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No change in wavelength of a monochromatic light

  1. Feb 5, 2014 #1
    I searched for the images of refraction on google images and i saw an image where a laser light was incident on a regular glass slab. Bending of light was clearly seen in the picture. According to Snell's law the refractive index of any particular medium is the ratio of angle of incidence to angle of refraction OR velocity of light in air to velocity of light in the medium. If we substitute all the given values in the formula, then the velocity of light in the glass slab comes out to be 2 X 10^8 m/s. As per the law, the frequency of the light remains constant. Now, we have the relation,

    v = n X λ...(where 'n' is the frequency of light and 'λ' is wavelength of light)

    If the frequency of the light remains constant, then velocity is directly proportional to the wavelength of light. So, if the velocity reduces in the glass slab then the wavelength must also reduce and so the color of the light must change in the glass slab. But on the contrary the color is not changing. So, what happens exactly in the glass slab? What is the phenomenon?


    please follow the above link to view the image.

    Posted By-

    Devendra s. Chavan
  2. jcsd
  3. Feb 5, 2014 #2
    The wavelength changes in glass. You are right.
    But the color you see depends on the frequency of the light incident on your retina. (Assuming that is a monochromatic beam of light).

    Anyway, before reaching your retina the light has to exit the glass slab, travel through air, enter you eye, travel through several different transparent components of the eye and finally excites the receptor in the retina. In every medium the wavelength will be different but the frequency will be the same.
  4. Feb 5, 2014 #3
    So, what if i was there in the medium? Which means if i was in the glass slab, can I see colour change?

    Devendra S. Chavan
  5. Feb 5, 2014 #4


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    Even if you're in the medium... the light still has to leave the medium to pass through your eyeball to hit your retina. Your retina, optic nerves, and brain will be responding to the frequency at the surface of your retina, and that's unaffected by the medium.

    Don't be confused by a different phenomenon: If the medium scatters different frequencies of light differently, you may be able to see that effect. For example, we're always in the medium of the earth's atmosphere, and we see things like red sunsets and blue sky - but that's a matter of different frequencies coming at us from different directions because they've been scattered differently by the medium.
  6. Feb 7, 2014 #5


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    Color is based on frequency; it is the frequency of the photons which determines the energy, and it is this energy which drives the electrochemical process in the retina!

    So even if you changed your vitreous humour for something with a higher (or lower) index of refraction, you should still see the same colors.
  7. Feb 7, 2014 #6


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    Let's be clear here. Your eye detects the colors of light by using cone cells. Each type of cone cells has a "color filter" that will only let certain frequencies through. So the medium the light passes through before it hits your cone cells is irrelevant.
  8. Feb 8, 2014 #7


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    At the boundary there must be continuity. The frequency of the transmitted light has to be unchanged. It is frequency that determines the perceived colour.

    Historically, it has been the4 practice to quote the wavelength of light - because it was the only thing that they could measure when they started to study it.
    When it has been practicable to measure frequency, at any particular part of the EM spectrum, the practice has been to use that as the prime quantity.
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