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Lots of questions about optics

  1. Apr 22, 2014 #1
    Hi, I am new here and I've got lots of questions about optics physics.
    I hope everybody can help me out.
    First, please check my understanding of diffuse reflection: http://www.physicsclassroom.com/Class/refln/u13l1d6.gif As shown in the diagram, I am sure that it will still follow the law of reflection but light will be reflected back from all different directions as the all the normals have different angle and photon is infinite small which means a beam of light will have infinite number of photons. Therefore, we will see the image no matter from what direction. Am I right so far? If not, please correct me.
    Question :
    1. If 10 people stand around an object (diffuse reflection), with a constant distance to the object. Will they see the same image if we ignore the effect of perspective? (include color, intensity, shape and so on)
    2. How do we calculate the intensity of lights that are reflected? What information do we need to know?
    3. If they all see the same image (question 1) and I want the calculate the light for all direction. Does it mean I can choose a group of light from a random direction and copy this group of light for other directions?
    Well. I just tried to look at an object from all direction (question 1), it looks almost the same but some area will have different intensity if I change my viewpoint. What cause this? Does it mean I need to calculate for all directions (question 3)?
    4. Usually if we want to calculate the whole image that will be formed, do we do it by separate a beam of light into single light rays and do the calculation? Or is there any better method?
    5. If we use the method that mentioned in question 4 and we want to calculate for diffuse reflection, how small does a light ray need to be? We can't even really see the roughness with our eyes unless we use microscope.
    6. When we say two waves are coherent, do we also refer that they have same amplitude?
    7. I found polarization of light hard to understand. How could a light ray have secondary direction? It is a line, not a plane. Though I know light is lots of wave and we can convert it into a plane but isn't there any method to deal with polarization of light with light ray?
    8. What is the different between scattering and diffuse reflection?
    9. Isn't thin films interference just reflection? Is there any interference?
    10. There are many types of scattering. If I want to run a simulation for real world, do i need to simulate all types of scattering?

    Thank you very much!
  2. jcsd
  3. Apr 22, 2014 #2


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    From where did you get this idea that photons are infinitely small and they are infinite in number?

    I don't thinks photons have any size(IMO).
    If photons are emitted infinitely in numbers,then the light source would be emitting infinite energy which is impossible.
    I suggest you to read this.
  4. Apr 22, 2014 #3
    Oh. Yeah. You are correct. Not infinite but just lots. Well, infinite small is like no size but it is matter so I would like to call to infinite small.
  5. Apr 22, 2014 #4


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    Optical physics can be done using at least four different models, each with its own set of equations and principles:

    1. Ray optics: this is what everybody studies first. Reflection, where the angle of the incident ray is equal to the angle of the reflected ray - where both are measured from the normal to the surface. The incident and reflected rays share the point of contact, and since rays are straight lines, the two rays and the point of incidence form a plane. Refraction, where the light path is bent when moving from one medium into another ... like air and glass, or air, glass, water. Here you use Snell's law and the measured index of refraction.

    From these rules you can design mirrors, lenses for spectacles, telescopes, and microscopes. If you start with just Snell's law and Fermat's Principle of Least Time - and some knowledge of advanced calculus - you can derive the eikonal equation; this allows you to calculate rays as they pass through materials with continuously varying indices of refraction - such as air at different temperatures or pressures.

    Now you can both explain and calculate the mirage images.

    There is no explanation for colors, but with the assumptions of Fermat you can understand the details of the prism; you just assume that each color has its own index of refraction.

    2. Wave optics; originally proposed by Christian Huyghens, it is based on a single principle: spherical waves are regenerated from each point. Now you have two additional phenomena which can be explained: diffraction, which is the bending of light around small holes and sharp edges, and interference, which leads to increases and decreases in the intensity of the light. These follow naturally from waves, and are easy to see with water waves.

    When the wave model is used it is clear that there must be a wave length for light: and you can measure it via interference effects. It quickly becomes clear that the phenomenon of color can be explained by each color having a different wave length - a continuously varying parameter.

    The wave model is a bit more complicated to make use of, but everything in ray optics can be explained via wave optics - this is primarily due to the fact that the "rays" are just the normal to the wave front - the propagation direction of the wave.

    In addition the wave model makes it possible to handle polarization - an otherwise mysterious property of light! - by having the wave vibrate sideways (lateral motion, like the string of a violin) - this vibration can be parallel to the ground, or perpendicular, or at any other angle - thus the polarization angle. Or you can have a mixture of different polarizations, or even light with continually "rotating" polarizations due to the source of the light.

    The fundamental equations are due to Fresnel, early in the 1800s.

    The modern theory of the microscope is based upon a detailed analysis of diffraction from a surface, conducted by Abbe: http://en.wikipedia.org/wiki/Ernst_Abbe

    Your questions about "diffuse reflection" can be answered by the theory of diffraction, though it is not needed for all of your questions.

    3. Physical optics is based upon Maxwell's electrodynamic theory - it tells us that the waves are electromagnetic in nature, with both electric and magnetic components oscillating. Interestingly Maxwell's theory is also the foundation for Einstein's Special Relativity. Physical optics is thus a branch of electrical engineering.

    This provides insight into how light can be generated and measured, as well as extending the ideas up and down the electromagnetic spectrum: from radio waves to microwaves to infrared, visible, and ultraviolet light, to x-rays and beyond. Additional properties of materials were defined and measured; it turns out that the "index of refraction" can be determined by the measurement of the electric permittivity and the magnetic permeability of the material: http://en.wikipedia.org/wiki/Refractive_index#Dielectric_constant

    Today we are able to engineer materials with enhanced or reduced reflectivity, and many other desirable properties. Think night vision, television, etc.

    4. Quantum theory of light: the properties of atoms cannot be explained in terms of Maxwell's electrodynamics - what works in the "large" does not quite work in the "small". Thus starting in 1900 with Planck's relation there has been a steady effort in the development of quantum theory. The best theory is Quantum Electrodynamics (QED) which as far as we know always gives correct answers for every question relating to light. But it is much more difficult to work with, so when you have a choice, always use one of the simpler theories.

    The idea of the photon is part of quantum optics. It plays no role in the earlier, simpler theories.
  6. Apr 22, 2014 #5


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    Some of your questions can be answered here: http://en.wikipedia.org/wiki/Diffuse_reflection

    In general a diffuse reflector will not scatter light equally in all directions.

    Nor will it reflect images, though there are gradations from "specular" to "totally diffuse"; but image quality degrades rapidly, as you will see by smearing a small bit of grease on your favorite mirror. You can clean it off with a bit of warm water and detergent.

    You can find the intensity of the light in each direction (for a fixed distance) with a light meter. There are many kinds, though most are based on photo-diodes today. Your smart phone has one.

    Or you can just take pictures - but turn of the "equalization" or "white balance" functions first! - and average the pixel values; this is a good proxy for intensity.
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