Question

A convex mirror used for the rear view on an automobile has a focal length of
2.5m. A car is
located at a distance of 4m from the mirror.
In the above scenario the sign that should be assigned to f and u is
a) u positive, f negative
b) f positive, u negative
c) f and u are positive
d) f and u negative

Answer 1

Given:

• Convex mirror being used as Rear-View mirror in car.
• Focal length is 2.5 metre.
• A vehicle located 4 metre behind this car.

To find:

• Sign of f and u ?

Calculation:

Always remember that for a convex mirror, value of 'f' (i.e. focal length) is always positive. This is because the length vector of focal length is directed in the same direction as the ray vector of light.

• Secondly, the object (i.e. vehicle) is behind the car (i.e. mirror). Hence, the sign of 'u' is negative.

• This is because the length vector of 'u' (i.e. object distance) is opposite to direction of ray.

Option b) is correct ✔️

$\star$ Hope It Helps.