CONVEX AND CONCAVE LENSES

CONVEX AND CONCAVE LENSES OBJECTIVE: To find the focal point and focal length of convex and concave lens.

CONVEX LENS – CONVERGING LENS

CONCAVE LENS - DIVERGING LENS

Convex LensA convex lens is shaped like two half circles, thin at the edge and thick at the middle. A convex lens is converging and can produce virtual and real images.

Concave LensA concave lens is thin at the center and thick at the edges. . Concave lenses only produce virtual images They are diverging lenses.

THEORY • When parallel light rays pass through a thin lens, They emerge either converging or diverging. • The point where the converging rays cross is the FOCAL POINT of the lens. • The FOCAL LENGTH of the lens is the distance from the center of the lens to the focal point. • If the rays diverge, the FOCAL LENGTH is negative.

TOOLS • LIGHT SOURCE • CONVEX LENS. • CONCAVE LENS. • METER RULER. • PENCIL.

STEPS 1.Place the light source in ray –box mode on a white sheet of paper.Turn the wheel to select three parallel rays. 2.Trace around the surface of the lens and trace the incident and transmitted rays.Mark arrows in proper direction. 3.The point where the outgoing rays cross is the focal point of the lens.Measure the focal length from the center of the lens to the focal point.

4.Repeat the procedure with the concave lens. Now the rays are diverging and do not cross. Use ruler to extend the outgoing rays straight back through the lens. 5.The focal point is where these extended rays cross. Record the focal length as a negative number. 6.Place the convex and concave lenses together in the path of the parallel rays. Trace the rays. Write the result. 7.Slide the convex and concave lenses apart by few centimeter and observe the effect. Then reverse the order of the lenses. Trace at least one pattern of this type. Note the changes and write.

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CONVEX AND CONCAVE LENSES