A pencil "slashed" by the light refr...

A pencil “slashed” by the light refraction in a bowl of water (Photo credit: Wikipedia)

Can You Bend A Pencil?

In this activity, it appears that you have bent a pencil without touching it.


  • Pencil (a straw works fine, too)
  • Water
  • Glass


  • Fill a water glass about 2/3 full of water.
  • Put a pencil in the glass, allowing it to lean against the side of the glass.

What Should Happen?

The pencil will appear to be bent at the water line.

That is, the part of the pencil underwater does not appear to be in a straight line with the part of the pencil above the water.

Why Is This Happening?

Light refracts, or bounces back to our eyes, at a different rate when it is going through water and when it is going through air.

It is similar to the phenomenon we saw in an earlier experiment when light refracted, or broke up into its respective colors, to make a rainbow, because it traveled through water and air at different speeds.

What Else Shows the Difference Between Air and Water and Light?

If you put a flat object in the water, like a ruler, you will see a slightly different effect.

The top part of the ruler will appear narrower than the part of the ruler in the water.

If you change the container, however, from a round water glass to a square glass container, like a vase, the bent effect you saw with the pencil happens again.

Why Is This?

The round glass acts like a convex lens, magnifying the light that comes from the flat, wide ruler that is underwater.

Again, the way the light is concentrated is different for air and water because of the different speeds that light travels through air and water.

Thanks to daviddarling.info for this explanation.

Is This Information Useful?

If you are trying to spear a fish underwater, knowing that the light is refracted helps you aim slightly below and to the side of where you see the fish.

Thanks to sciencekids.co.nz for this pencil-bending activity.

To you and helping your grandchildren understand the world.


Carol Covin, Granny-Guru

Author, “Who Gets to Name Grandma? The Wisdom of Mothers and Grandmothers

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