There are two types of spherical mirrors—concave and convex.

Concave mirrors have an inwardly curved reflecting surface. Concave mirrors are also known as converging mirrors because they collect or converge the rays of light parallel to the principal axis that falls on them to a single focal point. Concave mirrors concentrate light rays and can produce large, clear images.

A convex mirror has an angled(Curve) shape with a reflector that protrudes toward the light source, also known as a divergent mirror. The light in these types of mirrors is reflected outwards. Therefore, they are not used for focusing lights . Regardless of the distance between the mirror and the object, the image of a convex mirror is always reduced, erect, and virtual. Because the focal point and the curvature centre  are both hypothetical positions ‘within’ the mirror that cannot be accessed, virtual images are always formed.  As the images created using these types of mirrors  remain contained within the mirror, these cannot be used as screen projections.  The picture is smaller than the item initially, but as the same gets closer to the mirror, it gets bigger, but not bigger than the object.

What is focal length?

The distance between a lens or mirror’s focal point and that of convex lens or mirror’s is termed as focal length. It can also be understood as the convergence or meeting point of two parallel rays of light. Depending on the nature of the lens and mirror, the focal length varies with the sign (positive or negative) (convex or concave ).

The focal length:

• The concave mirror, just  as a regular mirror, adheres to the laws of reflection of light. 
• When rays parallel to the principal axis hit the concave mirror (same axis), the reflection converges at the Principal focus, which is the specific point of convergence
• An exact, smaller, and inverted  reflection gets formed where the mirror is focused
• The focal length of a concave mirror is the distance among the concave mirror’s pole P and F which is considered to be the focus.

The focal length of the convex mirror is positive, whereas the focal length of the concave mirror is negative. The mirror formula can be used to demonstrate the same point:

1/f = 1/v+1/u

Because it is apparent that the item is specifically situated, which is the left or the opposite direction of the mirror’s  ray of incidence, the distance of the item has to be -ve.

 u = -u

v = -v

 (Reflection generated through concave mirror are typically left side or the opposite direction of the mirror’s ray of incidence)

If you use the mirror formula:

 1/f =1/v+1/u 

Or f = (u+v)/uv

Or f = uv/u+v

The process of determining a concave mirror’s focal point is as follows.

• A distance of over fifty feet among the picked different items
• The mirror should be placed on a stand and the item to be reflected should face each other
• The position of the screen has to be in front of the mirror for proper reflection
• Appropriate tweaks should be done for proper,  and sharp image with clarity
• Here, the distance between focal length and Mirror equals the distance of screen and that of mirror, the same can be measured using unit of metre
• Mean of focal length can be measured through repetition of the above steps thrice.

Conclusion 

The focal length of a concave mirror is the distance between its pole P and its focus F. The focal length of a concave mirror can be estimated by obtaining a ‘real image’ of a distant object at its focus.