To Determine The Focal Length Of A Concave Mirror. | Grade Xi

APPARATUS REQUIRED:

• Concave mirror
• Meter rule
• Screen
• Light source (e.g., LED bulb)
• Pointer

THEORY:

A concave mirror is a mirror with a reflecting surface that curves inward. When light strikes a concave mirror, it is reflected toward the center of curvature of the mirror. The point where the reflected light rays meet is called the focal point. The distance between the center of curvature and the focal point is called the focal length.

PROCESS:

1. Set up the experiment as shown in the diagram. Place the concave mirror on a horizontal surface.
2. Place the light source at a distance greater than the focal length of the mirror.
3. Adjust the position of the screen until a sharp, inverted image of the light source is formed on the screen.
4. Measure the distance between the mirror and the screen. This is the image distance (v).
5. Measure the distance between the mirror and the light source. This is the object distance (u).
6. Calculate the focal length (f) of the mirror using the formula:

1/f = 1/u + 1/v

OBSERVATIONS:

• The image of the light source is inverted.
• The image is smaller than the object.
• The image is located at a distance from the mirror that is less than the object distance.

OBSERVATION TABLE:

Object Distance (u)	Image Distance (v)	Focal Length (f)
30 cm	20 cm	15 cm
40 cm	26.7 cm	20 cm
50 cm	33.3 cm	25 cm

Calculation:

Using the formula 1/f = 1/u + 1/v, we can calculate the focal length (f) of the mirror for each set of measurements:

• For u = 30 cm and v = 20 cm:

1/f = 1/30 cm + 1/20 cm 1/f = 0.033 cm^-1 + 0.050 cm^-1 1/f = 0.083 cm^-1 f = 12.0 cm

• For u = 40 cm and v = 26.7 cm:

1/f = 1/40 cm + 1/26.7 cm 1/f = 0.025 cm^-1 + 0.037 cm^-1 1/f = 0.062 cm^-1 f = 16.1 cm

• For u = 50 cm and v = 33.3 cm:

1/f = 1/50 cm + 1/33.3 cm 1/f = 0.020 cm^-1 + 0.030 cm^-1 1/f = 0.050 cm^-1 f = 20.0 cm

RESULTS:

The focal length of the concave mirror is approximately 16 cm.

PRECAUTIONS:

• Make sure that the light source is placed at a distance greater than the focal length of the mirror.
• Make sure that the screen is placed at a distance from the mirror that is less than the object distance.
• Use a sharp pointer to mark the position of the image on the screen.