Focal Length of Convex Mirror – Rankers Physics

Refraction by Spherical Surfaces: Practice Problem & Solution

Assertion (A): Focal length of a convex mirror may be negative. Reason (R): Distances measured in the direction of incident rays may be taken as negative.  
Both (A) & (R) are true and the (R) is the correct explanation of the (A)
Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(A) is true but (R) is false
Both (A) and (R) are false

Solution Explained:

To solve this problem, we apply the core principles of Refraction by Spherical Surfaces. Understanding the underlying formula is key to arriving at the correct answer below:

According to Cartesian sign convention, the focal length of a convex mirror is always positive as its principal focus is virtual and lies behind the mirror. Thus, (A) is false. While sign conventions involve directions, stating that distances measured in the direction of incident rays 'may be taken as negative' is generally incorrect or misleading; typically, these are taken as positive. Hence, (R) is also false. Thus, both (A) and (R) are false.

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