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Based on my understanding, frequency is dependent on the number of times that the wave passes through the resting point, and therefore the frequency is the speed of the wave. That makes sense to me. The thing that's tripping me up is that frequency is inversely proportional to wavelength, and the color of, for example, a laser, is shown by the wavelength of the output of the diode.

I don't understand how a laser of a higher wavelength would be traveling at the same speed as a laser of a lower wavelength since a different wavelength would change the frequency. Another thing I'm confused about is that light "doesn't have a color", but you can buy a light that shines a certain color, or a laser that shines red as opposed to cyan.

What am I missing?

ECThunderation
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  • You seem to be confusing the rules for particles with mass and the rules for particles without mass. – Carl Witthoft Mar 31 '22 at 13:12
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    "...the frequency is the speed of the wave..", this is blatantly incorrect. – Cross Mar 31 '22 at 13:12
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    Re, '...light "doesn't have a color"' Who told you that? Maybe you misinterpreted something that they said. It's true that color is not, in and of itself, a physical property of light, but the color that we perceive when we look at a light source is determined by the spectral power distribution of the source, and that is a physical property. – Solomon Slow Mar 31 '22 at 13:45
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    There's nothing in a light wave that moves back and forth (or up and down) through a "resting point." Waves in general, are more abstract than that. Wikipedia says, "a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities, sometimes as described by a wave equation." If we're talking about waves on the surface of a pond, the "quantity" is the height of the surface—the surface physically moves. But in the classical explanation of light, the "quantity" is the strength of electric and magnetic fields, not physical distance from a "resting point." – Solomon Slow Mar 31 '22 at 13:55
  • @SolomonSlow - I said light doesn't have a color. What is Gray, from a physics POV? – mmesser314 Mar 31 '22 at 15:02
  • This may help - How can a red light photon be different from a blue light photon? – mmesser314 Mar 31 '22 at 15:04
  • Right. Color is a matter of perception. When you look at one part of a scene, the color that you perceive depends on both, the spectral power distribution (SPD) of the light from the part in question and, on the SPD of light reaching your eye from other parts of the scene. The SPDs are what's physical. the color is mental. But, Let's consider a simple scene—a colored light shining on a white screen in a dark room. Everybody will agree on the color that they see because they're all looking at the same thing—the same physical SPD—and we can safely call that the "color of the light." – Solomon Slow Mar 31 '22 at 16:38

2 Answers2

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Wave speed is $f\lambda$

$c=f\lambda$

If the frequency increases, the wavelength decreases

Frequency is how frequent a peak travels across a certain point ( or freq of osscilations), if the frequency is increased, with a constant wavelength, then the same distance is covered in a shorter time making the wave speed up. So if the wavelength decreases, a shorter distance is traveled, in a higher time. Balancing out the change in frequency, causing the speed to be constant

jensen paull
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  • In principle good answer. Just one note, light speed depends on medium permittivity and permeability as declared by Maxwell $$c={\frac {1}{\sqrt {\varepsilon _{0}\mu _{0}}}}$$ and is $\text {const}$ for a given medium. So you can't speedup a light wave by making it's wavelength constant. Compressing more beats of EM field spatially automatically implies that single beat must travel shorter distance in one period. – Agnius Vasiliauskas Mar 31 '22 at 14:26
  • Thats why my answer also says that as frequency increases, wavelength decreases. I was using that hypothetical to explain what changing 1 thing does, not that it's possible. Also the speed of light in a medium isn't constant, it can vary with frequency/angle of indicence, such as birefringence, and dispersion in conductors – jensen paull Mar 31 '22 at 16:05
  • Yes, refractive index depends on frequency, but for 1 specific frequency,- phase speed is constant, unless optical material has unusual properties, like you say some isotropic optical axis, gradient of refractive index or even meta-materials. You can take any stuff to make things more complex, but I had in mind ordinary conditions, such as simple glass, air, etc and single frequency. – Agnius Vasiliauskas Mar 31 '22 at 17:05
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A wave is characterized by its amplitude $A$, frequency $\nu$, wavelength $\lambda$, and speed $v$. Stand at a certain point and allow the wave to travel past you. The amplitude is the height you measure between the highest and lowest intensity of the wave. Frequency is the rate that the signal oscillates between the highest and lowest intensity. For light, frequency defines the energy in the wave as $E = h\nu$, where $h$ is Planck's constant. Now run along side the wave until you see no oscillations in intensity. You are traveling at the speed of the wave. Determine the wave length by taking the frequency that you found when standing still and the speed when you moved with the wave using the equation $v = \lambda \nu$.

If you do this experiment in vacuum, you will find that $v = c$, the speed of light. If you do this experiment in anything other than vacuum, you will find that $v = c/n$, where $n$ is the index of refraction for the medium. The frequency of the light will not change between vacuum and any other medium. The wavelength will change (decrease) according to $\lambda_{n} = \lambda_{1} / n$, where $\lambda_n$ is the wavelength in the medium and $\lambda_1$ is the wavelength in vacuum.

Visible light is a combination of all frequencies (colors) of light. We select (pull out) frequencies (colors) using such objects as prisms.

Jeffrey J Weimer
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