Determination of the focal length of a concave mirror

 

The aim of the experiment

Experiment with the tools

Light source, concave mirror, convex mirror, object and screen

Laws

Steps to conduct the experiment

  :Method I Direct method

 Place the concave mirror facing the object, an arrowhead or a crosswire after a lamp, and screen with object in the same level as shown in Fig.(1)

 Adjust the position of mirror until a sharp and screen image coincides with the object

 Measure the distance between the bottom of the mirror and the object

  Find the focal length (f) of the mirror by applying equation (1)

   Find the percentage error (p.e) of the focal length: p.e= ((fth-fexp)/fth) × 100% 

f = …….cm

Method II Graphical method

 Place the object in front of the mirror

 Move the screen until a clear inverted image of the arrowhead is obtained (Fig. 2)

 Measure the distance So from the object to the mirror and measure the distance Si from the screen to the mirror

 Repeat steps 2 and 3 for different values of So and measure Si in each time

 Use the data recorded above for object and image distances to plot 1/so (y−axis) against 1/si(x−axis).

 Determine f from the reciprocal of the intercept of the straight line with the x- axis or the y- axis

 Find the percentage error (p.e) of the focal length: p.e= ((fth-fexp)/fth) × 100%

Virtual Lab