Why Shadows Change Size
A shadow grows longer when the light source is lower because the same-sized object blocks light rays at a shallower angle, stretching the shadow on the ground.
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Why does the same object cast a longer shadow when the Sun is low and a shorter shadow when the Sun is high?
A tall flagpole at noon casts a tiny shadow, but the same pole at sunset stretches across the playground.
It seems natural to think the shadow grows because the Sun moves closer, but the Sun is always far away—so what really stretches the shadow?
A side-by-side diagram of one object and its light rays at a high vs. low sun angle, showing how ray angle changes the shadow length.
A shadow's size depends on the angle at which light hits the object: a low angle spreads light rays out into a long shadow, while a high angle squeezes them into a short one.
- shadow color and penumbra
- seasonal Earth–Sun distance
- light sources other than the Sun
- 01The Same Pole, Two Different ShadowsslideSlot 1Hook
Show a flagpole at noon with a small shadow and the same pole at sunset with a very long shadow stretching across the ground.
- Same object, very different shadow lengths
- Only one thing changed: the Sun's position in the sky
PhenomenonAn object's shadow changes length throughout the day even though the object stays the same size.
QuestionIf the object hasn't changed, what makes its shadow grow longer or shorter?
- 02Is the Sun Getting Closer?slideSlot 2Tension
Surface the common guess that shadows stretch because the Sun is farther away at sunset, then show why distance is the wrong explanation.
- Common intuition: the Sun feels weaker at sunset, so it must be farther
- The Sun's distance barely changes during the day
- The real variable is angle, not distance
PredictionMany people predict that shadows grow longer because the Sun moves farther from Earth in the evening.
Tempting intuitionA dimmer, redder Sun feels more remote, so a 'farther Sun' explanation sounds reasonable.
- 03Light Rays and Shadow LengthinteractiveSlot 3Reveal
A simulation where the user changes the Sun's angle and watches the same object's shadow stretch or shrink, with light-ray arrows drawn from the Sun across the object to the ground.
- Drag the Sun up or down in the simulator
- Observe how ray angle changes shadow length
- Notice that shadow direction also flips across noon
EvidenceMoving a light source lower in the sky makes the shadow of a fixed object longer; raising it shortens the shadow.
ConclusionShadow length is set by the angle between the light rays and the ground, not by how far away the light source is.
Mechanism- 1Light travels in straight rays, and the object blocks any ray that tries to pass through it
- 2When the Sun is high, those blocked rays hit the ground almost straight down, so the shadow barely extends past the object's base
- 3When the Sun is low, the blocked rays graze the object at a shallow angle and travel far across the ground before hitting it, stretching the shadow
- 04Use Angle to Predict Any ShadowslideSlot 4Takeaway
Apply the rule to a new case: a streetlamp tilted toward the ground versus held upright, predicting which casts the longer shadow on the pavement.
- Lower light angle → longer shadow
- Higher light angle → shorter shadow
- Angle, not distance, is the deciding factor
TransferA tilted streetlamp sends light rays across the ground at a shallow angle, so its pole casts a long shadow just like a low Sun does.
Expected inferenceIf a new light source is placed closer to the ground, the learner should predict a longer shadow even if the bulb is brighter or nearer.
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