# Three models of light

1. Apr 16, 2014

### shayaan_musta

Hello experts!

I was taking class of optical fiber and communication.

My teacher was teaching the class via projector and slides (transparencies). He taught us light models and he just changed his slide after reading the few lines. I was not able to note down the main points of the slides.

The topic was models of light.
1) Ray model of light
2) Wave model of light
3) Quantum model of light

My questions are:
1) explain these 3 models with definition
2) obviously we can't use any model everywhere, so where to use the specific model? How to know when to use which one?

Thank you all.

2. Apr 16, 2014

### UltrafastPED

1. Ray model: light travels as a ray, in a straight line from here to there.

This model works fine for many applications, but fails to consider diffraction. Thus when you have small objects (like microscopic work) you will want to move to the wave model. The ray model also lacks interference; it also lacks polarization.

As you can see the ray model is a bit limited. But you can go a long ways with lenses and mirrors with ray optics.

2. Wave model: now you have diffraction and interference. Light can "bend" around corners. The theory of diffraction can be used to construct better microscopes. Fourier optics (spatial filtering) and interference filters can be studied. Better mirrors (dielectric) can be designed, as well as non-reflective coatings - these use interference effects. It is also possible to include polarization, but the wave model must be based on the physical waves from Maxwell's equations: lateral not longitudinal.

3. Quantum theory of light: now you can understand atomic spectra, and perhaps compute index of refraction and other "bulk parameters" of materials. The photo-electric effect can be understood, as well as most of non-linear optics. Lasers can be designed and utilized in precise experiments. All of these depend upon an understanding of the quantum properties of light.

PS: Next time pay better attention in class! Also read the text prior to the lecture. Then you are prepared for the material which you see, and can ask appropriate questions.

3. Apr 16, 2014

### shayaan_musta

PS: Next time pay better attention in class! Also read the text prior to the lecture. Then you are prepared for the material which you see, and can ask appropriate questions.

I try to understand and pay attention to the lectures but everytime students are not wrong. Some time teacher must consider either he or she was able to deliver for what he has been chosen. Well, I try my best.

Here is a statement, I want to know whether it is true or false?
When dimensions of the object are much larger than the wavelength of the light then we use ray optics
Is this true? This is what I get in the lecture and I noted it down in my notebook. But I am not sure about this. If this is true then also tell me which model is suitable to use for dimensions smaller than the wavelength of the light and which model is suitable to use for dimensions which are comparable to the wavelength of the light?
If my statement is false then kindly guide and correct me.

Thank you very much for you help sir.

4. Apr 17, 2014

### UltrafastPED

Ray optics breaks down for small objects or scratches/bumps/features which approach the scale of the wavelength of light. It also breaks down at sharp corners, small holes, etc.

So if the dimensions are much larger than the wavelength of light, and there are no sharp corners, or atomic effects, then yes, you can use ray optics.

For example, eye glasses, mirrors, windows, parabolic reflectors in electric torches - all of these can be modeled with ray optics.

5. Apr 17, 2014

### shayaan_musta

OK sir, thank you very much. Very good explanation.

And is there any kind of limitations for wave model and quantum model too like the ray model that it can be used when dimensions are much larger than the wavelength of light? Huh?

6. Apr 17, 2014

### UltrafastPED

The ray model is the most limited; the wave model incorporates all of the results of the ray model, plus diffraction, interference, and polarization; the quantum model incorporates all of the previous results.

You use the ray model when you can because it is simple to visualize and calculate.

Otherwise the wave model is used; the mathematical machinery is more involved, and requires more information about the materials, etc. For most applications the wave model is more than sufficient. For example, microscopy, fiber optics, designing anti-reflection coatings.

The mathematical machinery for quantum optics is more involved; I studied this in graduate school. Though some of the results can be plucked from quantum optics and used separately, calculations from the ground up are very involved. Thus you would not use it unless required.

7. Apr 17, 2014

### DrDu

I wouldn't say that there are three models of light. Rather 3 is the most fundamental description of light we know. 2 is an approximation for systems with large numbers of photons while 1 is an further approximation in the case that all objects (lenses, etc) are much larger than the wavelength of the light.

8. Apr 17, 2014

### shayaan_musta

Thank you all. Thanks a lot of you all.