Lens — Self-Conjugate PointHardQuestion 20248
Question
An object is placed at the centre of curvature of a concave mirror and a lens in between. Under what condition does the lens not affect the final image position?
- ALens is at the focus of mirror
- BThe object is at optical centre of the lensCorrect
- CLens at centre of curvature
- DThe lens has infinite focal length (absent)
Correct answer
The object is at optical centre of the lens
Explanation
A ray passing through the optical centre of a lens is undeviated. If the object is at the lens's optical centre, all rays pass undeviated through the lens (no refraction) — the lens effectively isn't there. The image is then determined purely by the mirror.