Question

Image of an object approaching a convex mirror of radius of curvature 20m along its optical axis is observed to move from 25/3 m to 50/7 m in 30 sec. What is the speed of the object in kmh-1? 

(a) 3 

(b) 4 

(c) 5 

(d) 6  

Answer 1

Correct option is (a) 3.

Explanation:

Using mirror formula,

[1/(+25/3)] + [1/(-u₁)] = 1/10 

or 1/u₁ = (3/25) – (1/10) 

or u₁ = 50 m 

and 1/(+50/7) +(1/-u₂) = 1/10 

1/u₂ = (7/50) – (1/10) 

or u₂= 25 m 

speed of object = (u₁ -u₂) / time = 25/30 ms-1 = 3 kmh^-1