Concave-Mirror Focal Length by Parallax
In the no-parallax method, the image position is located by making a movable needle coincide in depth with the real image; measured object and image distances then satisfy the concave-mirror relation with the object placed beyond the focus.
Why this shows up in the exam
Concave-mirror focal-length experiment · Locating real images on an optical bench · Removing depth-alignment bias
Learn the idea
Parallax disappears when the comparison needle and real image occupy the same spatial plane. If two viewed points are at different depths, they shift relative to each other when the eye moves sideways. Adjusting until that relative motion vanishes locates the mirror's real image.
🧠 Memory hook: No relative shift means no parallax and the same image plane.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- 1/f = 1/u + 1/v — magnitude form for a concave mirror producing a real image
- u = v = 2f — special coincidence at the centre of curvature
How to approach it
- 1Require a real image
- 2Move the eye laterally to test parallax
- 3Measure from the mirror pole and apply the stated method
Common slip-ups that cost marks
- •Placing the object inside the focal length for a real-image method
- •Judging overlap without moving the eye
- •Measuring from a stand edge instead of the pole
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
In a meter bridge, resistance 2 ohm is in the left gap and balance occurs at 40 cm from the left end. Find the right-gap resistance.
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