Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Verification of the Mirror Formula Using Concave Mirror

Introduction

The object distance \(u\), image distance \(v\), and the focal length \(f\) of the mirror are related by,

\begin{align} \frac{1}{v}+\frac{1}{u}=\frac{1}{f}. \nonumber \end{align}

pfa.png

The pole of the mirror is taken as the origin and principal axis as the \(X\)-axis. Usually, the positive of the axis is taken along the incident rays. The distance between the pole and the object (candle) is \(u\) and distance between the pole and the image (screen) is \(v\). Note that both \(u\) and \(v\) are negative in present case.

Apparatus

A concave mirror, V-stand, Screen, Screen-stand, candle, matchbox, scale.

Procedure

  1. Firstly, find the approximate focal length \((f)\) of a concave mirror. Then fix it vertically on a V-stand. Draw a long straight line on a table and place the mirror stand on it. The pole of the mirror should be exactly above the line.
  2. Light a candle and place it on one end of the line. The flame of the candle should be at the same height as the pole of the mirror.
  3. Fix the screen on its stand. The screen should be vertical. Place the screen between the candle and mirror.
  4. Analyse the nature of the image by moving the screen and/or mirror.
  5. For three different values of \(u\), find the value of \(v\). Calculate \(f\) by substituting in formula given above.

Discussion

Write the results in the form of a table.