Lens
Light, Sound and Waves

Reversing arrow

Classroom Activity for 11-14 IOP RESOURCES

Students explore how an arrow can look bigger and reversed through a glass of water. An introduction to the language used to describe images created by converging lenses.

Equipment

Each group of students will need:

  • A clear, straight-sided glass or beaker
  • A4 white paper or card
  • A felt tip pen and ruler
  • A jug or bottle of water for pouring

Procedure

Ask students to:

  1. Draw two short identical arrows on the A4 paper. They should be of a length equal to about a third of the diameter of the glass and pointing the same way, one above the other.
  2. Stand the paper upright – lean it against a book or wall if necessary.
  3. Place the empty glass/beaker so that it is touching the paper.
  4. Partly fill the glass so that one of the arrows is visible through water in the glass and the other can be seen through the air above the water.
  5. Gradually move the glass away from the paper.

Discussion prompts

  • How is what you see different from what you drew on the paper?
  • How does it change as you move the glass away from the paper?

Teaching notes

A glass of water is a very thick lens. It’s focal length (as measured from the centre of the glass) is equal to the diameter of the glass. When the glass is touching the paper the object is half a focal length away. Introduce the terms below to help students describe what they see.

WordMeaning
ObjectWhat is drawn on the paper
ImageWhat you see
MagnifiedBigger
DiminishedSmaller
InvertedReversed

To start, they will see a magnified image that is the same way around as the object. As they increase distance the image will becomes ‘left-right reversed’ – a bit like the image they see of themselves when they look in a mirror. As they move the glass away from the paper the inverted image will initially be a magnified one, then become the same size as the object, before becoming a diminished image.

Learning outcome

Students use the terms object, image, magnified, inverted and diminished when describing images formed by a converging lens.

Home learning

For a version of this activity for families and younger pupils to try at home, see Do Try This at Home: episode 4

This experiment was safety-checked in March 2020.

Lens
can be analysed using Refraction Formation of Images
can be analysed using the quantity Focal Length Power of a Lens
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