A plastic lens is made with material with index of refraction n=1.4. If the fron

Question

A plastic lens is made with material with index of refraction n=1.4. If the front lens is ground to a radius of 1.3 m and the rear lens ground to a radius of -0.9 m, what is the focal length of the lens?

a. 1.5 m

b. -7.3 m

c. 0.75 m

d. -0.14 m

A 1.2 cm figurine is placed 0.8 m in front of the lens in the previous problem. Where will the image be?

1.7 m from the lens on the opposite side from the object.

1.7 m from the lens on the same side as the object.

0.52 m  from the lens on the same side as the object.

0.52 m from the lens on the opposite side from the object.

A 1.2 cm figurine is placed 0.8 m in front of the lens in the previous problem. Will the image be upright or inverted?

There will be no image.

Upright

More information is required to answer this question.

Inverted

1.7 m from the lens on the opposite side from the object.

1.7 m from the lens on the same side as the object.

0.52 m  from the lens on the same side as the object.

0.52 m from the lens on the opposite side from the object.

Explanation / Answer

Problem 1)

using lens maker's formula,

(1/f) = (n - 1) [(1/R1) - (1/R2)]

(1/f) = (1.4 - 1) [(1/1.3) - (1/-0.9)]

(1/f) = 0.4 x 1.88 = 0.75

Focal length, f = 1.38 m ~ 1.5 m

Correct answer: Option (A)

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