Introduction
Newton's Ring Experiment was first performed by Isaac Newton in the year 1717 which also proved that light exhibits wave nature. This experiment help us to determine:-
- The wavelength of the monochromatic light source
- The radius of curvature of the plano-convex lens.
In this blog, we have discussed about:-
Objective:-
To study the Newton's ring experiment.
Materials required:-
- A plano-convex lens of larger focal length (or less thickness)
- Two plane glass plate
- A convex lens
- Monochromatic source of light (sodium lamp)
- A travelling microscope
Construction:-
Place the sodium lamp at the focus of the bifocal convex lens so that after
refraction the light beam becomes parallel and we obtain a plane wavefront.
One of the glass slab is kept at 45 degree to the horizontal light beam. The
plano-convex lens is placed above the glass plate as shown below. The
microscope is placed vertically upwards to this system and the inclined glass
plate.
Working:-
When the parallel beam of light strikes the inclined glass slab, it gets
reflected (towards the lens-plate system) as well as refracted (towards the
microscope). Focusing on the reflected one, when the light reaches the lower
portion of the plano-convex, due to refraction on light waves at its upper
surface, it is once again refracted and reflected. The latter one when reaches
the upper surface of the glass plate, again undergoes reflection and
refraction.
Interfering Waves:-
We have seen various waves but the two waves which are actually responsible
for the formation of newton's ring due to interference are:-
- The reflected ray emerging from the lower part of the plano-convex lens
- The reflected ray coming from the upper part of the glass plate
Important Questions:-
1. Why the Newton's ring are circular?
Sol:- The Newton's ring are circular because the locus of all the points,
with thickness of air film as constant, lies on a circle with point of
contact of lens and glass plate as center, when viewed in three dimensional
view.
2. Why the center of the pattern in Newton's ring experiment is dark?
Sol:- At the point of contact of the lens and the glass plate, path
difference of lamda by 2 occurs as the light gets reflected from a denser
medium i.e. glass plate. Since the path difference is an integral multiple
of half of the wavelength, the condition of destructive interference is
satisfied, so the central ring is dark.
