You can measure the approximate focal length of a concave mirror by forming the image of a distant object on a screen.
Measuring Focal Length: The Screen Method
This practical method utilizes the property that a concave mirror focuses parallel rays of light from a distant object (like the sun or a faraway building) at its focal point. By finding the position where this focused image is sharpest, you can determine the focal length.
Here's how to do it:
- Set up the mirror: Place the concave mirror on a stable surface, facing a distant object.
- Position the screen: The screen should be placed in front of the reflecting surface of the mirror.
- Adjust the screen: Move the screen back and forth until a clear, sharp image of the distant object appears on it.
- Measure the distance: Using a metre scale the distance between the concave mirror and screen can be determined. This distance represents where the parallel rays from the distant object converge.
- Determine the focal length: The distance is the same as the focal length of the given concave mirror.
Why This Works
When an object is very far away (theoretically at infinity), the light rays arriving at the concave mirror are nearly parallel. Concave mirrors are designed to converge parallel rays at their focal point. Therefore, the sharpest image of a distant object is formed precisely at the mirror's focal point. Measuring the distance from the mirror's surface to this sharp image on the screen gives you the approximate focal length ($f$).
Practical Tips
- Choose a very distant object for best accuracy. The sun (use caution and never look directly at the sun or its reflection) or a building visible through a window work well.
- Ensure the mirror and screen are perpendicular to the line connecting the mirror to the distant object (if possible) and parallel to each other for accurate distance measurement.
- The sharper the image, the closer the screen is to the true focal point.
This straightforward method provides a good approximation of a concave mirror's focal length using basic equipment.
