PSLE Science · Primary 5 and 6
Light and Shadows
Light is a form of energy and belongs to the Energy theme in PSLE Science. This topic explains how we see objects, how light travels and is reflected, how materials let different amounts of light through, and how shadows form and change size. Shadow questions often come with a table of distances and shadow heights, so you need to be able to spot the pattern and explain it using the fact that light travels in straight lines. This guide covers each idea with clear examples.
What the syllabus expects
- Explain that we see an object when light from it enters our eyes
- Distinguish between sources of light and objects that only reflect light
- Describe how light travels in straight lines and is reflected by surfaces
- Classify materials as transparent, translucent or opaque
- Explain how shadows are formed
- Predict how the size and shape of a shadow change with the positions of the light source, object and screen
Seeing and sources of light
A source of light gives off its own light. The Sun, a lit candle, a switched-on torch and a firefly are sources of light. The Moon is not a source of light, because it only reflects light from the Sun. We see an object when light from it enters our eyes. Light from a source travels straight to our eyes. For other objects, light from a source falls on the object and is reflected into our eyes. In a completely dark room we cannot see anything, not even a white or shiny object, because there is no light to be reflected. Light travels in straight lines. You can show this by lining up three cards with a small hole in each and shining a torch through them: you see the light only when the holes are exactly in a straight line.
Reflection and materials
All objects that we can see reflect some light. Smooth, shiny surfaces such as mirrors and polished metal reflect light very well, which is why mirrors are used in periscopes and in the side mirrors of cars. Materials also differ in how much light they let through. A transparent material, such as clear glass or clear plastic, allows most light to pass through, so we can see clearly through it. A translucent material, such as frosted glass or tracing paper, allows some light to pass through, so objects behind it look blurred. An opaque material, such as wood, metal or cardboard, does not allow light to pass through at all. The choice of material depends on the job: a window needs transparent glass, while a bathroom window often uses translucent glass for privacy.
How shadows form
A shadow forms when an opaque or translucent object blocks the path of light. Because light travels in straight lines, it cannot bend around the object, so the area behind the object receives less light or no light. A shadow is always on the opposite side of the object from the light source. The shape of a shadow is the outline of the object as seen from the light source, so turning the object can change the shape of its shadow. For example, a cone can cast a triangle-shaped shadow or a circle-shaped shadow depending on which way it faces the light. An opaque object casts a dark shadow. A translucent object casts a lighter shadow, because some light passes through it. A transparent object casts almost no shadow.
Changing the size of a shadow
Light spreads out from a torch or lamp. If the light source and the screen stay in place, moving the object closer to the light source makes its shadow larger, because the object blocks more of the spreading light. Moving the object closer to the screen makes the shadow smaller. A larger object also casts a larger shadow. Outdoors, the position of the Sun changes during the day. Shadows are long in the early morning and late afternoon, when the Sun is low in the sky, and shortest around midday, when the Sun is high. A shadow always points away from the Sun. In the morning the Sun is in the east, so shadows point towards the west, and in the evening the Sun is in the west, so shadows point towards the east.
Worked example
Amy placed a torch 60 cm from a screen. She held a card 4 cm tall between them, moved it to different distances from the torch and measured the height of its shadow. At 10 cm from the torch the shadow was 24 cm tall, at 20 cm it was 12 cm tall and at 30 cm it was 8 cm tall. (a) Describe the relationship between the distance of the card from the torch and the height of the shadow. (b) Explain this relationship. (c) Predict the height of the shadow when the card is 40 cm from the torch.
- For (a), read the trend: as the distance from the torch increased from 10 cm to 30 cm, the shadow height decreased from 24 cm to 8 cm.
- For (b), use how light travels: light from the torch travels in straight lines and spreads out. When the card is near the torch, it blocks a wider part of the spreading light, so the shadow on the screen is larger.
- For (c), look for a pattern in the numbers: 10 × 24 = 240, 20 × 12 = 240 and 30 × 8 = 240, so distance × shadow height is always 240.
- At 40 cm: 240 ÷ 40 = 6, so the shadow should be 6 cm tall.
- Check that it fits the trend: 6 cm is smaller than 8 cm, as expected when the card moves further from the torch.
Answer: (a) The further the card is from the torch, the shorter its shadow. (b) The card blocks more of the spreading light when it is closer to the torch. (c) 6 cm.
Common mistakes
- Saying the Moon gives off light. It only reflects light from the Sun, so it is not a source of light.
- Drawing the light going from the eye to the object. Light travels from the source to the object, then is reflected into the eye, so the arrows on your lines must point towards the eye.
- Thinking a shadow is a reflection or picture of the object. A shadow is an area where light is blocked, so it shows only the outline, with no colours or details.
- Saying translucent objects cast no shadow. They let only some light through, so they cast a lighter shadow.
- Getting the direction wrong: moving the object closer to the light source makes the shadow bigger, not smaller.
- Saying shadows are shortest in the evening. They are shortest around midday, when the Sun is highest in the sky.
Exam tips
- In 'how do we see' answers, describe the whole path: light from the source falls on the object, is reflected by the object and enters the eyes.
- For shadow size questions, say which distance changed and which stayed the same before you give the effect.
- Look for a number pattern in shadow tables, such as distance × height staying the same, before you make a prediction.
Practice questions
1. Easy: Classify these materials as transparent, translucent or opaque: clear glass, tracing paper, a wooden board, frosted glass.
Show answer
Transparent: clear glass. Translucent: tracing paper and frosted glass. Opaque: the wooden board. Transparent materials let most light through, translucent ones let some through and opaque ones let none through.
2. Medium: At 8 a.m., the shadow of a flagpole points towards the west. Where is the Sun, and how will the shadow change by 12 noon?
Show answer
The Sun is in the east, low in the sky, because a shadow always points away from the Sun. By noon the Sun is much higher in the sky, so the shadow becomes much shorter.
3. Harder: Explain why you cannot see a red book in a completely dark room, and how you see it when a lamp is switched on.
Show answer
In a completely dark room there is no light, so no light is reflected from the book into your eyes, and you cannot see it. When the lamp is switched on, light from the lamp falls on the book, the book reflects some of it, and the reflected light enters your eyes, so you see the book.
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