Determination of the focal length of a convex lens

 

The aim of the experiment

Experiment with the tools

Optical bench with stages

positive lens

negative lens

lens holder

image screen with holder

plane

mirror with holder

illuminated object

Laws

Where hi and ho are the height of the image and the object, respectively

Steps to conduct the experiment

Method I: Plane mirror method

 Place a vertical plane mirror just behind the converging lens

 Set an illuminated object (a pin or a slit of the shape of an arrow after a lamp) in front of the lens

 Keep the mirror, the lens, and the object at the same height above the bench

 Look past the illuminated object toward the lens. You should see an inverted image of the object

 Move the object toward or away from the lens until its image appears to “float” directly above the object. Then, the object and its image are a focal length away from the lens.

 Make three or more independent measurements, take the average of your results, and assign errors

 Draw a ray diagram to explain how this method works

Method II: Lens formula method and the length of the image

 

 Keep an illuminated object near one end of the optical bench 

Place the lens between the object and the screen and adjust the positions of both the lens and the screen  until a sharp magnified image appears on the screen (Fig. 2). Measure the object and image distances

  Measure the length of the image and calculate the magnification of the image

 Move the screen and readjust the lens to get a magnified image and measure the image length,  magnification, and distance

  Repeat the previous step for different positions of the screen

  Use the data recorded above for image magnification and distance to plot M (y−axis) against si (x−axis).

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

The intersection of the x-axis = ----, f = …….cm                          

  Method III: The lens formula and graphical plot 

 Keep an illuminated object near one end of the optical bench

 Place the lens between the object and the screen and adjust the positions of both the lens and the screen until a sharp image appears on the screen (Fig. 2). Measure the object and image distances

 Repeat step 2 many times at different object and image distances

 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

Virtual Lab