# Ordinary light and monochromatic light

What are these?
My notes give definitions of the two as follows
Ordinary light: Ordinary light consists of rays of different wavelength, vibrating in all possible planes, perpendicular to the direction of propagation of light.

Monochromatic light: Monochromatic light consists of rays of single wavelength, vibrating in different planes perpendicular to the direction of propagation of light.
I am unable to comprehend .

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Borek
Mentor
rays of different wavelength
rays of single wavelength

Ordinary light: vibrating in all possible planes, perpendicular to the direction of propagation of light.
vibrating in different planes perpendicular to the direction of propagation of light.

jtbell
BvU
Homework Helper
You can split up ordinary light (sunlight, for example) with a prism because there are a lot of wavelengths present (a spectrum). Monochromatic light only gives one angle of deflection (the spectrum is a single line).

FactChecker
Gold Member
Light can have different colors (frequencies) or can vibrate in different directions (planes perpendicular to its direction of propagation). "mono" means one and "chromatic" means color, so "monochromatic" means one color. "monochromatic" does not mean that all the light is in the same plane.

jtbell
Mentor

That's about unpolarized versus polarized, not about whether the light is monochromatic or not.

Ordinary light is polychromatic (= not monochromatic) and unpolarized. You can also have light that is monochromatic and unpolarized, polychromatic and polarized, or monochromatic and polarized.

gracy
What does it mean to have rays of different wavelength?

BvU
Homework Helper
Check your colour TV: there are small green, blue and red light sources. White light you get if all three light up, yellow if green and red, etc.
In short: different wavelengths = different colours

That's okay. But what I don't understand is the light composition. Is light made up of different rays or waves? Are rays and waves one and the same thing? Because as far as I know wavelength is the distance traveled by wave in Time period.

blue_leaf77
Homework Helper
But what I don't understand is the light composition.
A light beam, or a ray of light may contain more than one wavelength. That's very common around us, in fact you can never have light which only contains a single wavelength. However monochromatic a visible light may look like to our eyes, it actually has a range of wavelengths.
I would describe the "ordinary" light you copied into your note as the natural light under which our body is exposed to every single second, this light is not monochromatic and not polarized.

Fervent Freyja
Do all types of light i.e rays of light show vibration?

BvU
Homework Helper
There's nothing that vibrates, so: no.

There's nothing that vibrates, so: no.
Ordinary light: Ordinary light consists of rays of different wavelength, vibrating in all possible planes, perpendicular to the direction of propagation of light.
Monochromatic light: Monochromatic light consists of rays of single wavelength, vibrating in different planes perpendicular to the direction of propagation of light.

I am confused

BvU
Homework Helper
Mysterious, isn't it. OK, something vibrates far away -- at the origin of the light ray, so to speak.
What oscillates (yes, you could use the word vibrates instead, but it confuses some folks) are the electric and magnetic fields.
Electromagnetic radiation is propagated through the vacuum with the speed of light (Vacuum = nothing ).
I liked the video here and the picture here then there is this video and the next and a near infinity of more of them.

Reading title "physics for kids!" brought a broad smile on my face

BvU
Homework Helper
I liked it.

ok

This one is really exciting. But sadly I am not able to understand it.

Light is made up of rays and rays in turn is made up of waves. Right?

direction of propagation of light.
Is direction of propagation of light same as direction of propagation of waves the light is made up of?

blue_leaf77
Homework Helper
Light is made up of rays and rays in turn is made up of waves. Right?
Light is a wave. For now it's best for you to cling on it. The calling of light as being made up of rays is actually not so fundamental - the use of rays to describe light propagation is useful mainly in geometrical optics, outside this field as far as I know the introduction of rays is unnecessary. Moreover, rays actually is a bit loose term in its use in some field of science and math - you can find people talking about "rays" which has nothing whatsoever to do with light when discussing the so-called Hilbert space, besides have you also ever heard of the "cosmic rays"?. Cosmic rays represent not only EM radiation but also radiation due to other particles.
Is direction of propagation of light same as direction of propagation of waves the light is made up of?
Rather than "light being made up of waves", I would say "light as a form of wave".

Merlin3189
Is direction of propagation of light same as direction of propagation of waves the light is in form of ?

If we assume the image depicts light as electromagnetic wave then does the highlighted "direction of propagation of light" in the below sentence mean vector k in the above image?
Ordinary light: Ordinary light consists of rays of different wavelength, vibrating in all possible planes, perpendicular to the direction of propagation of light.

BvU
Homework Helper
#19: light is photons. We can describe the behaviour of light by treating them as rays (e.g. in geometrical optics), as waves (diffraction), as particles or as disturbances of fields (Quantum Field Theory). Mixing up metaphors is sometimes a bad idea, but at other times it helps to understand limitations of a paradigm.

#20 Any reason to think otherwise ? You've seen the videos.

 posts crossed. Let's try to reduce the rapid firing of questions by letting answers sink in. My mistake: I responded to the alert without seeing there were more posts.

Ordinary light consists of different waves with different wavelengths . Do all waves have same direction of motion?

Borek
Mentor
Ordinary light consists of different waves with different wavelengths . Do all waves have same direction of motion?

They can, they don't have to.

Light bulb sends light in all directions, laser sends light in only one direction.

You can collimate light send by the light bulb to make it go in one direction only, that's more or less how flashlights work.

blue_leaf77
Ordinary light consists of different waves with different wavelengths . Do all waves have same direction of motion?

Then should not we see different colors of light in different directions?

Borek
Mentor
Then should not we see different colors of light in different directions?

No, as long as all wavelengths are sent in all directions.

No, as long as all wavelengths are sent in all directions.
That will result in white light. Right?
No, as long as all wavelengths are sent in all directions.
What if it does not happen like that. What if all waves are sent in different directions?

Borek
Mentor
That will result in white light. Right?

Yes.

What if it does not happen like that. What if all waves are sent in different directions?

Then you will have different colors being emitted in different directions.

Not different from what happens when you split the white, collimated light, using a prism.

gracy
Ordinary light: Ordinary light consists of rays of different wavelength, vibrating in all possible planes.
Would it be correct to say"ordinary light consists of electromagnetic waves of different wavelength , vibrating in all possible planes (their vibration is shown by vector E and vector B) perpendicular to the direction of propagation of light?

I have used "electromagnetic waves" in place of "rays" .

But these vibrations aren't in all possible planes perpendicular the direction of wave propagation (vector K)
Can anyone post the picture of electromagnetic wave vibrations in case of light?

Borek
Mentor
For each SINGLE ray/photon/wave - whatever we decide to call it - fields vibrate in such a way that B is perpendicular to E and both are perpendicular to K. But if we take collimated light (that is, all have the same K) it doesn't mean B of one photon is parallel to B of another photon - they can be at any angle. We can filter the light to separate all photons of parallel Bs (this will also make their Es parallel) - that will be what we call polarized light.

gracy
Can you post the picture of electromagnetic wave vibrations in case of light? I tried to find it but failed.

Borek
Mentor
What is wrong with the one you posted? It looks perfectly OK to me.

What is wrong with the one you posted? It looks perfectly OK to me.
But these vibrations aren't in all possible planes perpendicular the direction of wave propagation (vector K)

Borek
Mentor
But these vibrations aren't in all possible planes perpendicular the direction of wave propagation (vector K)

Well, you liked the post where I have explained what is going on:

For each SINGLE ray/photon/wave - whatever we decide to call it - fields vibrate in such a way that B is perpendicular to E and both are perpendicular to K.

This is a picture of a single photon, you have quite a number of photons moving in the same direction, every one with its own E/B fields perpendicular to each other, but not parallel to the fields of other photons. While technically it is possible to draw a thousand photons, each at its own angle, such a picture won't be in any way better.

Fervent Freyja