Electromagnetic Spectrum – The Whole Nine Yards

We know already that all EM waves are transverse electromagnetic oscillations, travelling at 300 million m/s in free space. We also know that a glass block slows them down to about 200 million m/s.

We can only see a little bit of the whole spectrum – which means everything there is to see.  Our limit is anything from a wavelength of 0.0000004m (violet light) to 0.0000007m (red light). Bees can see in the UV, so this is how they see a yellow primrose, homing in on its UV emissions in the centre.

bee-flowerRadio and TV waves might have a wavelength of several hundred metres, whereas X-rays, gamma rays and cosmic rays might have wavelengths smaller than the diameter of an atomic nucleus. Here’s the picture that tells the whole story.  Everything you really need to know is on it. Dowload a copy or pick one up from me in school.

A few more little details. Microwaves are just low energy infrared rays. Their energies are such that water, fat and sugar molecules absorb them very efficiently. When you microwave food, all you’re doing is boiling the liquid inside it, which is why it’s so fast. For comparison, an hour a day for a year on a mobile phone produces about 10kJ of energy, equivalent to 10s in a microwave oven.

X rays are emitted when electrons are fired at metal targets. We won’t go into details here, except the only real difference between these and gamma rays is that gamma rays are emitted spontaneously from excited nuclei. X rays pass through human tissue, being absorbed by denser material.

In 1901, Wilhelm Roentgen was the first person ever to win the Nobel Prize for Physics. His discovery revolutionised the medical world.


A series of experiments helped him notice that barium platinocyanide emits a fluorescent glow –  X-rays.

In fact, he caught sight of the glow ‘out of the corner of his eye’. Had he been looking directly at the powder, he’d’ve missed it.

Combining his observation with a photographic plate and his wife’s hand, he made the first X-ray photo, and so made it possible to look inside the human body without cutting it open first. Here’s the first ever X-ray. Nice ring, Mrs Roentgen..

Microwaves have just the right amount of energy ( a wavelength of a few cm) to heat water very efficiently. So, a microwave oven gives the microwave energy to the water molecules in the food and the food ‘cooks itself”.

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