JAMB 2025 Physics · Question 96

A 25cm long pinhole camera produces a one-fifth of an object's size. Calculate the object distance

Options

A.20.49m
B.2.82m
C.4.23m
D.1.25m

Answer: D. 1.25m

Explanation written with AI and checked against the answer key

Analyze the question A pinhole camera forms an inverted image on a screen. The camera length (pinhole to screen) is the image distance, v = 25 cm. The image is one-fifth the size of the object. We need the object distance u. Concept For a pinhole camera, similar triangles give: magnification = image height / object height = image distance / object distance h_i / h_o = v / u Solution Given h_i / h_o = 1/5 and v = 25 cm: 1/5 = 25 / u u = 25 × 5 = 125 cm = 1.25 m Check against the options 125 cm = 1.25 m, which matches option D. The other options don't come from this relationship. For example, 2.82 m and 4.23 m would need a different ratio or camera length. Answer: D. 1.25 m Tip: In pinhole camera questions, always use the ratio image size/object size = image distance/object distance, and convert cm to m at the end (divide by 100). Watch the units, because a slip there leads to wrong options like 12.5 m or 0.125 m.

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