seegongsik
Grade 4 (age 9-10)

grade 4 Shadows and Mirrors

Switch on a torch in front of a wall, make a rabbit shape with your hands, and a rabbit shadow appears on the wall. Light travels in straight lines, so when an opaque object blocks it, a dark shadow forms behind it where no light reaches. Bring the torch close to your hands and the shadow grows; move it far away and the shadow shrinks. A mirror does not soak up light but bounces it back, and the angle at which light hits the mirror always equals the angle at which it bounces off. This is called the law of reflection. In the first picture, move the torch toward the object and measure the shadow; in the second, change the angle the light comes in at and compare it with the angle it reflects at.

Why do shadows form?

far
🌗 What a shadow needs
①Something that gives off light, such as a torch or the sun
②An object that blocks the light
③Light travels straight, so it cannot bend around behind the object
④Opaque objects like paper or wood make dark shadows, and clear objects like glass make very faint ones
⑤The closer the torch is to the object, the wider the light grazing its edges spreads, so the shadow grows

A mirror bounces light back

40°
🪞 Reflection in a mirror
①When light hits a mirror, it is not soaked up but bounces back
②Measured from a line at right angles to the mirror, the incoming angle is the angle of incidence and the outgoing angle is the angle of reflection
③The two angles are always equal; this is the law of reflection
④You see yourself in a mirror because light from your body reflects off the mirror into your eyes

Summary

What a shadow needs
Light + object that blocks it → shadow
A dark shadow forms behind the object where the light is blocked
The law of reflection
Angle of incidence = angle of reflection
Light leaves a mirror at the same angle it came in at
🎯 Key points
①Light travels in straight lines
②A shadow forms behind an opaque object that blocks light
③The closer the light source is to the object, the bigger the shadow
④Light bounces off a mirror at an angle of reflection equal to the angle of incidence
⑤Clear objects let most light through, so their shadows are faint

Examples and Unit Test

Example 1
If you move a torch closer to an object, what happens to the size of the shadow on the screen?
1
A shadow forms behind an opaque object that blocks the light.
2
The closer the torch is, the wider the light grazing the object's edges spreads, so the shadow on the screen gets bigger.
▸ The shadow gets bigger
The words under the first picture show how many times bigger than the object the shadow becomes as the torch comes closer.
Example 2
Light hits a mirror at an angle of incidence of 30°. What is the angle of reflection?
1
When light hits a mirror, it bounces off at the same angle it came in at (the law of reflection).
2
The angle of incidence is 30°, so the angle of reflection is also 30°.
▸ 30° (angle of incidence = angle of reflection)
Set the angle to 30° in the second picture and you will see the upper and lower arcs are exactly the same size.
Unit-test style
Which statement about shadows and mirrors is correct?
①Shadows form because light bends as it travels
②In a mirror, the angle of incidence equals the angle of reflection
③The farther the light source is from the object, the bigger the shadow
④Clear glass makes a dark shadow
⑤A mirror soaks up all the light
▸ ② In a mirror, the angle of incidence equals the angle of reflection
1
Light hitting a mirror reflects at the same angle it came in at, so the angle of incidence equals the angle of reflection.
2
Light travels straight, which is why shadows form (①), the closer the light source the bigger the shadow (③), clear objects let light through so their shadows are faint (④), and a mirror reflects light (⑤).
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