In the figure, a convex lens of focal length 10 cm has an object 15 cm in front of it. How far from the lens is the image formed? A convex lens stands on a horizontal principal axis, with F₁ and 2F₁ marked on the left of the lens and F₂ and 2F₂ on the right. The label reads f = 10 cm. An upright object arrow stands between F₁ and 2F₁.

In the figure, a convex lens of focal length 10…

In the figure, a convex lens of focal length 10 cm has an object 15 cm in front of it.

HardOriginal question

In the figure, a convex lens of focal length 10 cm has an object 15 cm in front of it. How far from the lens is the image formed?

6 cmCorrect answerYour answer15 cmCorrect answerYour answer25 cmCorrect answerYour answer30 cmCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer D

Correct answer: D

Lens formula: 1/v − 1/u = 1/f, with u = −15 cm and f = +10 cm.
1/v = 1/10 + 1/(−15) = 3/30 − 2/30 = 1/30, so v = +30 cm.
The image is 30 cm from the lens on the other side, which fits an object between F and 2F.

Worth noting

6 cm comes from adding 1/10 and 1/15, forgetting that u is negative. 25 cm simply adds the distances.

Ten questions from Everyday Physics, at your own pace.

Ten questions from Everyday Physics, at your own pace.