Shadow Experiment for Kids: Size, Distance and Light

A shadow puppet show is already a good evening. What most families miss is that it is also one of the few experiments where a five-year-old can make a prediction, test it with a ruler, and be right. Put a 10 cm cardboard cat 20 cm from a lamp, with the wall a metre away, and its shadow is 50 cm tall. Move the cat to 10 cm and the shadow fills the wall at 100 cm. Halve the distance, double the shadow, every time. This shadow experiment for kids turns a bedtime game into a measurement, and it also answers a question children ask constantly and rarely get a straight answer to: why are some shadows crisp and others blurry?

Key Takeaways

• Shadow height equals the object’s height multiplied by the screen distance divided by the object’s distance from the lamp. Move the object closer to the light and the shadow grows in exact proportion.
• Sharp shadows need a small light source. A big source creates a fuzzy border called the penumbra, and you can calculate its width.
• A glass of water does not make a simple shadow. It casts a bright line inside a dark patch, which is worth seeing.
• Best for ages 5 to 10 for the puppets, with the measuring and graphing from about age 8.
• What usually goes wrong: using a ceiling light or a phone torch, both of which are too large a source to give crisp shadows.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Small, bright light source	1	A bare LED bulb in a desk lamp with the shade removed, or an LED torch with a tight beam	A phone torch works for the puppet show but gives fuzzy edges; that becomes a measurement later
White wall or a hung sheet	1	n/a	A large sheet of white card taped to a chair back
Stiff card for puppets	2–3 sheets	Cereal boxes	Paper plates, or cut-up delivery boxes
Skewers or straws	1 per puppet	n/a	Pencils taped to the back
Tape measure and a 30 cm ruler	1 each	n/a	n/a
Masking tape	A few strips	n/a	To mark fixed distances on the floor
Red and green cellophane	2 small squares	Stationery	Only for the coloured-shadow extension; two layers each
A clear glass of water	1	n/a	Any clear tumbler. The surprise of the session
Tracing paper and clear plastic	A square of each	n/a	To compare opaque, translucent and transparent

Time: prep 20 min making puppets, experiment 25 min, cleanup 5 min Cost: nothing Mess level: low

Do the Experiment: A Shadow Experiment for Kids

Step 1 - Set up fixed distances and make the source small

Stand the lamp on a table and mark the floor or table with tape at 10, 20, 25 and 50 cm from the bulb, then put the screen or wall exactly 100 cm from the bulb. Remove the lampshade. The smaller and brighter the source, the crisper everything that follows will be.

Step 2 - Make one puppet exactly 10 cm tall

Cut a simple silhouette, a cat or a bird, exactly 10 cm from top to bottom, and tape it to a skewer. A known object height turns the whole activity from a game into an experiment.

Step 3 - Ask for the prediction, then measure three positions

Ask: “If I move the puppet twice as close to the lamp, what happens to the shadow? Stays the same, gets twice as big, gets half as big?” Write the answer down. Then hold the puppet at 50 cm, 25 cm and 20 cm from the lamp, and measure the shadow height on the wall each time.

What you should see: Roughly 20 cm, 40 cm and 50 cm. The rule is shadow = 10 cm × (100 ÷ distance from lamp). Move the puppet to 10 cm from the lamp and the shadow is a full metre tall. The proportionality is exact enough to check with a tape measure, which is unusual for a home experiment.

Step 4 - Compare three light sources and look at the edges

Keep the puppet at 20 cm and swap the light: bare LED bulb, then a phone torch with its diffuser, then daylight from a window.

What you should see: The bare bulb gives a shadow with a crisp edge. The phone torch gives a soft border a centimetre or two wide. The window gives a shadow so soft it is barely a shadow at all. Measure the width of the fuzzy border each time; it is the penumbra, and it scales with the size of the source.

Step 5 - Test things that are not solid

Hold up cardboard, then tracing paper, then clear plastic film, then the glass of water.

What you should see: Cardboard gives a black shadow. Tracing paper gives a grey one, because it transmits some light and scatters the rest. Clear plastic gives a barely visible shadow with a bright edge. And the glass of water casts a dark ring with a bright line down the middle, because the curved water surface acts as a lens and concentrates light into the centre of its own shadow. Children rarely see that coming.

Step 6 - Make coloured shadows

Put two layers of red cellophane over one torch and two layers of green over another, then shine both at the puppet from slightly different angles.

What you should see: Two shadows, one red and one green, with a black region where both torches are blocked. Each coloured shadow is the place where one torch is blocked but the other still reaches. This is the same additive mixing as in why paint mixing and light mixing disagree.

What’s Actually Going On

Light travels in straight lines, and a shadow is simply the region a straight line from the source cannot reach because something opaque is in the way. That is the whole mechanism, and the rest is geometry.

Draw a line from the lamp filament past the top of the puppet and on to the wall, and another past the bottom. Those two lines and the wall make a triangle similar to the one formed by the lamp and the puppet. Similar triangles give equal ratios of corresponding sides, so shadow height divided by object height equals screen distance divided by object distance. Written out: S = O × (D_screen / D_object). Nothing in that expression depends on brightness, colour or the material of the puppet, which is why the measurement is so reliable.

The edge is where it gets interesting. A genuinely point-like source throws a shadow with one region only: the umbra, where no light from the source arrives. A real source has width, and each point across that width casts its own slightly offset shadow. The overlap of all of them produces a fully dark umbra surrounded by a partly lit band, the penumbra, where some of the source is visible past the edge of the object and some is not. The same two names describe the shadow the Moon throws on Earth during an eclipse, which is why NASA’s description of the types of solar eclipse turns on whether you stand in the umbra or the penumbra.

The penumbra’s width on the screen is roughly the source width multiplied by (D_screen − D_object) / D_object. For a 5 mm LED with the puppet at 20 cm and the wall at 100 cm, that is 5 × (80/20) = 20 mm, or 2 cm of fuzz. Swap to a phone torch with a 15 mm diffuser and you get 6 cm. That calculation explains, in one line, why the crispest shadows happen on a sunny day with the object close to the ground, and why overcast light casts almost none.

Two misconceptions worth heading off, both documented in the research. Björn Andersson and Christina Kärrqvist’s 1983 study of how Swedish pupils aged 12 to 15 understand light, in the European Journal of Science Education, found that many students do not treat light as something that travels from a source to an object to an eye, which is the premise every shadow argument depends on. Younger children frequently talk about the shadow as a possession of the object, something it carries, rather than as a place light has not reached. The fastest cure is Step 3: if the shadow belonged to the cat, moving the cat would not change its size.

The second one is subtler. Many children assume a shadow must be the same shape as the object, and the glass of water in Step 5 breaks that cleanly. The water surface refracts light inward, concentrating it into a bright caustic line inside the dark region. The object is not blocking light so much as redirecting it, which is a genuinely different thing and a nice bridge to refraction.

How to Explain It to Your Kid

Ages 5–8: A shadow is where the light did not get to

Have them stand in front of the lamp and look at their own shadow, then put a hand up close to the lamp and watch the shadow swell to cover the wall. Say it plainly: light goes in straight lines, and the shadow is the place the light could not reach because they were in the way. Then run the puppet show, letting them discover that moving the puppet toward the light makes a giant. That discovery is the physics, and they do not need the formula.

Ages 9–12: Similar triangles, measured

Draw the two triangles and label the sides. Do the three measurements in Step 3 and write them in a table, then plot shadow height against 1 ÷ (distance from lamp). The points fall on a straight line through the origin, and the slope is the object height times the screen distance. A ten-year-old can read a prediction straight off that graph for a distance they never tested, which is the first time many children use a graph as a tool rather than a decoration.

Ages 13+: Penumbra width and extended sources

Give them the penumbra formula and have them predict the fuzzy band’s width for three sources whose sizes they have measured, then check with a ruler. Then push to the astronomical case: use the same geometry to work out why a total solar eclipse is visible only along a narrow track, given that the Moon’s umbra on Earth is at most a couple of hundred kilometres wide. This is the same ray geometry as a pinhole camera, where the hole is small and the object is the light source.

The question to ask: “If I put the puppet exactly halfway between the lamp and the wall, how tall will the shadow be compared with the puppet? Say the number before we measure it.”

Why Teaching This Early Pays Off

Shadows are the first place most children can be shown, rather than told, that light travels in straight lines. That single idea underlies reflection, refraction, image formation, eclipses and the entire ray-diagram apparatus of secondary physics. Jane Bray Nelson and Jim Nelson put shadows and straight-line propagation as the fourth activity in their geometric optics teaching sequence for exactly that reason: it is the foundation the rest stands on.

The proportional reasoning is worth as much as the optics. Shadow size varies inversely with distance from the lamp, which is a genuinely inverse relationship a child can verify by hand with a tape measure. Inverse proportion is a known stumbling block in school mathematics, and most children first meet it as an abstraction on a worksheet. Meeting it as a cat whose shadow doubles when you halve a distance gives it somewhere to live.

Be realistic about the scope. An evening of shadow puppets will not teach optics. What it does, reliably, is give you a shared physical reference you can return to for years, and give a young child the experience of making a numerical prediction that turns out to be right.

Before You Start: What Parents Should Know

Best age	5 to 10 for the puppets; 8 and up for measuring and graphing
Supervision	Light. An adult sets up the lamp and handles any bulb
Real hazards	An unshaded incandescent or halogen bulb gets hot enough to burn and to scorch paper; use an LED bulb, which stays cool, and keep card away from any hot lamp. Trailing lamp cables are a trip hazard for children walking backwards to look at shadows. Skewer points should be blunted or taped
Don’t	Don’t use the sun as your light source for the measuring part, and don’t let a child look toward the sun to find the light. Shadow geometry works indoors, and outdoor sun means someone will eventually look up

When It Doesn’t Work

• Every shadow has fuzzy edges. Your source is too big. A ceiling light, a lampshade or a phone torch with a diffuser all behave as extended sources. Use a bare LED bulb or a torch with a tight beam, and dim the room.
• The shadow is the wrong size compared with the formula. Measure the distances again, from the bulb rather than from the front of the lamp housing, and along the line to the wall rather than across the floor if the lamp and puppet are at different heights.
• The shadow is faint rather than dark. Too much ambient light. Switch other lamps off and close curtains. You want one source only.
• Two overlapping shadows from a single lamp. The lamp has more than one LED. That is two sources, and each one casts its own shadow. Cover all but one LED with tape, which is an informative fix rather than a workaround.

Take It Further

• Build the graph properly. Measure shadow height at six distances from 10 cm to 60 cm, three repeats each, and plot against the reciprocal of the distance. A straight line through the origin confirms the model, and the spread between repeats is your error bar.
• Measure the penumbra and predict an eclipse. Measure your light source’s physical width, predict the penumbra width at two puppet positions, then check. Then apply the same geometry to the Sun, Moon and Earth and discuss why eclipse tracks are narrow.
• Make a shadow clock. Put a vertical stick in a pot of soil on a sunny windowsill, with your back to the sun, and mark the shadow tip every half hour on a sheet of paper. The tip traces a curve, not a straight line, and the direction and length encode the time of day. Pair it with the frame-by-frame thinking in a homemade thaumatrope for a whole afternoon of light.

What to Watch For Over the Next 3 Months

• Week 4: They predict the size of their own shadow before stepping into a streetlight’s beam, or complain that an overcast day has “no shadows”.
• Month 2: The words “umbra” and “penumbra” appear, or at least “the fuzzy bit”, used about a real shadow outdoors.
• Month 3: If they ask why the shadow of a tree’s leaves has round spots of light in it during an eclipse, they have connected shadows to pinhole images. Those spots are images of the Sun, and they go crescent-shaped when the Sun does.

Frequently Asked Questions

Why are shadows outdoors sometimes sharp and sometimes blurry?

Because the Sun is a small source on a clear day and effectively a huge one when the sky is overcast. A small source gives a narrow penumbra and a crisp edge; a large source spreads the penumbra until the shadow fades away entirely.

How do I calculate the size of a shadow?

Multiply the object’s height by the screen distance and divide by the object’s distance from the light. A 10 cm object 20 cm from a lamp, with the wall 100 cm from the lamp, gives 10 × 100 ÷ 20 = 50 cm.

Why does a glass of water make a bright spot in its own shadow?

Because the curved water surface refracts light inward like a lens, concentrating it into a bright caustic line. The glass is redirecting light rather than only blocking it, which is a different mechanism from a simple shadow.

Can we do this with a phone torch?

For the puppet show, yes. For sharp-edged measurements, no: the diffuser makes it an extended source and the edges blur by several centimetres. Comparing a phone torch with a bare LED is itself a good experiment.

Why does my child’s shadow have fuzzy feet but a sharp head, or the other way round?

Because the parts of the body at different distances from the screen have different penumbra widths. The closer a part of the object is to the surface, the sharper its shadow. Watch a hand approach a wall and the edges snap into focus.

---

About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

---

Sources

1. NASA Science. “Types of Solar Eclipses.” https://science.nasa.gov/eclipses/types/
2. Andersson, B., & Kärrqvist, C. (1983). “How Swedish pupils, aged 12–15 years, understand light and its properties.” European Journal of Science Education, 5(4), 387–402. https://doi.org/10.1080/0140528830050403
3. Nelson, J. B., & Nelson, J. (2020). “Activity 4: Shadows and Straight-Line Propagation of Light.” In Teaching About Geometric Optics. AIP Publishing. https://doi.org/10.1063/9780735422179_004
4. Galili, I., & Hazan, A. (2000). “Learners’ knowledge in optics: interpretation, structure and analysis.” International Journal of Science Education, 22(1), 57–88. https://doi.org/10.1080/095006900290000
5. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165