# Ordinary light and monochromatic light

• gracy
In summary, Ordinary light consists of waves with different wavelengths that vibrate in all possible planes, perpendicular to the direction of propagation of light. Monochromatic light, on the other hand, consists of waves with a single wavelength that vibrate in different planes perpendicular to the direction of propagation of light. Light can be described as a combination of rays, waves, particles, or disturbances of fields. The direction of propagation of light does not necessarily have to be the same as the direction of motion of the waves. A light bulb sends light in all directions, while a laser sends light in only one direction. Light can also be collimated to travel in a single direction.

#### gracy

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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gracy said:
rays of different wavelength
gracy said:
rays of single wavelength

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

jtbell
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).

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.

gracy said:

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?

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.

gracy said:
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?

There's nothing that vibrates, so: no.

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

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

I liked it.

ok

BvU said:
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?

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

gracy said:
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.
gracy said:
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
gracy said:
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?
gracy said:
Ordinary light: Ordinary light consists of rays of different wavelength, vibrating in all possible planes, perpendicular to the direction of propagation of light.

#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?

gracy said:
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
gracy said:
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?

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

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

Borek said:
No, as long as all wavelengths are sent in all directions.
That will result in white light. Right?
Borek said:
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?

gracy said:
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
gracy said:
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?

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.

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

Borek said:
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)

gracy said:
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:

Borek said:
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