Light goes straight

The idea

Light seems to travel in straight lines, as shown by the sharp shadow a small source casts of an opaque object; this straight-line path is indicated as a ray of light.

A card in front of a bulb

A small bulb shines towards a wall. You hold an opaque card (light cannot pass through it) between the bulb and the wall.

What will the wall show behind the card?

• A dark shadow with sharp edges, a little bigger than the card
• No shadow at all: the light goes around the card
• A soft glow with no clear edge

Fit the shadow

Drag the card. The shadow on the wall moves with it. The orange lines are the rays that just miss the card.

A ray is a straight line

Light seems to travel in straight lines. A small source of light casts a sharp shadow of an opaque object. This straight-line path of light is called a ray of light.

A ray is drawn as a straight line with an arrow that shows the way the light travels.

• What you saw | What it tells us
• The shadow has sharp edges | Light does not bend around the card
• Each edge of the shadow sits where a straight ray past the card lands | Light travels in a straight line
• The shadow grows when the card is nearer the bulb | Straight rays spread out from the bulb

Notes

Light seems to travel in straight lines, as shown by the sharp shadow a small source casts of an opaque object; this straight-line path is indicated as a ray of light.

Check yourself

What is a ray of light?

The bulb is 20 cm from the card. The wall is a further 40 cm behind the card. The card is 10 cm tall. How tall is its shadow on the wall?

Answer: 30 cm

The wall is 20 + 40 = 60 cm from the bulb, which is 3 times the card's distance of 20 cm. Straight rays spread out in the same ratio, so the shadow is 3 × 10 = 30 cm.

You move the card nearer to the bulb. The shadow on the wall …

Which of these would not make a proper shadow on the wall?

• A thick beam of many colours. A ray is a thin path, not a coloured beam.
• A curved path that light follows around objects. Light seems to travel in straight lines, so a ray is straight.
• The straight-line path along which light travels — correct. Yes! It is drawn as a straight line with an arrow.
• gets bigger — correct. Yes! Near the bulb, the card blocks a wider fan of rays.
• gets smaller. It is the other way round: near the bulb the card blocks a wide fan of straight rays.
• stays the same size. The rays spread out as they travel, so the size changes with the card's place.
• A wooden card. Wood is opaque. It stops the light, so it casts a shadow.
• A thin sheet of clear glass — correct. Yes! Light passes through clear glass, so there is no dark shadow.
• A steel plate. Steel is opaque. It stops the light, so it casts a shadow.