IOP Institute of Physics

# Swing thing

Ingredients

• string (approx 1m long)
• 2x 2p coins
• BluTack
• old pair of dark sunglasses

Instructions

1. Use the BluTack to stick together the 2p coins to make a weight. Tie the string
to the weight – you now have a pendulum.

2. Ask an audience member to watch the pendulum. Get them to stand ~2m
away from you, and swing the pendulum so that it is perpendicular to the line
between them and you. Ask them what direction the pendulum is swinging in.

3. Break the sunglasses in half at the nose so that the two lenses are separate.
Get your volunteer to hold one lens in front of one eye. Note: they need to
keep both eyes open! Swing the pendulum again, exactly as before. Ask
them again what direction the pendulum is swinging in (they will see it going in
a circle).

4. Get the volunteer to hold the lens in front of the other eye – they will see the
pendulum going in a circle in the opposite direction to the previous one.

How does it work?

Sunglasses block some of the light travelling towards them, so that when you wear a
pair of sunglasses, less light actually reaches your eye. This makes your eye more
sensitive to light. If you think about how a camera works, in darker conditions it is
necessary to increase the size of the aperture and the exposure time in order to get a
decent photo. In the same way, your eye’s aperture (the pupil) automatically
increases in size, and the timing of the signals being sent from the eye to the brain is
delayed when you wear sunglasses. You don’t normally notice this effect when you
wear sunglasses because both eyes are covered up, and therefore both signals are
delayed. But in this experiment only one eye is covered, so you can distinguish the
difference between the two.

The eye with the sunglasses lens in front of it sees the pendulum delayed with respect
to the normal eye – and therefore in a different position. This has the effect of tricking
your brain into thinking that the pendulum is now moving in three dimensions instead
of two, i.e. in a circle (or ellipse) instead of a straight line. When you swap the lens to
hold it in front of the other eye you swap which signal is being delayed, thereby
changing the apparent direction of swing.

Tips for Success

Some people don't see the effect so quickly – they may need to move the lens back
and forth in front of one eye in order to see the difference.

If you don't have an old pair of sunglasses lying around, but do have access to an
optics lab then neutral density filters will of course work just as well – if you don't mind
them being held by the audience members!

Serving Suggestions

This trick works best one at a time as the volunteer needs to be perpendicular to the
direction of the pendulum swing. It therefore works well with small audience sizes,
where everyone can have a turn to see the effect.