Measuring Focal Length of Gel Lenses



•Meter stick (1)

•Lens made from gel

•Lamp which projects an image

•Ring stand (or another object to support the lamp)

Apparatus set up:

•Procedure for Making Lamp:

+To make the lamp which projects an image, you will need four things: (1) A piece of paper, (2) your flashlight, (3) a thick marker, and (4) a rubber band or some tape.  Draw an image on the piece of paper as in the following picture:






+Secure this image to the end of your flashlight with either your tape or rubber band.  You now have a great object for the experiment!

•Set up your apparatus as in the following picture (includes (1) the lamp, (2) the meter stick, and (3) the lens):

Experimental Setup for measuring focal length



•Measure the height of the lamp from the ground using the meter stick.  This value is ____________.

•Make gels using the recipe and pour into watch glass molds (See “Recipe for Making Gel”).  Allow to gel.

•Take one of the watch glass lenses and move it up and down underneath the lamp.  Find the location of the lens at which the clearest image is formed.

•Measure the distance, called the image distance, from the lens to the ground .  This value is ­­­____________.

+To find the object distance, subtract the image distance from the height of the lamp.  Object distance = lamp height - image distance = ________ - ________ = ________.

•Calculate the focal length of the lens using the following equation:

•Now you have calculated the focal distance of your lens.  This value should be the same for different object and image distances.  Try it.

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