6 mistakes learners make with light and shadows, each one named and explained.
Light travels out from a source in straight lines, and we see an object when light from it reaches our eyes.
You list a mirror, the Moon or a lit page as sources of light.
They are the brightest things in view, and brightness is what a source looks like.
Question. Which of these is a light source: a candle, a mirror, the Moon?
A common answer. All three, because all three are bright
The answer. The candle. The other two send back light made somewhere else
Why. A source makes light. The rest pass it on.
Ask where the light was made.
You describe the shadow as coming out of the object, or as the object's reflection on the ground.
A shadow moves with the object and looks like it, which makes it feel like a possession rather than a gap.
Question. What is a shadow?
A common answer. Something dark the object sends onto the ground
The answer. The area where the object has blocked the light
Why. Nothing is sent out. Light is stopped, and the dark patch is where it did not reach.
Describe a shadow by what is not there.
You say a little light will curve round the edge and fill in the middle of the shadow.
Sound does travel round corners, and shadows in real rooms are soft at the edges because the light comes from more than one place.
Question. Why is the middle of a shadow dark?
A common answer. Some light bends round the object and fills it partly
The answer. Light travels in straight lines, so none of it reaches behind the object
Why. The soft edge comes from the size of the lamp, not from light bending.
Draw the rays with a ruler and see what is blocked.
You say a mirror reflects light but a book does not, although you can see the book.
Reflection is taught with mirrors, so it comes to mean a clear image rather than light bouncing off.
Question. Does a page of a book reflect light?
A common answer. No, only shiny things reflect
The answer. Yes. You see the page because light bounces off it into your eye
Why. A mirror reflects evenly and gives an image. A rough surface scatters, but it still reflects.
If you can see it and it is not a light source, light is bouncing off it.
Your ray diagram has arrows going from the eye to the object.
Looking feels active, like reaching out, and the phrase casting a glance says the same thing.
Question. How do you see a book?
A common answer. Light goes from your eye to the book
The answer. Light from the lamp bounces off the book and into your eye
Why. If eyes sent out light you would see in a completely dark room.
Draw every arrow ending at the eye.
You say moving the torch closer will not change the shadow, because the object has not changed.
A shadow looks like a copy of the object, so it seems to belong to the object rather than to the arrangement of light, object and screen.
Question. What happens to the shadow when the torch moves closer to the object?
A common answer. Nothing, the shadow stays the same size
The answer. The shadow gets bigger
Why. The closer light spreads out more steeply past the object, so it blocks more of the screen.
A shadow is made by three things, not one.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.