Focal Length Of Lenses
Objective
The objective of this experiment was to find the focal length of two converging lenses separately and also to find the focal length of combination of two converging lenses. Another objective was to study the image formed by a converging lens plus diverging lens.
Background
This experiment focused on a simple lens system. A simple lens is a piece of glassware that is spherical on one side and flat on another side, or spherical on both sides. There are two different types of lenses. A converging lens, also called a positive lens, and a diverging lens, also known as a negative lens. A converging lens thicker middle area than top and bottom, and the diverging lens has a skinnier mid-section.
When a parallel ray passes through the lens, the rays will be diffracted so that it will always intercept the focal point. When a ray passes through the optical center, there will be no diffraction. The three basic rules about the rays are:
1) Rays travelling parallel to the lens will diffract towards the focal point
2) Rays travelling at optical centre will have no diffraction.
3) Rays travelling through the focal point towards the lens will have a parallel diffraction to the plane.
Focal length f varies from lens to lens and it is characterized by:
1/f = 1/do + 1/di (equation 1)
do represents the distance of the object to the lens and di represents the distance of the image formed from the lens. Rearranging for f, the equation becomes:
f = dido / (d¬i + do) (equation 2)
The theory behind the experiment Part I is that when an object that is extremely far away, the focal length is equal to di. Using equation 1, this idea is supported. When 1 is divided by do (infinity), the value will get closer to zero, so it’s possible to assume the limit as zero. If this idea holds, the new equation will become 1/f = 1/di. Thus f = di. Using this theory, the...
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