Year 3 Science · Light and shadows · Lesson 8.6

Year 3 Science: The size and shape of shadows

Part of the Year 3 Science light and shadows chapter. Chapter overview · whole chapter on one page

8.6

The size and shape of shadows

Year 3
L5: find patterns in the way that the size of shadows change; the shape of a shadow depends on the object and the light

Story. Ruby and Aisha hold a card in front of a torch and a wall. Ruby says, “When I move the torch closer, my shadow gets bigger.” Aisha says, “Let’s measure it, so we know.”

The size of a shadow changes when the distances between the light source, the object and the surface change. We can find patterns by doing a fair test: change only one thing, keep everything else the same, and measure the thing that changes.

In the set-up, the card is 4 cm tall. The shadow height is measured on the wall with a ruler. The picture is not to scale.

A torch, a card and a wall. The distance from the torch to the card, and from the card to the wall, are the two distances that change in the fair tests. Not to scale.

Table 1. The card stays 20 cm from the wall. Only the torch distance changes.

Torch to card (cm)	Shadow height (cm)
10	12
20	8
40	6

Table 2. The torch stays 20 cm from the card. Only the wall distance changes.

Card to wall (cm)	Shadow height (cm)
10	6
20	8
40	12

A bar chart of the shadow height for each torch distance in Table 1.

The pattern is clear. The closer the torch is to the card, the bigger the shadow. The further the card is from the wall, the bigger the shadow.

The shape of a shadow depends on the shape of the object, and on the way it faces the light. When the light comes from directly above, the shadow looks like the outline of the object seen from above. A ball gives a circle. A box standing on its end gives a square. A box lying on its side gives a rectangle.

The shadows of a ball, a box standing on its end, and a box lying on its side, with the light from directly above.

Shadows change through the day as the Sun moves. The Sun is low in the sky in the morning and late afternoon, and it is high at midday. Shadows point away from the Sun.

Time of day	Sun in the sky	Shadow
Morning	low in the sky	long
Midday	high in the sky	short
Late afternoon	low in the sky	long

A stick and the Sun at three times of day. The shadow is long in the morning and late afternoon, and short at midday.

Key ideas

A shadow gets bigger when the object is closer to the light source.
A shadow gets bigger when the object is further from the surface it falls on.
The shape of a shadow depends on the shape of the object and the way it faces the light.
Shadows are shortest at midday, when the Sun is highest in the sky.

Worked example 8.6.1

Look at Table 1 in this section. What is the shadow height when the torch is 40 cm from the card? Write the number.

Method. Find the row for 40 cm in Table 1.

Read the row. When the torch is 40 cm from the card, the shadow height is 6 cm.
6 cm
Write the number. So the shadow height is 6.
6
Write the answer.
6

Check. The shadow is smaller when the torch is further away. ✓

Worked example 8.6.2

Look at Table 2 in this section. How much bigger is the shadow when the card is 40 cm from the wall than when it is 10 cm from the wall? Write the number.

Method. Find the two shadow heights in Table 2.

Read the two rows. The shadow is 12 cm when the card is 40 cm from the wall, and 6 cm when it is 10 cm from the wall.
12 and 6
Subtract. The difference is 12 minus 6, which is 6.
6
Write the answer.
6

Check. Check: 6 + 6 = 12. ✓

Watch out. Subtract the smaller shadow from the bigger one.

Worked example 8.6.3

Look at the day table in this section. At which time of day is the shadow shortest? Write the time.

Method. Find the short shadow in the table.

Read the table. The shadow is short at midday, and long in the morning and late afternoon.
short
Write the time. So the shadow is shortest at midday.
Midday
Write the answer.
Midday

Check. The Sun is highest in the sky at midday. ✓

Watch out

Watch out: moving the torch closer to the card makes the shadow bigger, not smaller. Do not change two things at once, or the test is not fair.

Try it at home or school (with a grown-up). With a grown-up, set up a torch, a card and a wall in a dim room. Keep the card 20 cm from the wall, and put the torch 20 cm from the card. Measure the height of the shadow with a ruler. Then move the torch to 40 cm and measure again. Keep everything else the same. Never shine the torch into anyone’s eyes.

Exam tip

Grown-up tip. Ask: “What did you change? What did you keep the same? Which shadow was bigger?” Let your child use the words fair test and shadow height.