Notices
Results 1 to 20 of 20

Thread: Optics

  1. #1 Optics 
    Forum Professor
    Join Date
    Apr 2007
    Location
    In the circuitous haze of my mind
    Posts
    1,028
    (I have a few interrelated questions)

    To begin, I want to know what makes a prism a prism (shape, refractive index).

    -What would happen if you were to cut the prism down the middle so that you had two right angle triangles and then separated the two by a millimeter? What if you did this in a vacuum?


    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    For the first question, a prism is a prism (that is, it creates a spectrum) because refractive index is actually a function of the wavelength of the light. So light of different wavelengths is bent slightly differently. This basic property isn't particularly dependent on shape, and nearly any shape will produce a spectrum, just not a particularly pronounced one. The triangular shape is there because it makes a nice, wide spectrum and also because of internal reflection.


    Reply With Quote  
     

  4. #3  
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by MagiMaster
    For the first question, a prism is a prism (that is, it creates a spectrum) because refractive index is actually a function of the wavelength of the light. So light of different wavelengths is bent slightly differently. This basic property isn't particularly dependent on shape, and nearly any shape will produce a spectrum, just not a particularly pronounced one. The triangular shape is there because it makes a nice, wide spectrum and also because of internal reflection.
    What you said is correct, except that the function of prisms is not necessarily to demonstrate the spectrum. Prisms are normal elements in some optical instruments, porro prism binoculars for instance. Their function is to direct light to to the eyepieces, which are not colinear with the objective lenses and to lengthen the optical path while maintaining a compact device.

    The OP probably had the idea of spreading the spectrum in mind, but he did not actually ask about it.
    Reply With Quote  
     

  5. #4  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    Ah right. Most cameras use prisms to reverse the inverted image from the lenses. In those cases, they try to minimize the spread by a combination of shape and material.
    Reply With Quote  
     

  6. #5  
    Suspended
    Join Date
    Apr 2008
    Posts
    2,176
    Quote Originally Posted by MagiMaster
    For the first question, a prism is a prism (that is, it creates a spectrum) because refractive index is actually a function of the wavelength of the light. So light of different wavelengths is bent slightly differently. This basic property isn't particularly dependent on shape, and nearly any shape will produce a spectrum, just not a particularly pronounced one. The triangular shape is there because it makes a nice, wide spectrum and also because of internal reflection.
    Then how can the prism take the different types of light and send all of each type from the entire width of the beam, to only one area. And a single ray of single colored light?

    How do you explain the second prism, being able to take broken up light and turn it back into white light?

    Sincerely,


    William McCormick
    Reply With Quote  
     

  7. #6  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    It doesn't "send each type from the entire width of the beam to a single area." It sends each color (roughly) to an area slightly larger than the original beam. This means that a somewhat wide beam will smear across the output. A very narrow beam will smear less.

    The interface between two surfaces will bend light away from the normal, so by turning the other prism, you can make it bend the divergent beams back towards one another. All this is very well understood and has been for hundreds of years (since about 984, so over a thousand years ago).
    Reply With Quote  
     

  8. #7  
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by William McCormick
    Quote Originally Posted by MagiMaster
    For the first question, a prism is a prism (that is, it creates a spectrum) because refractive index is actually a function of the wavelength of the light. So light of different wavelengths is bent slightly differently. This basic property isn't particularly dependent on shape, and nearly any shape will produce a spectrum, just not a particularly pronounced one. The triangular shape is there because it makes a nice, wide spectrum and also because of internal reflection.
    Then how can the prism take the different types of light and send all of each type from the entire width of the beam, to only one area. And a single ray of single colored light?

    How do you explain the second prism, being able to take broken up light and turn it back into white light?

    Sincerely,


    William McCormick
    The whole thing is based on the fact that the index of refraction varies with wavelength (color). There are not really different types of light, only different wavelengths, or if you like, photons of various energies. White light is nothing more than the perception of the eye to a mix of photons of many different energies.
    Reply With Quote  
     

  9. #8  
    Forum Professor
    Join Date
    Apr 2007
    Location
    In the circuitous haze of my mind
    Posts
    1,028
    So, even a square cube (perfect) of glass will produce a spectrum?

    I noticed a while ago on my camera that sometimes when the light comes from a certain angle, a violet haze appears around the edge of the shot; is this essentially 'the prism effect' where it is not intended? How do you get around this?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
    Reply With Quote  
     

  10. #9  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    Yes, a cube held at an angle to the light should produce a (usually fairly narrow) spectrum.

    In cameras this is called chromatic aberration. You'll have to look up your specific camera to see if there's anything you can do about it (at least, as far as I know).
    Reply With Quote  
     

  11. #10  
    Forum Radioactive Isotope mitchellmckain's Avatar
    Join Date
    Oct 2005
    Location
    Salt Lake City, UTAH, USA
    Posts
    3,112
    Quote Originally Posted by Cold Fusion
    So, even a square cube (perfect) of glass will produce a spectrum?
    MagiMaster was saying to use a corner of the cube and the reason is this: The light gets split apart on each refraction but in a perfect cube, the bending of light as the light enters one face of the cube is undone when it goes out the opposite face of the cube, BECAUSE these two faces are parallel. This is why there is no distortion when seeing through a pane of glass. However there is a seperation of colors so if you have a thick enough piece of glass there will be cromatic distortions. The angular piece of glass works better because the different colors (wavelengths) of light leave the glass at different angles so the separation of colors continues after the light leaves the glass.


    Here is a related physics challenge. Why are there no green stars?
    See my physics of spaceflight simulator at http://www.relspace.astahost.com

    I now have a blog too: http://astahost.blogspot.com/
    Reply With Quote  
     

  12. #11  
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by mitchellmckain
    Quote Originally Posted by Cold Fusion
    So, even a square cube (perfect) of glass will produce a spectrum?
    MagiMaster was saying to use a corner of the cube and the reason is this: The light gets split apart on each refraction but in a perfect cube, the bending of light as the light enters one face of the cube is undone when it goes out the opposite face of the cube, BECAUSE these two faces are parallel. This is why there is no distortion when seeing through a pane of glass. However there is a seperation of colors so if you have a thick enough piece of glass there will be cromatic distortions. The angular piece of glass works better because the different colors (wavelengths) of light leave the glass at different angles so the separation of colors continues after the light leaves the glass.


    Here is a related physics challenge. Why are there no green stars?
    This site shows the black body spectrum and also the sensitivity of the human eye. The explanation is a bit fuzzy, but what actually happens is that for a star that is of the temperature to emit a spectrum that peaks in the green it also has a significant red component and the human eye sense both the red and green so that we see yellow (or white).

    http://www.astronomycafe.net/qadir/q72.html
    Reply With Quote  
     

  13. #12  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    Quote Originally Posted by mitchellmckain
    Quote Originally Posted by Cold Fusion
    So, even a square cube (perfect) of glass will produce a spectrum?
    MagiMaster was saying to use a corner of the cube and the reason is this: The light gets split apart on each refraction but in a perfect cube, the bending of light as the light enters one face of the cube is undone when it goes out the opposite face of the cube, BECAUSE these two faces are parallel. This is why there is no distortion when seeing through a pane of glass. However there is a seperation of colors so if you have a thick enough piece of glass there will be cromatic distortions. The angular piece of glass works better because the different colors (wavelengths) of light leave the glass at different angles so the separation of colors continues after the light leaves the glass.


    Here is a related physics challenge. Why are there no green stars?
    Actually, the spread from one face to the opposite face won't get completely canceled out. Chances are the spread afterwards will be fairly small though, so you might only be able to see a little red and blue around the edges. Looking through a pane of glass, there would still be some spread, but it'd be very tiny because of both the thickness and the angle (not much of either).
    Reply With Quote  
     

  14. #13  
    Forum Professor
    Join Date
    Apr 2007
    Location
    In the circuitous haze of my mind
    Posts
    1,028
    (sorry for the late responses lately, but my schedule has become very inconsistent)

    So a parallelogram should not create much of a spectrum?

    I find the idea of the initial angle of incidence (as I think it is called) as a means to determine the angle that the various colors bend very interesting. If you look at a prism on a nanoscopic level, it is like looking at a circle on a t.v. very closely; all smooth curves are not actually smooth, but are jagged. The photon is at the size of the supposed plank length, much smaller than even a proton. When it impacts an atom and determines its bend at that point, it is only hitting the single atom, which is not "curved", yet still the resultant angle acts as if it totally felt the relative curve, defined by the surrounding atoms. I suppose the nearby atoms project their wave functions onto each other and come up with a resultant virtual curve that tells the photon the surrounding conditions. or, maybe its just the photons probability wave the "feels out" the surface and determines the curve.

    What determines the index of refraction of a material? Is it the density?

    Why do different colors bend differently? I was told it is because of the difference in energy....but why would that change anything?

    http://en.wikipedia.org/wiki/Prism_(optics)

    One picture depicts a resulting spectrum coming out in a straight line, yet one model shows the different colors coming out at different angles which would not result in what is shown in the first picture. What causes this difference?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
    Reply With Quote  
     

  15. #14  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    William, that little sliver of red light coming out of a prism isn't all the red that's in the original beam. Take a piece of paper. Draw a circle. Now draw another circle of the same size mostly overlapping the first. Now draw a third circle of the same size so that centers are equally spaced along a line.

    Label the center circle yellow, and the others red and blue. Where all three overlap, you get white again. Red and yellow gives orange, yellow and blue gives green. Hmm... this look suspiciously like the spectrum produced by a wide beam of white light going through a prism. Notice how there's a lot more to the red circle than what shows up as just red.
    Reply With Quote  
     

  16. #15  
    Forum Cosmic Wizard SkinWalker's Avatar
    Join Date
    Apr 2005
    Location
    Grand Prairie, TX
    Posts
    2,377
    Moderator Note: This thread has been cleaned up. Fringe/speculative & unfounded posts removed.
    Reply With Quote  
     

  17. #16  
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by SkinWalker
    Moderator Note: This thread has been cleaned up. Fringe/speculative & unfounded posts removed.
    Nice job. The thread is a lot shorter than it used to be, but with no loss in value.
    Reply With Quote  
     

  18. #17  
    Forum Sophomore
    Join Date
    Sep 2008
    Posts
    174
    Ne? Where is my post about the prisms spreading of light because of incident angle?

    That sums up everything.
    Reply With Quote  
     

  19. #18  
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by CoolEJ
    Ne? Where is my post about the prisms spreading of light because of incident angle?

    That sums up everything.
    I stand corrected. That would be a loss.
    Reply With Quote  
     

  20. #19  
    Forum Professor
    Join Date
    Apr 2007
    Location
    In the circuitous haze of my mind
    Posts
    1,028
    A seemingly unrelated question: How does changing the medium that the projected particle travels through in the double slit experiment change the outcome? In other words, would we see different behaviors if the medium were water, or argon instead of the air mix?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
    Reply With Quote  
     

  21. #20  
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by Cold Fusion
    A seemingly unrelated question: How does changing the medium that the projected particle travels through in the double slit experiment change the outcome? In other words, would we see different behaviors if the medium were water, or argon instead of the air mix?
    You should get the same qualitative behavior. What would change are the dimensions of the pattern. This is because light frequency remains constant while the speed varies depending the medium, so the wavelength changes. But if you immersed the entire experiment in a new medium and used light of the same wavelength, rather than frequency, as in a vacuum or air there would be no difference.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •