Diffraction in Water

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Diffraction in Water

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Question:

Here is a question that appeared on my optics midterm. The prof did not post the answers and was unavailable for help. It has been driving me crazy... "An unexpanded beam from a He-Ne laser is incident on a diffraction grating. The straight through and the first spectral orders are observed on a screen. Now the entire setup (laser, grating and screen)is immersed in water. The pattern on the screen will... Spread out? Crowd together? Disappear? Remain the same? What will it do???

Replies:

I am not very sure but i think that the pattern will spread out. My reasoning is this : In water the wavelengths will be reduced as the velocity of light is less, and smaller wavelengths disperse more. This reasoning will hold only for a diffraction grating as the dispersion mechanism is independent of the surrounding medium. In case of a prism, the dispersion will be less as the relative change in refractive index is small, in which case i think the pattern on the screen will remain the same.

jasjeet s bagla

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Update: June 2012