Does a radio wave or gamma radiation have a faster speed? I know that all light travels at $\pu{3E8 m s-1}$, but does that include these forms of electromagnetic radiation? Or do radio waves travel at $700~\mathrm{nm}$ and gamma rays $400~\mathrm{nm}$?
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1$\begingroup$ travels the same speed in the same medium. $\endgroup$– CognisMantisCommented Dec 1, 2014 at 19:24
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2$\begingroup$ nm is short for nanometers which is 1x10^-9 meters. It's a unit of distance not speed $\endgroup$– RobChemCommented Dec 1, 2014 at 22:28
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1$\begingroup$ The 700nm and 400nm are wavelengths. An electromagnetic wave of wavelength $\lambda$ still travels at the speed of light $c$, but has a different frequency , $f=c/\lambda$ and hence energy $e=hf$. $\endgroup$– theoCommented Dec 3, 2014 at 5:41
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$\begingroup$ The wavelengths of radio waves are actually in the range $10^{-3}$ to $10^5$ m, whereas those of gamma rays are less than $10^{-11}$ m. As mentioned in one of the answers, $700$ nm and $400$ nm are the wavelengths of red and violet visible light respectively. $\endgroup$– orthocresolCommented Jan 2, 2016 at 13:44
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3 Answers
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In a vacuum all light travels the same speed no matter what the wavelength or frequency. Gamma radiation has a smaller wavelength and larger frequency, radio waves a smaller frequency and larger wavelength.
Wavelength $\lambda$ and frequency $f$ are inversely proportional. As $\lambda$ gets bigger, $f$ gets smaller. But if you multiply them together they always equal a constant, the speed of light $c$: $$\lambda \cdot ν = c$$
[...] do radio waves travel at $\pu{700 nm}$ [...]
The value $\pu{700 nm}$ is the measure of wavelength; meter is a measure of distance; speed would be meters per second.
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All light travels at the same speed in vacuum. What makes different kinds of light different is their wavelength. The electromagnetic spectrum gives you the range of wavelengths that the different kinds of light have. So, $\pu{400 - 700 nm}$ would be the visible light range. Basically, the light is moving in a particular direction at speed $\pu{3x10^8 m/s}$, while the electric field is moving up and down and the magnetic field is moving left and right, both perpendicular to the direction the light is moving and perpendicular to each other. If the electric field went up as high as it can go, then went as low as it can go, and then came back up, the distance the light traveled during this time will be the wavelength, and the time it took will be the period ($t$), and the amount of times it happens in one second is the frequency($1/t$ or $f$). More energy makes the electric field oscillate faster up and down, so the wavelength is shorter. The speed of propagation remains the same.
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All electromagnetic waves will travel at around the same speed in a vacuum. The differences are negligible so we just refer to the speed as $c$, or the speed of light which is $\pu{3.00E8 m/s}$.
The $\pu{700 nm}$ and $\pu{400 nm}$ you are referring to is probably referring to wavelength which is usually measured in $\pu{nm}$ because it is so small. A wavelength of $\pu{700 nm}$ and $\pu{400 nm}$ will place those waves in the visible light spectrum because the visible light runs from $\pu{390 nm}$ to $\pu{700 nm}$. Frequency also plays a big role and it is usually measured in $\pu{Hz}$ or hertz which is just another name for $\pu{1/s}$, because it is measuring how many times the wave cycles per second. Wavelength is inversely related to frequency. Gamma rays have one of the highest frequencies and smallest wavelengths in the electromagnetic spectrum while radio waves are at the opposite end with low frequency large wavelengths.
