This is One-of-the-Best:

A point source of light radiates monochromatic light with an intensity 1 Cd.

Now, take a convex lens (f=10cm) and place this source at its focus. (You have one focus left.)

Simple, whats the intensity at the other focus?

Assume lens is too small..:P

18 comments:

  1. AmitDecember 19, 2010 at 7:37 PM

    I wonder what to do and why would it change? So, I think 1 Cd only.

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  2. vmcianDecember 19, 2010 at 7:57 PM

    bhaiya, as you are bringing some good changes to blog so, also change that time zone from US one to Indian timings. :)

    PS- bhaiya how could you make so good pictures, any software. And if you dont mind, I want to ask should I also recommend some good questions for your blog, I will just mail you them and you can short-list(if you want).If yes, then please give me your email id.

    ReplyDelete
  3. shouryaDecember 19, 2010 at 8:02 PM

    i think it is 1/(4{pie}(20)^2).

    ReplyDelete
  4. shaanDecember 20, 2010 at 3:32 PM

    o acc to me.

    ReplyDelete
  5. vmcianDecember 20, 2010 at 7:47 PM

    I am really very very sorry bhaiya if you felt bad. Cheer up!

    ReplyDelete
  6. AmitDecember 22, 2010 at 7:52 PM

    bhaiya please tell the answer

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  7. shaanDecember 22, 2010 at 9:49 PM

    @shourya
    please tell how u got that ans.
    (i hope u did not take area of focal point as 1!!)

    ReplyDelete
  8. AnujDecember 23, 2010 at 1:37 AM

    shourya's answer is right..try reading your NCERTs more carefully...

    ReplyDelete
  9. AnujDecember 25, 2010 at 1:10 PM

    Note this:

    If the waveform is spherical, its intensity decreases with the square of the distance from the point source.

    However, intensity in a planar waveform remains constant.

    Here, the role of the convex lens is to convert the spherical waveform into a planar waveform.

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  10. ShivankerDecember 25, 2010 at 1:46 PM

    ok.. really a nice one bhaiya!

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  11. ShivankerDecember 25, 2010 at 1:50 PM

    Just one thing bhaiya.. shudn't it be "*power* emitted by the source is 1 Cd"??

    dimensions confused me a lot when i tried this earlier.. so i was thinking of multiplying by the area of the focal point :D and crash!

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  12. AnujDecember 25, 2010 at 3:38 PM

    Cd is the unit for luminous intensity, which is not the same as the W/m^2 intensity which we're used to.

    My fault.

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  13. somay jainDecember 25, 2010 at 7:40 PM

    just one doubt here..
    why are we taking 1/4pi(20)^2?
    shouldnt it be 1/4pi (10)^2?

    ReplyDelete
  14. ShivankerDecember 25, 2010 at 7:43 PM

    ok.. thnx bhaiya..

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  15. Sambhav GuptaDecember 26, 2010 at 12:26 AM

    same doubt as somay

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  16. AnujDecember 26, 2010 at 12:50 AM

    my fault again...1/4pi(10)^2

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  17. AnujDecember 27, 2010 at 10:07 PM

    This comment has been removed by the author.

    ReplyDelete
  18. shouryaDecember 28, 2010 at 10:56 PM

    yes it is 1/4pi(10)^2. i wrote 20 by mistake.

    ReplyDelete

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