Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have studied Electronics Engineering and of course the LED. I was wondering that they claim that the speed of light is $3\cdot10^8$ m/s, but do they claim the speed of the light of the sun rays ? or any light ? Because the speed of light that comes out of semi-conductors like LEDs can be controlled, correct ?

share|cite|improve this question
up vote 1 down vote accepted

The speed of light is of course $3\cdot10^8$ m/s in a vacuum. This is the Universal limit for the speed of light, which all electromagnetic waves (Radio, Ultraviolet Light, Visible, etc.) travel at. Where they differ is wavelength and frequency.

Where frequency is determined by the speed of light in a vacuum and the wavelength: $$f = \frac{c}{\lambda}$$

And you can determine the wavelength in the same manner: $$\lambda = \frac{c}{f}$$

However this only in a vacuum. It changes once we introduce different materials for these electromagnetic waves to travel through.

Permeability changes based off of the material. In a vacuum this permeability is the Permeability of Free Space denoted as: $$\mu_{0} = 4\pi \cdot 10^7 \frac{Wb}{A\cdot m}$$

If passing through a non-magnetic material, $\mu_{0} = 1 $ and you focus on permittivity. Permittivity of Free Space is: $$\epsilon_{0} = 8.854 \cdot 10^{-12} \frac{F}{m}$$

So the speed of light with respect to permittivity of free space is: $$ c = \frac{1}{\sqrt{\mu_{0}\epsilon_{0}}} $$

And with respect to relative relative permeability and permittivity the phase velocity is: $$ v_{p} = \frac{1}{\sqrt{\mu_{r}\epsilon_{r}}} $$

Relative permeabilities and permittivities you can find listed in many tables.

So in your case you're asking if you can alter the speed of light in a LED, the answer is no. This is because the speed of light itself is a constant.

However you can alter the relative speed of light, or the phase velocity (the velocity the wave propagates at), because the medium that you send a signal or a wave through alters the speed that the wave will travel through the given medium.

So physicists and engineers can alter the phase velocity of the light emitted from a LED by changing the material and amount of layers of the material that the light passes through. For this look into Planar Waveguides which is a large focus in optics.

share|cite|improve this answer
I'm just wondering, how the light that comes from the LED is constant, regardless the material, what semi-conductor is used or whatever... and how the light comes from the sun has the same speed. Both sources are totally different, and they have the same speed of light ? How come ? – Ahmed Saleh Sep 23 '13 at 19:49
@AhmedSaleh light quanta (photons) have no mass, thus they travel at maximum possible velocity, which is velocity of light. This means that no matter what the source, light will always propagate at this velocity in free space. – Ruslan Sep 23 '13 at 19:56
The speed of light is the universal speed limit. Visible light is an electromagnetic wave that exists in wavelengths that our eyes can see and distinguish. Take a look at the electromagnetic spectrum and the equation for the speed of light within a vacuum. As the wavelength of a wave goes up, the frequency must go down, and vice versa. It's a law of nature that protects the speed limit (applies the same way in relativity by slowing time). Wavelength and frequency balance to remain at the speed of light for all electromagnetic waves. – Signus Sep 23 '13 at 20:00
@Ruslan Thanks so much!, yea that would really solved my confusions. – Ahmed Saleh Sep 23 '13 at 20:02

The speed of light in vaccuum is the same to all observers, and is about 300 Mm/s.

No, the speed of light coming out of LEDs can not be controlled. I can't even guess where you think you heard that from.

The speed of light can be different in different media (not vaccuum). This change in speed when entering or leaving something transparent, like glass, causes bends in the direction of the light. This bending is the basis for how lenses work. This speed in media can be dependent on wavelength. That is the basis for how prisms work.

share|cite|improve this answer… What about that paper ? – Ahmed Saleh Sep 23 '13 at 19:12
Could you cite what the paper is trying to say? You technically don't control the speed of light, you simply alter the relative speed of light by sending it through a specific waveguide (glass, plastic, etc). – Signus Sep 23 '13 at 19:14

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.