# Visible light wavelength discrepancy on the EM spectrum?

Hello all,

I'm not all too familiar with the electromagnetic spectrum, nor generally with physics but there is something that keeps me up. I learned, and also read now everywhere that visible light (to humans) lies in the wavelength range of about 400-700 nm. No I have a science book here with a picture of the EM spectrum with a scale for the wavelengths of all the different types of EM radiation and on this scale, visible light is marked at the range of µm. However, in the text below it says of course that it lies in the nm range. Now it's the same here http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg which gives me the slight suspicion that it's not the scales but me who's not getting it right here. Could someone please fill me in on that?

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Hi there.

I'm not quite sure where you see this discrepancy. The picture you linked from wikipedia, clearly shows visible light is in the range of 0.5 μm, which is the same as 500nm.

ah you're right there, that was my own stupidity. however in that book I have, they really noted it at 1x10^-6 m, but I guess that's just a mistake then...
Thank you

A range is defined by two numbers. If one number is 1 μm, what is the other one?
Is the scale maybe logarithmic?

well it goes from 10^7 to 10^-14 on the gamma-end of the spectrum. distances between the numbers on the picture are equal and "visible light" is clearly marked at 10^-6 and it says next to it "1 µm" so no decimal notation there.

Redbelly98
Staff Emeritus
Homework Helper
It sounds like they are just giving a rough figure for visible light. On a logarithmic scale, the wavelength 0.5 μm is closer to 10-6 m than it is to 10-7. So it's not really a mistake, it's just an approximation to the nearest power of ten -- which is 10-6 m for visible light.

If you want a more precise figure, then use 0.4 to 0.7 μm. But to indicate it within a wide range of the EM spectrum on a logarithmic scale, 1 μm is an acceptable figure too.

oh ok, but shouldn't it be close to 10^-9 for nm? at least when the base is not a zero decimal number? sorry, really bad with math as well....

Redbelly98
Staff Emeritus
Homework Helper
When you see "500 x 10-9 m" (or 500 nm, which is the same thing), you can't just look at the "10-9 m" or "nm" part and say it's close to 1 nm. You have to consider the "500x" part as well.

Just for example, if you had "1000 nm" (or 1000 x 10-9 m), that is actually the same as 1 μm. So we wouldn't say it is close to 1 nm.

500 x 10-9 m happens to be the same as 10-6.3 m. It really is closer to 10-6 than it is to 10-9.

Oh, I think I see your problem.
The wavelength of light is not of the order of a nm but of a few hundreds of nanometers.
So 100nm = 100 x 10(-9) = 10^(-7).
And 500 nm = 5x10^(-7) or 0.5x10^(-6) which is 0.5 microns.

Sorry but the Redbelly was faster.

yes! thanks thanks ;) did not consider.