How many bright rings are produced in Newton's Rings experiment?

[threewingedfury]

Figure 35-46a shows a lens with radius of curvature R lying on a flat glass plate and illuminated from above by light with wavelength . Figure 35-46b, a photograph taken from above the lens, shows that circular interference fringes (called "Newton's rings") appear, associated with the variable thickness d of the air film between the lens and the plate. The radius of curvature R of the lens is 5.0 m and the lens diameter is 17 mm.

(a) How many bright rings are produced? Assume that = 555 nm.

(b) How many bright rings would be produced if the arrangment were immersed in water (n = 1.33)?



I used the equation: m=r^2/R*lambda - .5

Definetly not getting the right answer

 

[hage567]

Can you show more of your calculation?

 

[threewingedfury]

m=r^2/R*lambda - .5

m=.017^2/(5*555*10^-9) - .5
m=104

m=(1.33).017^2/(5*555*10^-9) - .5
m=138

I've tried 103,104, and 105 for a

and done the same for b

but the method can't be right because in the book r=20mm, R=5m lambda=589nm and the answers are a. 34 and b. 46

 

[hage567]

17 mm is the diameter, not the radius.

but the method can't be right because in the book r=20mm, R=5m lambda=589nm

I don't understand, these are given in the answers?