CBSE Class 12 Physics Practical Notes with Readings Section B Experiment – 1

Experiment – 1

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

  \frac{1}{f}=\frac{1}{u}+\frac{1}{v}

Or 

f=\frac{uv}{u+v}

Ray Diagram

Observations

Calculation

f_{1}=\frac{30.0×30.0}{30+30}=\frac{-900}{60}=−15.0\;cm

f_{2}=\frac{25.0×37.2}{25.0+37.2}=\frac{-950}{62.2}=−15.0\;cm

f_{3}=\frac{22.5×42.6}{25.0+37.2}=\frac{-950}{62.2}=−15.0\;cm

Mean=\frac{f_{1}+f_{2}+f_{3}}{3}

Mean=\frac{15⋅0+15.0+15.0}{3}

Mean=15.0\;cm

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.

We hope that provided CBSE Class 12 Physics Practical File Notes will be helpful to you. If you have any queries regarding this notes, please drop a comment below, and we will get back to you as soon as possible.