What a signal is
A signal is something you do on purpose so somebody else knows something.
Signal Something you do on purpose to send a message.
Step 1: Something to Notice
Watch first. The explanation comes later.
A lighthouse flashes on and off all night. Ships miles away know exactly which lighthouse it is and where the rocks are.
How can a light that only goes on and off say anything at all?
Step 2: Find Out
Use the lamp and the block to make a signal that somebody could see.
Step 1 — Predict
To make a flash that somebody can see, which do you need?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Light goes a long way
A shout fades after a street or two. A light can be seen from miles away.
Agree first
Agree what it means before you send it. One flash for yes, two for no.
On and off
You need not switch the torch off. Put your hand in front and the light is blocked.
Step 4: A Common Mistake
Lots of people think
“To tell somebody something far away, you have to shout louder.”
Step 5: Signals People Really Use
The same idea somewhere new.
Traffic lights, a lighthouse, the indicator on a car, a torch flashed from a tent. All of them are light used on purpose to say something.
Step 6: Think It Through
Practice makes it stick.
Across the Field
Problem 1 of 1
Your friend is far across a field at night. What is the best way to signal to them?
Sending a Message
1 of 3
Which one could send a message a long way at night?
2 of 3
What has to happen before a flashing signal can mean anything?
3 of 3
Put each picture in the right box.
Tap something to move it.
- Empty
- Empty
Step 7: Quick Check
Show what you know.
Question 1 of 1
Why can a lighthouse tell a ship something from miles away?
What You Found Out
- A signal is light used on purpose to say something.
- Light travels much further than a shout.
- Both people have to agree what the signal means.
For the grown-up
People send messages with light — A lighthouse flashes to warn ships. Traffic lights tell cars to stop or go. A torch flashed on and off can spell out a message. The light itself carries the meaning.
Both people must know the code — A flashing light only means something if the person watching knows what it means. Red meaning stop works because everybody was taught the same thing.
What a signal is — A signal is something you do on purpose so that somebody else knows something. Waving is a signal. So is a flashing light. The thing itself is not the message — the message is what you and the other person agreed it would mean.
Light goes a long way — A shout fades away after a street or two. A light can be seen from miles off, and at night from further still. That is why a lighthouse uses a light and not a bell: ships far out at sea would never hear it.
Agree first — Agree what the signal means before you send it. One flash for yes, two for no. If the other person has not been told, they see a light going on and off and learn nothing at all. Red means stop only because everybody was taught the same thing.
On and off — You do not have to switch a torch off to make it flash. Put your hand in front and the light is blocked; take it away and it gets through again. The person watching sees on, off, on — which is enough to carry a message.