To Find the Focal Length of a Concave Mirror (Class 10 Physics Practical)
To determine the focal length of a concave mirror using the mirror formula and distance measurements.
TL;DR: To determine the focal length of a concave mirror using the mirror formula and distance measurements.
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Aim: To determine the focal length of a concave mirror using the mirror formula and distance measurements.
Materials Required
- Concave mirror
- Plane mirror
- Object pin
- Mirror holder
- Meter scale
- Optical bench or wooden stand
- Screen
- Candle and matches
Theory
A concave mirror follows the mirror formula: 1/f = 1/u + 1/v, where f is focal length, u is object distance, and v is image distance. When a real, inverted image is formed on a screen, the object distance and image distance can be measured, and the focal length can be calculated.
Procedure
- Set up the concave mirror on the optical bench with the mirror holder
- Place an illuminated object (candle) in front of the mirror at various distances
- Move the screen until a sharp, inverted real image is formed on it
- Measure the object distance u (from mirror pole to object) and image distance v (from mirror pole to image)
- Record at least 5 sets of u and v values by varying the object distance
- For each set, calculate focal length using 1/f = 1/u + 1/v
- Calculate the mean focal length from all observations
- Verify using u + v versus u plots
Observation
When the object is placed at different distances from the mirror, real, inverted images are formed on the screen. The object and image distances are measured for each position. A typical observation shows that as the object distance increases, the image distance decreases.
Result
The focal length of the concave mirror is determined as [value] cm, which represents the distance from the mirror pole to the focal point where parallel rays converge.
Precautions
- Keep the mirror clean and free from dust
- Measure distances from the pole of the mirror accurately
- Ensure the object, mirror, and screen are on the same horizontal axis
- Darken the room for clearer image observation
Viva Questions
What is the focal length of a mirror?
Focal length is the distance between the mirror pole and its focal point, where parallel rays converge after reflection.
Why must the image be real to find focal length in this method?
Real images can be projected on a screen and their position can be accurately measured, unlike virtual images.
What is the mirror formula?
The mirror formula is 1/f = 1/u + 1/v, relating focal length, object distance, and image distance.
Can you find focal length if the image is virtual?
No, virtual images cannot be projected on a screen; we would need other methods like using a plane mirror to locate the image.
More Class 10 Physics Practicals
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- To Find Resistance Using a Metre Bridge
- To Study the Laws of Reflection
- To Determine the Density of a Solid
- To Verify the Laws of Refraction (Glass Slab)
- To Study a Simple Pendulum and Find the Acceleration Due to Gravity (g)
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