KS3 · Physics
How shadows form
A dark shape follows you everywhere on a sunny day, copying your outline but never your colours. So what is it made of? Nothing at all.
Physics · Light and shadows
Layers
Explore
Tap a part · switch layers on and off
The shadow is the unlit patch on the screen, exactly between the two edge rays, on the far side of the object from the lamp.
A side view of a lamp, an opaque object and a screen. Tap each part to see what it tells you, and switch the layers off and on to build the diagram yourself.
Physics · Light through materials
Which materials block light?
Pick a material, then choose what it does to light.
Still to sort
Transparent (0)
Lets light travel through it, so you can see clearly through it.
Where the line is: Transparent and translucent both let light through. The difference is whether you can see clearly through it.
Translucent (0)
Lets some light through, but not in a clear way.
Where the line is: Translucent isn't opaque: light still gets through, even though you can't see clearly through it.
Opaque (0)
Doesn't let light through. It blocks it.
Where the line is: Opaque means no light gets through at all, so an opaque object leaves an unlit area behind it.
A shadow needs something that stops light. Sort each material by what it does to light.
The shadow chain
Reason it through
Why is there an unlit area behind an opaque object?
First link · your turn
Light leaves the lamp. How does it travel?
Predict, then check
A shadow-puppet show. The lamp and the screen stay exactly where they are. Only the puppet moves.
You slide the puppet closer to the lamp. What happens to its shadow on the screen?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- Transparent materials let light through clearly, translucent materials let some light through but not clearly, and opaque materials block light.
- Light travels in straight lines at an extremely high speed, spreading out as it goes, so places further from a lamp are less brightly lit.
- A light ray is an arrow on a diagram showing the direction some light is travelling. It's a model: light isn't made of rays.
- A shadow is an unlit area in the shape of an opaque object, on the opposite side of the object from the light source.
The big picture
Light travels in straight lines. When an opaque object blocks it, the light can't bend around the object, so the area behind it is left unlit. That unlit area, in the object's shape and on the opposite side from the light, is a shadow. The rays that just skim the object's edges show exactly where the shadow ends.
Key points
Worked example
Problem
A torch shines at a wall. Between them you hold a card cut into the shape of a capital letter L. Describe, using a ray diagram, where the shadow forms and what it looks like.
⚠ Watch out
Drawing rays that curve round the object's edges, or putting the arrowheads pointing back towards the lamp. Rays are always perfectly straight, and they point away from the light source, because that's the direction the light travels.
Memory hook
A shadow is a hole in the light, not a thing on the wall. Straight lines can't bend, so the object leaves a gap exactly its own shape.
Check yourself
Cover the page. Using the words 'straight lines' and 'opaque', explain why a shadow lands on the far side of an object from the lamp. Then sketch the two rays that fix its edges.
Flashcards
(14)What does a transparent material do to light?
What does a translucent material do to light?
What does an opaque material do to light?
How does light travel?
Why are places further from a lamp less brightly lit?
What is a light ray?
What happens when light hits most surfaces?
What is a shadow?
Why can't light reach the area behind an opaque object?
Which rays fix the size of a shadow on a ray diagram?
What is the difference between a shadow and a reflection?
What colour is a shadow?
What does light from a second lamp do to a shadow it reaches?
What happens to a shadow when the object moves closer to the lamp?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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How this lesson was checked. This KS3 Physicslesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.