How was the electromagnetic spectrum discovered?

[seriously disabled]

Since we can only see visible light, how was the electromagnetic spectrum discovered? How were X-rays, gamma rays, microwaves and radio waves discovered?

[insane_alien]

well, they're all the same thing. once you've discovered one you've discovered them all. the different bits are just wavelength regimes.

[ydoaPs]

Since we can only see visible light, how was the electromagnetic spectrum discovered? How were X-rays, gamma rays, microwaves and radio waves discovered?

 

Send a light through a prism. The place a thermometer next to where the red appears.

[swansont]

The individual parts were discovered in bits and pieces before the realization that it's all EM radiation. Properly addressing your question would require a very lengthy answer.

[seriously disabled]

well, they're all the same thing. once you've discovered one you've discovered them all. the different bits are just wavelength regimes.

 

But how did they discover that these bits are just different wavelength regimes?

[McCrunchy]

Hi,

 

That's an interesting, but ambiguous question - I don't know whether "discovery" is the right term, but there are definitely periods in history where certain wavelength domains were investigated more intensely than others. For example, I heard Terahertz radiation generation (between 1 millimeter (high-frequency edge of the microwave band) and 100 micrometer (long-wavelength edge of far-infrared light)) was long neglected and is currently the object of intense research.

 

The first question one has to address is when radiation was first recognized to be an oscillatory phenomena, ie, to which you can associate a frequency. This would be Young 1803 ( http://en.wikipedia.org/wiki/Double-slit_experiment ). This was the definite proof, but a lot of scientists had already gotten very familiar with this notion well before that : Huygens and his famous principle (1695, http://www.mathpages.com/home/kmath242/kmath242.htm ), Fresnel, etc.

 

They assumed light travelled as a wave, however I don't who first discovered that red was lower frequency than blue. They all obviously understood why a prism decomposes light - the refractive index of a material depends on the colour of the impinging light -, but the relation between refractive index and frequency (see the graph in http://en.wikipedia.org/wiki/Refractive_index : refractive index decreases with increasing wavelength ) is not a thing one can easily deduce, I think, unless you know that light is an EM wave, and that came later with Maxwell ...

 

 

Some other key milestones would be:

 

- 1800, Herschel's discovery of IR radiation. Or let's rather say : he discovered some kind of invisible rays on the red side of the spectrum that heat stuff, see

http://www.practicalphysics.org/go/Resources_16.html

 

- ????, discovery of UVs ? I mean, someone must have discovered that you don't tan behind a glass window !

 

- 1864, Maxwell publishes his famous equation : light is electromagnetic radiation.

 

- 1888, Hertz, antennas, which considerably boosted the discovery of "new" wavelength domains. From there on, "discovering" a new wavelength domain is more of a technological problem than anything else: it's equivalent to mastering the oscillation frequency of the current, at least for low-frequency to microwave type radiation.

 

- 1895, Roentgen, X-Rays, citing Wikipedia:

" He knew the cardboard covering prevented light from escaping, yet Röntgen observed that the invisible cathode rays caused a fluorescent effect on a small cardboard screen painted with barium platinocyanide"

10-20 years later Laue discovered that X-rays were none other than high frequency EM waves

 

- 1900 , Paul Villard, gamma rays. "It wasn't until 1914 that Rutherford showed that they were a form of electromagnetic (EM) like light only with a much shorter wavelength than x rays."

 

- 1903, N-Rays : that's a fun one, see http://en.wikipedia.org/wiki/N_ray

 

- later : microwaves, cellphones

 

- now : terahertz

 

 

I hope this helps. I'm sure there are many more details and versions to be given of this history. I really do wonder when the picture of the radiation spectrum" as we know it ( http://en.wikipedia.org/wiki/File:Electromagnetic-Spectrum.png) first emerged in textbooks. It would be fun to compare what this picture looked like back in 1920, 1950s and now.

 

Cheers,

 

McCrunchy

[Mr Skeptic]

But how did they discover that these bits are just different wavelength regimes?

 

Basically because Maxwell's equations explained them all by considering them electromagnetic waves of a certain wavelength/frequency. The easiest giveaway was that they all traveled at c.