# To study the variation of image position with incident angle for an object at infinity

## Objective

To study the variation of the image position with incident angle for an object at infinity.

## Introduction

A parallel beam going parallel to the principal axis of a concave mirror meets after reflection at the focus of the mirror. What will happen if the parallel beam is not parallel to the principal axis?

## Apparatus

A Light box giving two parallel narrow light beams, a concave mirror, scale, graph paper, pencil.

## Procedure

On a plane paper draw the position of concave mirror and its axis. From the pole draw lines making angles \(\theta=\) 10 degree, 20 degree, 30 degree, 40 degree, 50 degree, 60 degree, 70 degree with the axis. Place the light box on the platform showing the two light beams. Treat this as a pair of rays. Put the concave mirror and let the rays fall on it such that one of the rays goes along the line at 10 degree. On the sides the rays will meet and then diverge. You have to accurately locate this point of intersection. Normally the intersection is not sharp. To get it we suggest the following. Put a graph paper and let the reflected light go on it. Mark two positions on the either side of the intersection where the gap between the two rays are equal, say 8 mm or so. The mid-point between these marks should be the intersection point

Measure the distance \(v\) between the pole of the mirror and the intersection point using a scale. Repeat for other angles and make a table of \(v\) and \(\theta\). Calculate the projection on the principal axis \(v\cos\theta\) and put in same table. Draw a graph of \(v\) versus \(\theta\).

Write your observations from this table.