WBBSEClass XPhysical ScienceLight – Reflection and Refraction03 Mark2026 Moderate

QQuestion

Establish the relationship between radius of curvature (r) and focal length (f) for a convex mirror. Write one use of a concave mirror. [2+1]

Exam-ready answer 03 Mark

Answer

Establish the relationship between radius of curvature

Let P, F and C be the pole, focus and centre of curvature of the convex mirror.

According to the law of reflection, ∠MQC’ = ∠NQC’

Since MQ ∥ PC, ∠CQC’ = ∠FCQ

Therefore, ∠FCQ = ∠FQC

Hence, FC = FQ

For paraxial rays, FQ ≈ FP = f

Therefore, FC = FP = f

Now, CP = CF + FP = 2f

Since CP = r,

r = 2f or, f = \frac{\text{r}}{2}

Use of concave mirror: It is used as a reflector in vehicle headlights.

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