- Hold a concave mirror perpendicular to the direction of sunlight.
- Take a small paper and slowly move it infront of the mirror.
- Find the point where you get smallest and brigh test spot, which is the image of the sun.
- 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.
- Measure the distance of this spot from the pole § of the mirror.
- This distance is the focal length (f) of the mirror.