# Newton's Rings experiment

1. Apr 13, 2008

### steven10137

1. The problem statement, all variables and given/known data
In a Newton's Rings experiment, involving a curved lens on a glass surface, what might have happened to the set-up to see a bright spot at the centre?

2. The attempt at a solution
Well I know that normally we get a dark spot at the centre because of the lambda/2 phase difference which causes destructive interference, but I am a little confused with how a bright spot could be seen at the centre.

My first thought was that perhaps an air gap was left between the centre of the lens and the glass but i'm not really sure.

cheers
Steven

2. Apr 13, 2008

### steven10137

Anyone got any suggestions?

i was thinking it must have something to do with the phase differences on exit, but no idea how to relate this to the experiment.
http://physics.bu.edu/~duffy/semester2/c26_newtons_rings.html

I know that the equation for the radius of the m'th Newton's bright ring is:
$$x_m = \left[ {\left( {m + \frac{1}{2}} \right)\lambda R} \right]^{1/2}$$
where:
R is the radius of curvature of the lens the light is passing through,
m is 0,1,2,3... which is dependent upon the number of light spots,
λ is the wavelength of the light passing through the glass.

Last edited: Apr 13, 2008
3. Apr 13, 2008

### steven10137

nevermind I found the solution elsewhere.
Turns out it is because of the air gap between the lens and the glass.
Upon reflection, there is a further phase difference, meaning that it will be > lambda/2
and hence not give fully destructive interference and give a bright spot.

thanks neways

4. Apr 13, 2008

### mysqlpress

anyway, you can change the result of Newton ring.
Just view at the bottom of the ring. the transmission of light should be opposite to the surface