PHYSICS LENSES

2010-10-06 5:01 am

An object O is placed at distance u from the optical centre of convex lens A. The focal length of the lens of f1 ,where f1>u. An image I' is formed.

(A) write the equation which relates the image distance v' with u and f1.

(B) Another convex lens B is added in contact with lens A. The focal length of lens B is F2. The lenses are thin, such that the distance between the two optical centres is small compared to the object distance and the image distance. The image I' is acting as the object for lens B,

(I) What is the relation between the object distance u' for lens B and the image distance v' for lens A?

(II) hence write the equation which relates the image distance v and v' and f2.

(III) Consider the whole lens system. Find its focal length f in terms of f1 and f2.

HELP~

回答 (1)

天同

2010-10-06 6:58 pm

✔ 最佳答案

(A) Applying the thin lens equation:1/u + 1/v' = 1/f1 ------------------ (1)i.e. v' = u.(f1)/(u-f1) = -u.(f1)/(f1-u)

(B)(I) u' = -v'

(II) I suppose f2 denotes the focal length of lens BApply the thin lens equation,1/u' + 1/v = 1/f2i.e. -1/v' + 1/v = 1/f2 ------------------- (2)solve for v gives v = v'.f1/(v' + f2)

(III) Object distance = uImage distance = vhence. 1/f = 1/u + 1/v ----------------- (3)but from (1), 1/f1 = 1/u + 1/v'
from (2), 1/f2 = -1/v' + 1/v
adding, 1/f1 + 1/f2 = 1/u + 1/v
compare with (3), 1/f = 1/f1 + 1/f2

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