Question

Which type of mirror has negative focal length?

A. Convex mirror

B. Concave mirror

C. Plane mirror

D. None of the above

Answer 1

Hint: We can solve this problem with the concept of reflection of light at the spherical mirror. Negative focal length gives us an idea about the position of the focus point i.e. negative focal length means both  the focus point and object are on the same side and in front of the lens or mirror. In the positive focal length, the focus point is on the opposite side to the mirror or lens.

Complete answer:

A plane mirror is the only mirror which always has a virtual, erect, and same size image of the real object. While virtual objects produce real images. The real object presents in front of the mirror and its image appears to be behind the plane mirror. For a plane mirror angle of reflection equals the angle of incidence.

Concave and convex mirror is a spherical mirror with a curved reflecting surface. Convex mirrors and concave mirrors are defined by their reflecting surfaces. if the concave side of the spherical mirror is reflecting, it is called a concave mirror and if the convex side of the mirror is the reflecting surface, it is called a convex mirror. 

Focal length formula for mirror is $\dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u}$, where $v$ is the distance from pole to image and $u$ is the distance from pole to object and $f$ is the focal length. Here we use sign conventions. Focal length is positive for a convex mirror and negative for concave mirror. 

So, the correct answer is “Option B”.

Note:

We know that images formed by the concave mirror may be inverted or erect, real image or virtual image and it can be smaller, larger or of the same size depending on the situation of the object. While the image formed by the convex mirror is a virtual and erect image and also  it is smaller than the object always.