# What is the wavelength of light in a practical sense?

Okay, so I know that a wavelength is a distance of crest and a trough added together which makes a whole wave, but what is it in the real sense? Like in the case of sound I know when we say "frequency" we mean how fast the air molecules are going away from their mean positions and coming back which is how fast they are vibrating basically. And in the case of light I assume it would be how fast a charged particle is vibrating to produce electric and magnetic fields.

But what is wavelength in sense of the electric/ magnetic fields. Like if I say there are two waves of light with a long wavelength and a short wavelength. I can imagine it in my mind that the distance between the crest and trough of the magnetic/eletric field for the longer wave is greater than for the short wave. But what does it mean practically? Does it mean that the electric fields (and magnetic) are created for the longer distance?

Another confusion this creates in my mind is that what exactly is a trough for electric (magnetic) field? Does it mean a field in the opposite direction? Lets say theres an electron vibrating with changing velocities. It is creating all around a changing electric (and magnetic) field around it. But how do we connect this with the electric (and magnetic) field in the wave image of light?

• Re, "I know that a wavelength is a distance between a crest and a trough..." That's half right—literally! The distance between "crest" and "trough" is 1/2 the wavelength. Wavelength is the length of one full period of the oscillation. May 14 at 12:18
• I'm sorry, what I meant was distance of crest and trough added together. English is not my first language so bare with me May 14 at 12:22
• This might be a good visualization for you. Your impression of sound waves is incorrect. Frequency of sound has nothing to do with the air molecule velocity; it refers to how often (times per second) a high pressure region will pass a certain point. For light, or any EM wave, it means how often the maximum electric field value (and magnetic field with it) will pass a certain point. youtu.be/YHS9g72npqA May 15 at 3:12
• And since you asked specifically about wavelength, if you were able to freeze time for an instant as an EM wave is passing by, if you measured along it with a very sensitive volt meter, you would see repeating high and low electric field values. Taking a ruler and measuring the distance between adjacent max values is the wavelength (for light, in the 100s of nanometers). The reality is more literal than you may think. May 15 at 3:16
• But the faster a particle oscillates per second, the more the number of time it oscillates per second. So how can the speed not effect the frequency? May 16 at 7:49

Lets look at this Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. The electric and magnetic fields in such a wave are in-phase with each other, reaching minima and maxima together.

Before the brilliant formulation of Maxwell's electrodynamics, the understanding of light was not clear. It was the bringing together the various disparate laws of electricity and magnetism into one mathematical framework that led to the wave properties of light, and the measurements fit the theory very well. One measures the frequencies of visible light from the interference patterns but the whole spectrum of electromagnetism is very large.

When the solutions of a wave equation fit the data then one has the mathematical description of a wave, as seen in the animation.

Define light. Visible light on the electromagnetic spectrum, More simply, the range of wavelengths is called visible because We can see it, and as such the human eye can detect wavelengths from 380 to 700 nanometer wavelengths

• Yes but that isnt what i asked. I asked what is wavelength in terms of electric and magnetic fields May 14 at 12:06
• There is no theoretical physical limit on the maximum wavelength threshold as it only would eminate and not occupy a distance equivalent to diameter of the current universe.... May 14 at 12:10
• In many textbooks you find explanations how oscillating charges create a electric and magnetic fielt, which propagate with the speed of light. With sound its al little easier to understand, how the pressure or the velocity propagate, for electromagnetic waves you have to better understand how changing electric field create changing magnetic field an vice versa. But here is not the place to write a textbook May 14 at 13:46

The question seems to focus on the relation between the source of an EM wave and the wave itself.

Without think of sources, the wave length of a light beam can be identified by interference effect. The split of colors of the sunlight by a CD or DVD relates the dimensions of the periodic microdents to the wavelengths of the colors.

But how that white light, composed of a continuous of wavelengths are produced in the sun's surface?

Talking about vibrating charges in a given known frequency is to relate the human made radio waves for example to what happens in the sun. It is certanly not the case. Ionized particles are moving there (accelerated) in a chaotic fashion, and the white light is the generated field.