REPOST: This was originally written in 2007 but is being reposted because of some discussion it generate elsewhere.

There is a lot more to the universe than the light you and I can pick up with our eyes and brains. Although its a shame that we can’t see them naturally we can use technology to reveal the wavelengths of light normally invisible to us. Visible light is only part of the electromagnetic spectrum (a very small part) and I thought it would be interesting to see some familiar objects in unfamiliar ways.

The electromagnetic spectrum is usually split into seven parts: the radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray. You can find a good schematic of the EM spectrum over on Wikipedia. We obviously see things in the visible, but some creatures, such as bees, see some of the ultraviolet (one reason there are so many purple flowers). Here is a picture of a Geranium in the visible and then the UV. You can find more images like these, for all kinds of flowers over at this website.

But this is an astronomy blog and so here are some objects seen in multiple wavelengths. Some of them may surprise you. First up is the Moon. Here we have it in the radio, microwave, infrared, visible, ultraviolet and x-ray.

The infrared image (top right) shows various bright spots. These are warm areas on the Moon. The brightest spot, in the bottom-center of the Moon is the crater Tycho, which also shows up brightly in the visible and UV images. The X-Ray picture is from ROSAT (bottom-right). Here we see the Moon shown in its quarter phase to highlight that it is only reflecting X_Rays from the Sun and not giving any off itself. The Submillimetre (microwave) image (top-centre) was taken using the SCUBA camera on the JCMT in Hawaii. This is a camera normally used to image dust in nebulae and faint galaxies.

Next let’s look at something more exotic but still familiar, the Andromeda Galaxy. Also called M31, this is closest neighbouring galaxy in the wider universe and is just about visible from a good site. We are seeing this in the radio, microwave, infrared, visible, ultraviolet and x-ray.

You can really start to see now why observing objects in different wavelngths can tell us something about them that we didn’t know before. The different details in each image are coming from different parts of the galaxy. In the top-center image, which is taken at 175 micrometres, we are seeing the dust lanes between stars in the galaxy. The X-Ray image in the bottom-right shows us only the extremely energetic objects, which as we can see, are located predominantly toward the centre.

Another easy to spot object, which you may know of is M45, also known as the Pleiades or Subaru. Once again here it in the radio, microwave, infrared, visible, ultraviolet and x-ray.

The most impressive image here in my opinion is the X-Ray image from ROSAT (bottom-right). It looks to me like something from Doctor Who. It is also interesting to note that M45 doesn’t show up as much in radio frequencies. In the top-centre image we can see only some of the stars. This is because M45 is a young cluster and some of the surrounding dust still obscures the stars in this far-infrared image.

I thought I’d save the best til last: here is the Earth. Now its tricky to get pictures of our own planet - because we’re on it. We’re lacking in any good radio images of the Earth because you can do radio astronomy perfectly well without putting telescopes in space. However I really like these images because they really took me by surprise. In order, these images show the Earth in infrared, visible, ultraviolet, extreme-ultraviolet, x-ray and gamma rays.

The first image shows the infrared emission on the Earth from water vapour. Like the two UV images, this picture is from an extensive network of Earth-observing satellites that are attempting to investigate how our planet operates. Tectonics, oceanography and climate change are just three of the many topics being studied to ever-more depth by NASA, ESA and researchers the world over. The X-Ray image (bottom-centre) shows an aurora on the Earth’s north pole. Other than this high-energy interaction between the atmosphere and the Sun, the Earth seems to be invisible at this wavelength.

Finally we have the gamma ray image (bottom-right). What you’re seeing here are extremely high-energy particles, mostly from outer space, reflecting off the Earth’s atmosphere. The edge of the globe is seen to be much brighter than the center because cosmic rays hit the atmosphere at a shallow angle and are more likely to create detections. There is an imbalance btween the East and West due to the Earth’s magnetic field, which is asymmetrical.

I hope you’ve enjoyed this little collection, if you know of any other cool multiple wavelength images - astronomical and otherwise - then please leave a comment with a link.

Astronomer studying star formation, like myself, use telescopes that can see though the pretty optical exteriors of nebulae into the interiors, where very cold dust radiates in the submillimetre and microwave regimes.

Microwaves, fall on the electromagnetic spectrum, between radio waves and infrared waves. They are usually around the size of a few centimetres and you may well be very familiar with them as they are produced by the microwave oven that might just be sitting in your kitchen.

Microwave ovens use a particular microwave frequency to excite molecules of water. Since water is present in lots of food and drink, this means that microwaves heat up lots of useful stuff - and they do it quickly. The fact that microwaves are now readily available to most of us in the western world and they are only a few centimetres in length, means that you can measure the speed of light in your very own home.

What You Need:

The quickest and tastiest way to perform this little experiment is with marshmallows, but chocolate chips also work. You’ll obviously need a microwave oven as well, and a large, microwaveable dish. You will need a ruler, too.

What to Do:

Get your large, microwaveable dish and place a layer of marshmallows at the bottom of it.Remove the turntable from the bottom of the microwave oven. If you don’t, then this experiment will not work at all. If your microwave doesn’t have a turntable, it means that the turning mechanism is elsewhere and you’ll need to find a regular microwave oven to try this experiment.

Cook the marshmallows on a low heat for a couple of minutes, or until you see parts of the marshmallows starting to bubble. When you do, remove the dish and take a look at the marshmallows.

You ought to see that they have not melted evenly. In fact you should be able to see a regular pattern has formed, drawn out in melted-mallow. It depends on your microwave oven, but you should see a melted/unmelted pattern across the dish in some direction. When I tried it at home, my oven created long melted strips next to long unmelted strips (see above).

This regularity is caused by the same mechanism that heats up the food you place into your microwave oven. The appliance generates microwaves which very quickly form standing waves (see animation above) inside the cavity inside, where you put food. As the food rotates around, it passes through the standing wave nodes and this excites the water molecules, heating the food.

Measure the Microwaves:

Take your ruler and measure the distance between the melted parts of the marshmallows. You should find that there is an even pattern of melting and that the distance between them is something like 5 or 6cm. Why? Because that is the distance between the nodes of the standing waves.

Without the rotating mechanism, the food does not move around and cook evenly, instead it just heats at the nodal points. Using your marshmallows you have created a ‘map’ of the microwaves in your microwave oven!

Find the Frequency:

Finally you need to know the frequency at which your microwave oven operates. It is usually written on the back somewhere in small writing. Most standard microwave ovens operate at 2450 MHz. If you cannot find the value on the back of the oven, you can take it for granted that 2450 MHz is about correct.

Measure the Speed of Light:

Now you have what you need to measure the speed of light. You just need to know a very fundamental equation of physics:

Speed of a Wave (c) = Frequency (f) x Wavelength (L)

The distance between the melted sections of the marshmallow is in fact L/2, because there are two nodes for each wave (see animation). So if you have measured 6cm and your oven operates at 2450 MHz, then your measured speed of light is (0.12 x 2450,000,000) 294,000,000 metres per second.

The agreed value of the speed of light through a vacuum is 299,792,458 metres per second. See how accurately you can measure it? what could you do to make the experiment better, and thus get a closer answer?

Now You Can Eat the Gooey Melted Marshmallows:

…and make yourself sick. Yay!