Light and shadows

Light travels from a source in straight lines. When it hits an opaque object it is blocked, leaving a shadow the same shape as the object. We see things because light from a source bounces off them into our eyes – our eyes do not send anything out. Shadows change size with the position of the light, and length with the time of day.

Key facts

  • Light sources (the Sun, flames, bulbs, screens) make their own light; the Moon and most objects only reflect it.
  • Light travels in straight lines, very fast: about 300 000 km every second.
  • Transparent: lets light through clearly. Translucent: lets some through, blurred. Opaque: blocks it, making a shadow.
  • Moving an object closer to the light makes its shadow bigger.
  • Outdoor shadows are shortest at midday, when the Sun is highest, and point away from the Sun.
  • Never look directly at the Sun – it can permanently damage your eyes.

Worked examples

Example 1

Sort these: clear glass, tracing paper, a wooden board. Which are transparent, translucent and opaque?

  1. Clear glass: you see straight through it, so transparent.
  2. Tracing paper: light gets through but blurry, so translucent.
  3. Wood: no light gets through, so opaque – it makes a dark shadow.

Answer: glass – transparent; tracing paper – translucent; wood – opaque

Example 2

At the same moment, a 1 m stick casts a 1.5 m shadow and a tree casts a 9 m shadow. How tall is the tree?

  1. Each 1 m of height gives 1.5 m of shadow.
  2. Divide 9 ÷ 1.5 = 6 m

Answer: 6 m

Example 3

A 6 cm puppet is 20 cm from a small torch. The screen is 80 cm from the torch. How tall is the shadow?

  1. Scale factor 80 ÷ 20 = 4
  2. Shadow height 6 × 4 = 24 cm

Answer: 24 cm