why parallel incidence is required in newton's ring experiment?
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In a Newton rings experiment, a parallel beam from an extended source of light falls on a glass plate at an angle of incidence of 45°, making the reflected ray fall on a convex lens ( mostly a plano convex lens) put on a smooth flat glass plate normally with curved surface of the lens in contact with the flat glass plate. Circular rings are formed in the air gap in between the flat glass plate and the plano convex lens.
An extended source can be regarded as a collection of large number of points sources Each point source gives a beam of parallel rays, all these parallel rays from one point give rise to one point of the Newton's ring. So for the whole ring to be formed we need a number of parallel rays from different points of the source. That is why we need an extended source to form the Newton's rings.
When I was a student we called the source an extended source, instead of a broad source. I guess the nomenclature has changed now.
The Newton's rings experiment is done using a Sodium lamp. It gives yellow light of wave lengths 5890A° , 5896A°, which can be regarded as monochromatic.
While doing experiment did you notice that if power goes off, the sodium lamp goes off and does not start emitting light even when power comes after say a few seconds. Do you wonder why this happens? Or why is the light of the sodium lamp red in the beginning and changes to yellow much later. Why this happens?
Newton rings are Alternate dark and bright rings formed due to the presence of air film when the plano-convex lens is placed on the glass plate is called newtons rings.
The angle between the incoming ray and the glass plate is 45 degree to make turns the light rays to 90 degrees and that's why the rays fall normally on the plano-convex lens. Finally forms circular rings.