Make a rainbow tank

Warning: This activity uses small panes of glass. The edges of the glass can potentially cut if care isn’t taken – remember to use gloves while handling it. If you drop and break any glass, be sure to clean it up immediately with a dust-pan and brush and wrap the shards in paper before putting it into the bin. Have an adult assist with this activity.

You will need

• Two 10 x 15cm picture frames
• Two 8 x 13cm picture frames (check out a ‘dollar store’ for cheap stock)
• Epoxy glue
• Silicon sealant for glass
• Methylated spirits
• Water
• An old rag
• Old newspaper
• Sticky tape
• Scissors
• Blu-tac or plasticine
• Gloves (simple dishwashing gloves work well)

What to do

Take the backs off each of the picture frames and remove the pane of glass.

Tip: Keep the frames – you never know when another Science by Email activity might use them!

Put on your gloves to protect your fingers and cover the work area in newspaper.

Take the two longest pieces of glass and form an upside-down ‘V’, with their long sides touching. It should look like the roof of a house. You might find it difficult to keep them in this position while gluing (and holding) them together, so press several large lumps plasticine around their bases to keep them in place. Keep the area at the ends free of plasticine so it doesn’t get in the way at step 5.

Prepare the epoxy glue as per the instructions on the packet. Once you are satisfied that the glass panes won’t move easily, run some glue along the length of the join.

Run some glue along the short edges of the ‘roof’, and place a smaller glass pane at each end to form ends to the tank. They will also act as legs to hold the tank upright. Leave the glue to dry for a day.

Use sticky tape to cover all exposed edges of the glass panes, making it safer to handle.

Run the sealant along the inside corners of the tank, plugging any small holes (you might need to repeat this after you fill it with water and notice leaks).

Once the sealant is touch-dry, clean the glass with methylated spirits and a rag, and fill the tank with water.

Put the tank on a desk and look through it. What can you see?

What’s happening?

Looking through the tank, you should notice that objects several metres away appear to be edged in rainbow colours.

Visible light is part of what we call ‘electromagnetic radiation’ (EMR). EMR also consists of gamma rays, x-rays, ultra violet light, infra red light, microwaves and radio waves. All forms of radiation act as waves, only some types have longer wavelengths than others. Gamma rays have very, very short wavelengths, and are packed with energy. Radio waves are at the end other end, with big long wave lengths. Visible light is somewhere in between.

As EMR moves through a material, it interacts with its particles. This means when light moves from something thin (such as air), to something dense (such as water), its velocity slows down slightly, a little like a person being slowed down by a dense crowd. This can make it change its direction as well, or ‘refract’, especially if it hits at an angle. The lower the wavelength, the more it refracts.

Each colour is really just a different wavelength we can see, with violet at one end (smaller wavelengths) and red at the other (longer wavelengths). The triangular shape of a prism means light refracts as it enters, and then refracts even more as it leaves, spreading the white light out into a full range of visible colours.

Applications

Even though the rainbow you see is made of visible colours, there are ‘colours’ of light you can’t see.

Early in the 20th century, the astronomer William Herschel was following the latest craze amongst scientists – he was playing with prisms and testing the different colours of light they produced. In this experiment, he was using a thermometer to compare the temperatures of each colour.

To test the room’s temperature, he held the thermometer outside of the spectrum, just past the red line…and was amazed to find it was warmer than when he held it in the light. He had discovered a form of light which could not be seen below the colour red.

We now know there are wavelengths of radiation we can’t see, above violet (ultra violent to gamma rays) and below red (infra red to radio waves). The night sky might seem pretty empty with huge expanses of blackness pocked with bright white stars. Yet astronomers often use special telescopes to see other wavelengths, from powerful x-rays down to quiet radio waves, and some to spy on the radio waves from other galaxies.

 

Source:  Science by Email