# Frequency Of Light

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The frequency of light refers to the relation $$\lambda\nu = c$$ where $\lambda$ is the wavelength, $c$ the speed of light and $\nu$ the frequency of the light (also sometimes denoted by $f$). Note that this seems to suggest that light consists of waves. However, this is misleading at best. Light also exhibits particle-like behaviour. The photo-electric effect is a typical example, since it was this experiment that led Einstein to postulate the existence of photons, "light-particles" if you will.$^1$

More in detail, when light shines on the surface of some metals, emission of electrons is observed - the electrons get "knocked out of the metal surface" so to speak. This is known as the photo-electric effect. Before Einstein's explanation in terms of photons it was considered paradoxical that this emission of electrons did not depend on the amplitude of the incident light, rather it depended on its frequency. Einstein clarified this problem by assuming the existence of photons, individual entities (quanta) with particle-like attributes, specifically with their energy given by

$$E = h\nu,$$

where $h$ is Planck's constant. This relation is exactly what Einstein used to resolve the paradox: only a photon with a sufficiently high frequency $\nu$ has enough energy $E$ to knock an electron out of the material it is incident on.

As for your second question, why $c$ is the maximum speed and if there is a relation between this and the frequency of light, this answer could be a place to start. Indeed, $c$ is called the speed of light and light in a vacuum always travels at speed $c$, but this is not a unique property of light: all massless particles must travel at speed $c$. It just so happens that we discovered this in light first, which is why $c$ is commonly called the speed of light. The group velocity of a single-frequency wave is $v_g=\lambda\nu$ and $v_g=c$ for light waves in a vacuum. As to why nothing can travel faster than $c$, this is actually a postulate of special relativity and as such we don't have a theoretical justification but we do have loads of strong experimental evidence in the form of results that match the predictions of special (and general) relativity exceptionally well. Some reading material: If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light? and Why does the (relativistic) mass of an object increase when its speed approaches that of light?.

$^1$ Although you hear people call them that, it's misleading to speak of particles because it suggests light consists of particles. A more careful term to use is quanta, and the physical nature of these quanta appears to be some blend of particle-like characteristics and wave-like properties.

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### devWaleed

Updated on December 04, 2020

• devWaleed almost 3 years

I am confused on few topics...

What is meant by "Frequency of Light"? Does the Photon(s) vibrate, that is known as its frequency? If the Photons vibrate, then they have a specific frequency, then What is meant by "Higher frequency light" as used in Photo-electric Effect? In which directions/axis do they vibrate to have a specific frequency. Why is it that nothing can go faster than the Speed of light. Does these two things have any relation to each other?

-Thanks.

• DJBunk over 10 years
You might want to divide this up into 2 questions, 1 for photons and frequency, and another for wave particle duality.
• joshphysics over 10 years
possible duplicate: physics.stackexchange.com/q/30402
• Solomon Slow almost 8 years
What is meant by the frequency of a radio wave? As far as anybody knows, light and radio waves are the same phenomenon.
• John Duffield almost 8 years
I'm voting to close this question as off-topic because it shows no research effort.