# Why do all electromagnetic waves travel at the same speed when travelling through vacuum?

What does my teacher mean when he says that all electromagnetic waves travel at the same speed when travelling through a vacuum? If you may, please answer as simple as possible.

• They mean that all electromagnetic waves travel at the same speed through vacuum. What's unclear about that? – ACuriousMind Mar 27 '17 at 12:19
• @ACuriousMind I thought that all the different EM waves travelled at different speeds as they all have different wave lengths and frequencies etc. so it confused me how they would all be able to travel at the same speed in a vacuum – Pottypie123 Mar 27 '17 at 13:38
• Its easier to understand if you think of photons. They can have different frequencies but they all propagate at the speed of light. – Bill Alsept Mar 28 '17 at 22:05
• A photon considered red has a frequency of more than 428 trillion oscillations per second. As the photon travels along at the speed of light it will complete one of these oscillations in a distance of 700 nanometers. The so called wavelength. – Bill Alsept Mar 28 '17 at 22:28

Electromagnetic waves include visible light, radio waves, X-rays, and so on. What distinguishes these different bands of light is their frequency (or wavelength). But what they all have in common is that they travel at the same speed in vacuum.

The reason for qualifying 'in vacuum' is because EM waves of different frequencies often propagate at different speeds through material.

The speed of a wave $c$, its wavelength $\lambda$ and frequency $f$ are all related according to $c=\lambda f$. So if $c$ is the same for all EM waves, then if you (say) double the frequency of a wave, its wavelength will halve.

• I know this information already, what I dont understand is that the different waves have different wavelengths and frequencies. Radiowave--> Gamma rays have increasing frequencies. SO if each of them have different wave lengths and frequencies how do they all have the same wave speed? – Pottypie123 Mar 27 '17 at 11:39
• @Pottypie123 I have updated my answer - is it clearer now? – lemon Mar 27 '17 at 12:13
• yes that is very helpful thx!! – Pottypie123 Mar 27 '17 at 13:36
• Perhaps the issue is a matter of cause and effect. f and λ are not the cause of c. For instance, in the case of sound, the velocity is determined by the medium (air, water, metal) not by the frequency of the source or the wavelength of the standing waves that might be present in the source. What causes the speed of light to be what it is, is of course more complicated than the speed of sound, but that might serve as a starting point. – Paul B Mar 27 '17 at 16:42

Assume you are walking down the road. You carry a little stick with you. Just for fun you decide to wiggle the stick rhythmically up and down at the rate of one up/down wiggle per second (you are a bit of an olympic expert at stick wiggling so it's very accurate and reliable). Your stick wiggling is at 1Hz. The speed at which you walk down the road is not related to the rate at which you wiggle the stick. You can walk down the road at any speed you like and still wiggle at 1Hz.

Have you got it now ?

The speed of transmission of an e-m wave is not related to the frequency/wavelength.