1) Hold a concave mirror perpendicular to the direction of sunlight.
2) Take a small paper and slowly move it infront of the mirror.
3) Find the point where you get smallest and brigh test spot, which is the image of the sun.
4) The rays coming from the sun parallel to the
concave mirror are converging at a point. This point is called focus or focal point (F) of the concave mirror.
5) Measure the distance of this spot from the pole (P) of the mirror.
6) This distance is the focal length (f) of the mirror.