JEE Main 2026 — Ray Optics Question with Solution
JEE Main 2026 (22 January Shift 2)
Question
In parallax method for the determination of focal length of a concave mirror, the object should always be placed:
Choose an option
Show full solutionCorrect option: D
Correct answer
Dat any point beyond the focus of the mirror
Step-by-step explanation
In the parallax method for determining the focal length of a concave mirror, we use an object pin and its real image formed by the mirror.
The method of 'no parallax' involves aligning the tip of the object pin with the tip of its real image. This is only possible if the image formed is real.
For a concave mirror, a real image is formed only when the object is placed at any position beyond the focus (), i.e., .
If the object is placed between the pole () and the focus (), the image formed is virtual, and it cannot be caught on a screen or used for the standard parallax alignment with a physical pin in front of the mirror.
Therefore, the object should be placed at any point beyond the focus () of the mirror to ensure a real image is formed.
The method of 'no parallax' involves aligning the tip of the object pin with the tip of its real image. This is only possible if the image formed is real.
For a concave mirror, a real image is formed only when the object is placed at any position beyond the focus (), i.e., .
If the object is placed between the pole () and the focus (), the image formed is virtual, and it cannot be caught on a screen or used for the standard parallax alignment with a physical pin in front of the mirror.
Therefore, the object should be placed at any point beyond the focus () of the mirror to ensure a real image is formed.
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This is a previous-year question from JEE Main 2026, covering the Ray Optics chapter of Physics. PrepSharp catalogues every PYQ from JEE Main with a verified answer key and step-by-step solution prepared by IIT alumni — so you can search by chapter, topic or year and revise efficiently.