1>Jill is farsighted and cannot see objects clearly that are closer to the eye t

Question

1>Jill is farsighted and cannot see objects clearly that are closer to the eye than 65.0 cm. What is the focal length of the contact lenses that will enable her to see objects at a distance of 25.0 cm from her eyes?
A) +40.6 cm B) -40.6 cm C) -23.3 cm D) +23.3 cm

2> A double-slit interference experiment is performed in the air. Later, the same apparatus is immersed in benzene (index of refraction, n = 1.50), and the experiment is repeated. When the apparatus is in benzene, you observe the interference fringes to be
A) more closely spaced as when the apparatus is in air.
B) equally spaced as when the apparatus is in air.
C) more widely spaced as when the apparatus is in air.
D) Impossible to tell without knowing the wavelength of light in use.

Explanation / Answer

The near point of the Jill q = 25 cm object distance p = 65cm now by apply thin lens equation 1/p +1/q = 1/f plug the given values in the above equation to get focal length 1/f = 1/65 + 1/-25 = 1/f f = Pq/P+q    = 40.625 cm ................................................................... (2) In interference pattern fringe width = n/d refractive index n = c/v                               = speed in air / speed in medium                               = f/1f                               = /1 so refractive index is increases then the wavelenth will decrease. Those are in inveresly proportional. so refractive index is increases then the wavelenth will decrease. Those are in inveresly proportional. fring width = D/d so the spacing betweeen the springes will decrease. so the answer is A.

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