A)- A camera is supplied with two interchangeable lenses, whose focal lengths ar

Question

A)- A camera is supplied with two interchangeable lenses, whose focal lengths are 35.0 and 150.0 mm. A woman whose height is 1.80 m stands 7.00 m in front of the camera. What is the height (including sign) of her image on the film, as produced by each of the following.

(1) the 35.0 mm lens

Incorrect: Your answer is incorrect. .

(2) the 150.0 mm lens

Incorrect: Your answer is incorrect. .

B) A nearsighted person cannot read a sign that is more than 7 m from his eyes. To deal with this problem, he wears contact lenses that do not correct his vision completely, but do allow him to read signs located up to distances of 12.7 m from his eyes. What is the focal length of the contacts?

Explanation / Answer

A)

Let u be the object distance, v be the image distance, and f be the focal length, and m be the magnification, then according to lens equation:
1/u + 1/v = 1/f
m = v/u.

(1)
1/7+ 1/v = 1/0.035
v = 3.51 x 10-2 metre.
m = v / 7 = 5.02 * 10^(-3).

(2)
1/7 + 1/v = 1/0.15
v = 0.153 metre.
m = v / 7 = 2.17 * 10-2

B)

The object will be at 12.1m
So u = - 12.1 m
A virtual image of the object will be developed from where the person is able to read.

So v = -7 m

Use lens formula:
1/f = -1/7 + 1/12.1 = -0.14 + 0.08 = -0.06
f = -16.6m

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