Number 25: 'DNA' diffraction with a spring and a laser pointer

Photo 51

Time for my second DNA post and I thought I'd take a look at the data that allowed Watson and Crick to work out the famous molecule's structure.

The crucial bit of information came from a photo taken by Rosalind Franklin and Raymond Gosling.  The image is now quite famous and is known as photo 51.

But photo 51 doesn't much look like a picture of DNA. And thats because it is in fact an X-ray diffraction image taken by shining X-rays at a crystal of DNA. Its a bit of a leap from the photo to the DNA structure but luckily there's a really easy way to demonstrate how an image like this comes from a helical structure.

You'll need:

- a laser pointer

- a retractable ball point pen

Safety:
Adult supervision required here. Be careful with the laser and don't shine it in anyone/anythings eyes.

What to do:

1) Unscrew the pen and remove the spring. The spring is of course a helix, so its going to act as our model for DNA.

2) Shine the laser through the spring, and then onto a white wall or card about 3 meters away. Best to do this at night with the lights dimmed.

You should see an image on the wall that looks a lot like this.

Which also happens to look a lot like photo 51. And that's because the same processe generates both images.

What's going on:

Both photo 51 and the cross you've just made on the wall are formed by a process known as diffraction. To explain what that is we need to remember that light is a wave.  Now imagine two waves meeting each other. If the waves overlap so that he peaks are in the same place then they combine and the result is a wave that is twice as high. But if the peak of one wave meets the trough of the other they cancel each other out, and in the case of light you get a dark spot (you can also see this happening if you shine the laser at a CD). So some of the laser light that diffracts off the spring interferes with other waves of light giving you a cross and the spots. And from the distance between the spots and the angles of the cross you can work out the shape of the spring (or DNA).

Exactly how its done is explained very nicely here.

And a hat tip to Suzie Sheehy who told me about this fab demo.

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'!