Pre-Engineering Physics-Optics Lab

Finding the Focal Length of a Concave Lens

 

Using the equipment provided, light source (connected to a 5V power source; possibly use circuit trainer), object, 4 different lenses, screen, and optics bench, set up as illustrated in the photo below.  Use only one lens at a time. 

 

Equipment Setup

 

Place the lamp at the 20 cm mark, the object on the object stand at the 35 cm mark, and the screen at the 75 cm mark.  Hook up the lamp to a 5-V power source, turn it on, and line up the equipment for viewing.  Then by sliding the lens along the bench, adjust the position of the lens until the image is as clear as possible on the projection screen.  Best viewed when room is dark with very little ambient light.

Note:  Slide the object stand and lens left and right to adjust until a clean-edged image forms on screen.   

 

 

 

Use the caliper to measure the heights down to the 1/1000th of the inch.   The heights should be as accurate as possible.  Use only the part of the image h’ that can be clearly viewed for measurement.  For example, select certain distinctive features to define the heights.  One such feature could be the top of the object down to the circular ring geometry which is slightly above the base.  Also, measure the h’ in a dark lab so the edges are better defined.  Then use this same part of the object to measure for the h.                                                                                            

 

 h                                                                                 h’        

                                                                                             

 

 

 

Measure the p and q distances down to the tenth of a centimeter. 

 

 

 

 

 

 

 

 

 

 

 

 

Left-Right Arrow: p Left-Right Arrow: q
 

 

 

 

 

 

 

 

 

 

 

 

 

 


p is the distance from the object to the lens.

q is the distance from the lens to the screen. (This value is negative, since the image is past/behind the lens.)

h is the height of the object as measured with a caliper. (to the 1/1000th inch)

h’ is the height of the image as measured with a caliper on the screen.  (to the 1/1000th inch)

 

 

 

 

 

 

Complete the table of data below with measured and calculated values. 

Use the lens equation 1/p + 1/q = 1/f to find the focal length of each lens.  Use M=h’/h to find magnification.    

Show work for each lens in this space provided below table. 

Lens type

Plano Concave

Double Concave

Plano Convex

Double Convex

p (cm)

 

 

 

 

q (cm)

 

 

 

 

h (in)

 

 

 

 

h’ (in)

 

 

 

 

f (calculated in cm)

 

 

 

 

M (use inches/inches)

 

 

 

 

 

Show work here.  Show diagrams on next page. 
Diagram position of image, lens, and object below.  Mark approximate position in cm as it is on the optical bench.                                                                                               

 

1.

 

 

 


2.

 

 

 


3.

 

 

 


4.