Why Does a Pencil Look Broken in Water? Refraction Experiments for Kids

Stand a pencil in a glass of water and look from the side. It looks broken. This is one of the oldest and best science tricks, and it leads straight to one of the key ideas of light: refraction.

Experiment 1: the broken pencil

Fill a clear cup with water and stand a pencil in it. Look from the side. The pencil seems to snap at the water’s surface. Look from above and the pencil looks normal. Now compare with an empty cup.

Experiment 2: the vanishing and appearing coin

Put a coin in the bottom of an opaque mug.
Step back until you just can’t see the coin because the rim blocks your view.
Stay still while a friend slowly pours water into the mug.
The coin comes into view, though nothing moved.

Experiment 3: a water-drop magnifier

Put a clear plastic sheet (a page protector) over some small print. Place a single drop of water on the sheet, right over a letter. Look through the drop. The letter looks larger.

The science

Light travels in straight lines through air. When it passes into water, it slows down and changes direction, an effect called refraction. Your brain assumes light always arrives in a straight line, so it places the pencil where the light seems to come from, which is not where the pencil really is. That’s why it looks bent.

In the coin trick, light from the coin bends at the surface as it comes out of the water, so it reaches your eye over the rim. Your eye sees the coin higher than it really is.

A curved drop acts like a lens: the curved surface bends light so that it spreads the image out and magnifies it. That’s the principle behind magnifying glasses, spectacles and cameras.

Questions to ask

What did you predict? What actually happened?
Does it matter how much water is in the cup?
Does the effect happen with oil or with a clear jelly?
Where else have you seen light bent? (Rainbows, swimming pools, glasses.)

Common mix-ups

“The pencil really bends.” Take it out: it’s straight.
“Water is a mirror.” Mirrors bounce light (reflection). Water bends it (refraction), though calm water also reflects a little.
“Light speeds up in water.” It slows down.

Try the lesson Why does a pencil look broken in water? for all three activities with a student sheet and teacher guide. It sits in Bouncing Light: Mirrors, Water and Colour with How can a mirror steer a beam of light? and What colours are hiding in white light? Sign up for three lessons free.