Straight Shooters: How Light Travels (and the Ghost Coin)

We are learning to identify sources of light, to explain that light travels from a source in straight lines, and to describe how shadows are formed when that path is blocked.

Before you startAbout 55 minutes · what success looks like · notes for the helper

About 55 minutes. It's fine to stop after the Quick Check and come back another day: the lesson opens where you left it, at the Quick Check.

You'll know you've got it when

I can sort objects into light sources (which make light) and reflectors (which only bounce it)
I can explain that we see something only when light from it travels in a straight line into our eyes
I can predict how a shadow will change when the light, the object or the surface moves, and explain why

For the helperhow to sit beside this lesson, and what this step is for

This step: Spark

Start with the question at the top and chat about it. There is no wrong answer here: it gets your learner thinking about what they already know.

Getting ready: A mug, a coin and a jug of water for the Ghost Coin, a torch, and a room that can be made dark. Shadow Puppet Theatre needs a torch, a wall and cardboard for cut-outs; the Three-Card Proof needs three pieces of card with a hole punched in each, blu-tack or pegs and cups, and a skewer or a piece of dry spaghetti; the Source and Bouncer Muster needs a look around the house and just outside at night, with you there if the child goes outdoors. Nothing in this unit involves looking at the Sun. All of it takes about 55 minutes; many families stop after the quick check and do the explore task as a second sitting, in a room that can be made dark.
What understanding sounds like: Listen for the three-step trip said aloud, 'lamp to page to my eyes', and for sources and reflectors sorted with the dark-box test: 'it wouldn't glow in a sealed box, so it's a reflector'. With the torch, listen for a prediction and its reason: 'closer to the torch, the shadow gets bigger, because my hand blocks more of the spreading light'.
A likely mix-up, and something that helps: The two big ones are that eyes send out sight-rays, and that the Moon is a light source. A shadow being 'given off' is a third. You might ask, 'If eyes sent out rays, could you see in a pitch-black room?' and 'Why is only the Sun-facing side of the Moon ever lit?'
If it comes easily: For a child who is ahead, the trickiest object in the house to classify, such as a glow-in-the-dark star, makes a good case to argue using the dark-box test.
If it's a struggle: If it is a struggle, you might do the dark-room torch test together first and say each rule as you see it happen, one at a time. If the child shuts down, a few minutes of shadow puppets is real evidence of the third criterion, and you might note what they predicted as a Notice in the Parent Hub. Talking through 'source', 'reflect' and 'opaque' first, in your home language if that is easier, can help.

Step 1 of 6 · Spark

Lesson 1 of 4 · Light Detectives: Shadows, Mirrors and Bending Beams

The ghost coin

For the helper

Start with the question at the top and chat about it. There is no wrong answer here: it gets your learner thinking about what they already know.

Try this RIGHT NOW — it takes one mug, one coin and thirty seconds. Drop the coin in the empty mug. Step back slowly until the mug's rim just barely hides the coin from view. Freeze — don't move your head. Now have your helper slowly pour water into the mug... and watch. The coin RISES BACK INTO VIEW. You didn't move. The coin didn't move — it's sitting flat on the bottom the whole time. Yet somehow, a coin you could not see is now a coin you can see. Something invisible between the coin and your eye has changed its behaviour. By the end of this unit you'll be able to explain the whole trick — and today you'll crack the first half: why the rim could hide the coin in the first place.

No water: coin hidden behind the rim. Add water: the coin reappears. Nothing moved. The Ghost Coin case is now open.