A luminous object is placed at a distance of 40 cm from a converging lens of focal length 25 cm. The image obtained in the screen is
- A.erect and magnified
- B.erect and smaller
- C.inverted and magnified
- D.inverted and smaller
Correct answer
Explanation
The correct answer is C, inverted and magnified. The object lies between F and 2F of the convex lens (25 cm and 50 cm), and such an object always gives a real, inverted and enlarged image beyond 2F.
Using the lens formula 1/v − 1/u = 1/f with u = −40 cm and f = +25 cm gives 1/v = 1/25 − 1/40 = 3/200, so v is about +66.7 cm. The positive value shows a real image on the other side of the lens, which is why it can be caught on a screen. The magnification m = v/u is about −1.67: the minus sign means the image is inverted, and a size above 1 means it is larger than the object. This is the arrangement used in a slide or film projector.
Option A is wrong because an image formed on a screen is real, and a real image from a single lens is never erect. Option B is wrong for the same reason, and it is also enlarged, not smaller. Option D is wrong because a smaller inverted image needs the object beyond 2F, that is, more than 50 cm away.
Exam tip: convex lens, object between F and 2F = real, inverted, magnified image beyond 2F (projector).
Practice Questions
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- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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