A student incorrectly calculates a positive object distance while using the lens formula for a real object. Based on the sign convention, what is the conceptual error in this calculation?
Object distance should be negative for a real object as it is placed to the left of the lens.
In the New Cartesian sign convention for lenses, the optical centre is taken as the origin and the object is placed on the left of the lens, so light travels from left to right. Distances measured in the direction of the incident light (to the right) are positive, and distances measured against it (to the left) are negative.
A real object lies on the left of the lens, so its distance is measured against the direction of incident light and the object distance \(u\) must be negative. Taking it as positive is therefore the conceptual error.
The sign of the object distance depends only on the side of the lens where the object lies, not on the focal length or on the object height (heights have their own sign rule: upward positive, downward negative). Saying the object distance is always positive for a real object repeats the student's error.
Hence, Object distance should be negative for a real object as it is placed to the left of the lens.
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A. Rear View mirrors in vehicles
B. Glass windows
C. Makeup Mirror
D. Kaleidoscope
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The power of a lens is -2.5 D. The type of lens and its focal length are respectively:
