya knows that light travels at different speeds in different materials. In air, it moves fast. In water, it slows down. When it crosses from air into water, that speed change makes it bend.

The pencil isn't broken. The light coming from the submerged part of the pencil is bending as it exits the water — so your brain, which assumes light travels straight, places the bottom of the pencil in the wrong position. The pencil looks displaced.

Priya's grandfather can't read the newspaper without his glasses anymore. His eyes have lost the ability to bend light strongly enough to focus it onto the retina — the image forms behind the retina instead of on it.

His glasses have thick lenses that are fatter in the middle than at the edges. They're convex lenses — converging lenses — and their job is to add extra bending so that the focal point shifts back onto the retina where it belongs.

One afternoon, Priya picks up his glasses and holds them above a patch of sunlight on the floor. She moves them up and down slowly.

At a certain height, the scattered sunlight converges into a bright point on the floor. A burning dot. She's found the focal point of the lens.

Priya's neighbour is building a periscope to spy on the cricket match visible from the terrace — but she can't go up because it's exam season and her mother has banned rooftop trips.

She builds a cardboard tube with two plane mirrors angled at 45°. Light from the match enters the top, bounces off the first mirror, travels down the tube, bounces off the second, and exits sideways into her eye.

The image is clear, right-way-up, and the same size as reality. She can see the boundary from her room.

Two reflections, two right-angle bends, and light has traveled a path it wasn't supposed to. No lenses, no refraction — just the first law of reflection applied twice.