Number 9: The Camera Obscura

In my opinion a home made room sized camera obscura¹ beats TV hands down. Its really simple to make one and the results are just extraordinary. Me and the kids love lying on the bed watching the outside world projected onto the walls and ceiling.

What you'll need:

• A sunny room, preferably with one window.
• A bright day.
• A roll of thick plastic refuse sacks.
• Masking tape.
• Scissors.

What to do.

1) Cut open the sacks and stick them together into a sheet large enough to cover the window(s).

2) Tape the sheet of sacks up against the window.

3) Cut a 1cm hole in the middle of the sheet.

4) Sit back and let your eyes get used to the dark.

5) After a few minutes you should start seeing the outside world projected (upside down) all around your room. Cars will wizz along the walls and you'll see people in the street walking along your ceiling!

View inside the room with the camera obscura set up. It you want to take a photo like this you'll have to use a long exposure (2-4 seconds should do the trick).

For comparison here's the same view out the window.

What's going on?

Basically we've just made ourselves a great big pinhole camera! Light travels in a straight line and crosses over as it goes through the hole in the bags. The light carries on until it hits the wall in the room. So everything on your wall appears upside down.

There are loads of camera obscura's you can visit all over the world. My favourite is the observatory near the Clifton Suspension Bridge in Bristol. It has splendid views of the Bridge and the City.

¹The term camera obscura comes from the latin for 'dark room'. So the word camera is actually just latin for 'room'!

Number 7: The magnetic grape

You remember how in school your teacher told you that only things with iron in them are magnetic; spoons, nails that sort or thing? Well your teacher didn't get that quite right. Because it turns out that  everything is magnetic to some extent. Even grapes. And that's why this happens:

If you want to try this yourself..

You'll need:

• 2 grapes
• A wooden skewer (NOT A STEEL ONE)
• A pin
• Something to stick the pin through so that it points upwards on a stable base. I used an old film canister, but bit of cork would do just as well.
• A neodymium magnet. The stronger the better but you probably want one with at least 20 Kg pull.

Safety:

THIS IS REALLY IMPORTANT

The neodymium magnets are really powerful. So...

1. Don't let kids play with them.
2. Don't put them near your credit cards, phone, watch or any other electrical equipment.
3. Don't put 2 magnets anywhere near each other because they'll fly towards one another, shatter and send chunks flying.
4. If you have any medical implants don't go anywhere near them.
5. Read the safety instructions that come with the magnets.

What to do:

1. Push the 2 grapes onto either end of the skewer

2. Push the pin through the cap of film canister, so that its pointing upward. Put the cap back on the canister.

3. This is the tricky but. You need to balance the skewer and grapes on the point of the pin. If you get close then adjust the balance by pushing one grape further onto the skewer.

4. Once you've managed that just put the edge of the magnet near one of the grapes and watch them spin.

What's going on?

There are actually numerous types of magnetism. At school we only learn about ferromagnetism. Then there's paramagnets (which are attracted to ferromagnets) and diamagnets (which are repelled by ferromagnets). These 2 types of magnetism are much weaker than ferromagnets so you generally can't see the effect. Which is why you need a set up like this and a powerful magnet to observe it.

Since the water in the grape is diamagnetic its repelled by the strong ferromagnet near it and that makes the grapes spin on the pin. 

You can demonstrate the same principle another way shown here.

Incidentally this effect is central to how MRI scanners in hospitals work.