Objective
To find the value of v for different values of u in case of a concave mirror and to find the focal length.
Apparatus Required
A knitting needle, a half metre scale, an opticle bench with three uprights, two needles and concave mirror with mirror holder.
Formula Used
Ray Diagram
Observations
Calculation
Result
Mean value of focal length, f = 15.0 cm
(as per sign convention the focal length of concave mirror is negative) =- 15.0 cm
Focal length of concave mirror = 15 cm
Precaution
- Tip to tip parallax should be removed between needle and the image of the object needle.
- Index correction for u and u should be properly determined and must be applied.
- All the uprights should be vertical and principal axis of the mirror should be horizontal.
- To locate the position of the image, the eye should be at least 30 cm away from the needle.
- Tips of the needles and the pole of the mirror should be at the same height as that of the pole of the mirror.
Sources of Errors
- The parallax removal may not be perfect.
- The uprights may not be vertical.
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