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Ray of light

  1. Dec 25, 2005 #1
    What is meant by a ray of light?
  2. jcsd
  3. Dec 25, 2005 #2


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    Does a beam of light help? More or less it's just a part or section of light. If you were to wake up in a shinny morning, draw your room curtains aside and be blinded by sun light, you would in other words see infinite rays of light. Imagine a lamp on its own switched on, if you were to put it in a box with a front cover which has a fine slit, you would therefore see a ray or beam of light passing through the slit.
    Last edited: Dec 25, 2005
  4. Dec 26, 2005 #3
    I agree with all that. But, Physically what is meant by a ray of light? i mean, how can we divide light into straight lines, when it spreads in every direction from the source?
  5. Dec 27, 2005 #4


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    Ever used a laser?
  6. Dec 27, 2005 #5


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    A ray of light is light going all in one direction. What you ask is in fact a fundamental question. If you consider light to be particles, there is no conceptual problem. But light is also waves, so you need to think a little more deeply. Have you done experiments of waves on water? If so, you would have noticed that waves coming through an aperture whose size is smaller than about one wavelength will come out in all directions. However, if the aperture is large compared with the wavelength, the waves appear to channel in a more or less straight line. This effect is called diffraction. In the case of visible light, the wavelength is 0.0005 mm. So even if you shine light on a 1mm pinhole, it will appear to shine through the hole as a ray with little spreading. The way to think of this is that there are waves that emanate in all directions from the aperture, but outside the ray, these waves have phases such that they add up to nothing.
  7. Dec 27, 2005 #6
    E.g., if you put lots of point sources in a line, the only constructive interference of each of these spherical waves appears like wavefronts of a plane wave, look at this diagram http://theory.uwinnipeg.ca/physics/light/node3.html
  8. Jan 9, 2006 #7

    Claude Bile

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    A ray is essentially a theoretical construct used as an aid to solve problems in geometric optics (much like electric field lines can be used to describe an electric field). A single ray is used to define a single path that light may take.

    Using rays is subject to some restriction, for example, rays cannot be used for problems where diffraction effects are important (for example where small apertures are involved) because they do not take into account the e/m field nature of light. The term 'Ray optics' or 'Geometric optics' is sometimes used to denote the field of optics where using rays is acceptable.

  9. Oct 4, 2008 #8
    define the term ray of light
  10. Oct 4, 2008 #9
    what is a beam of light?
  11. Oct 4, 2008 #10


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    Have you even read the previous replies?
  12. Oct 5, 2008 #11

    Andy Resnick

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    If the OP is referring to the ray-optics model of light, then a ray of light is an infinitesimal amount if light which has zero area and propagates in a particular direction. Being infinitesimal, the ray optics model also approximates light as having an infinite frequency (wavelength goes to zero), which is consistent with a classical model. In the ray optical model, points in object space are mapped onto points in image space.

    Note the fundamental flaw in ray optics- a ray of light carries no energy.
  13. Oct 5, 2008 #12
    Ray of light not carrying energy is not a flaw. It is geometric optics and you do not speak about energy while dealing with it.

    Why do you care what a ray of light is? Ray optics is just observation - it is not a theory. Wave optics is theory. Wave optics is put forth as a theory and it is used to explaining effects like interference and diffraction. First of all the question was about ray of light and i do not know why the topic of wave nature of light came up. Secondly this question need not be dealed in such vigour. A ray of light is just an idealized (narrow) beam of light.
  14. Oct 6, 2008 #13
    Geometrical optics, and its' extension: "geometrical theory of diffraction" have been in use for a long time to calculate scattering effects (i.e. interference/diffraction).

    Physical optics (what you call "wave optics"), works in a similar manner - but without the need to calculate image sources.

    Both methods employ ray optics to determine such things as angle of incidence, and incident field.


  15. Oct 6, 2008 #14
    A ray of light is a line which runs perpendicular to the wavefront. They're useful in geometric optics.
  16. Oct 6, 2008 #15

    Andy Resnick

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    Perhaps "flaw" is a pejorative word to use when the intent was not there. That said, people often rely on a mental picture of light rays to construct unphysical optical systems: optical concentrators of arbitrary efficiency, for example.

    Geometrical optics is a very well-developed theoretical construct. Many optical design programs (Zemax, Code V, Oslo, etc) use ray optics to model optical systems. Using ray optics concepts is essential to understand certain specifications: field of view, primary and secondary aberrations, vignetting, and magnification (among many).

    Vigorous, polite, discussion of essential concepts is no vice.
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