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# physics 2

1.What is the wavelength of the light entering an interferometer if 648 bright fringes are counted when the movable mirror moves 0.205 mm?

2.What is Brewster's angle for an air-alcohol (n = 1.36) surface?

3.A total of 33 bright and 33 dark Newton's rings (not counting the dark spot at the center) are observed when 530 nm light falls normally on a planoconvex lens resting on a flat glass surface (Fig. 24-31). How much thicker is the center than the edges?

### 1 Answer

- 魏王將張遼Lv 71 decade agoFavorite Answer
(1) According to the given from the question, it is known that for a distance of 0.205 mm, the path difference has changed by 648λ, so:

648λ = 0.205 × 10-3

λ = 316.4 nm

(2) Using the formula n = tan θp, where θp is the Brewster's angle:

tan θp = 1.36

θp = 53.67°

(3) The equation for the radius of the mth Newton's bright ring goes as follows:

圖片參考：http://upload.wikimedia.org/math/d/3/a/d3a9fbfc395...

where

R is the radius of curvature of the lens the light is passing through,

m is 1,2,3... which is dependent upon the number of light spots,

λ is the wavelength of the light passing through the glass.

So, for the 33rd ring, x33 = √(33.5 λR)

Also, with the aid of following diagram:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywun...

and the formula x332 = 2Rt where t is the central thickness of the lens.

So, 33.5 λR = 2Rt

t = 16.75 λ

= 8.88 μm

So the center is 8.88 μm thicker than the edges.

Source(s): My Physics knowledge