Motion and Rotation of a Plane Mirror
For motion normal to a fixed object, image speed relative to the object is twice the mirror speed. Rotating a mirror by theta rotates a reflected ray by 2 theta when the incident ray is fixed.
Why this shows up in the exam
Scanning mirrors · Optical galvanometers · Laser steering
Learn the idea
A plane-mirror image responds twice to mirror translation and twice to mirror rotation in the appropriate geometry. Moving the mirror changes both the object-to-mirror and mirror-to-image gaps, so the image can move twice as much.
🧠 Memory hook: Mirror moves or turns once; the image effect is twice.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- v_image/object = 2 v_mirror — mirror translates normally while object is fixed
- Delta ray = 2 Delta mirror — fixed incident direction and rotating plane mirror
How to approach it
- 1Choose a reference frame
- 2Write the image as the reflection of the object
- 3Differentiate position or compare normals
Common slip-ups that cost marks
- •Using the factor two for arbitrary translation
- •Confusing image speed in ground and object frames
- •Rotating both incident ray and mirror while applying 2 theta
🌟 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.
A real object is placed 30 cm from a converging lens of focal length 10 cm. Find the real image distance.
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