fresnel biprism - why is the slit kept narrow? Any specific reason

Question

why is the slit narrow in the fresnel's biprism experiment? Almost all books state this fact without explaining it. Please help

Duplicate https://physics.stackexchange.com/q/331572/104696 – Farcher Sep 19 '17 at 21:33 — Farcher, Sep 19 '17 at 21:33

score 0 · Accepted Answer · answered Sep 19 '17 at 16:37

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To get interference, you need a precise path difference between two beams of light. If you have a wide slit, light could have passed in many places - and from there to the screen there are many different path lengths possible.

When this happens, you don't get a clear interference pattern. We say that the narrow slit makes the source "spatially coherent".

answered Sep 19 '17 at 16:37

Floris

118,905

Thanks for answering. i have another doubt. it is stated that-"Motion of eyepiece should be perpendicular to the lengths of the bench ?". Why is it so? – james gem Sep 19 '17 at 16:40
Not sure where the eyepiece is, or how the setup is relative to the "bench". So not easy to answer that question. In general if you are observing fringes, you want to move perpendicular to the fringes to measure their spacing. – Floris Sep 19 '17 at 16:52
why do we move perpendicular to fringes? – james gem Sep 19 '17 at 17:00
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I imagine you do this to measure their spacing . But I am guessing at the details of your setup now so don't push me for details. Or provide a diagram, at least... – Floris Sep 19 '17 at 17:01

score 0 · Answer 2 · answered Apr 25 '19 at 03:16

A broad source is equivalent to a large number of narrow source placed side by side.Now if slit is broad then the two virtual source will also be broad. Now each pair of conjugate points on the virtual sources will give rise to an interference pattern .These interference pattern are slightly displaced from one another.An overlapping of such pattetns results in general illumination.

fresnel biprism - why is the slit kept narrow? Any specific reason

2 Answers2