Compound Lens Two Lenses A single convex lens

Compound Lens

Two Lenses ] A single convex lens produces a real image. ] That image can be acted on by a second lens. • Second image can be real or virtual

Final Image ] Two thin convex lenses with focal lengths 0. 30 m and 0. 50 m are separated by 0. 20 m. ] An object is placed 0. 50 m in front of the first lens. ] First apply the lens equation to the first lens. • f 1 = 0. 30 m, so 1 = 0. 50 m ] The intermediate image is so 2 = ] Find the image in the second lens. 0. 75 – 0. 20 = 0. 55 m beyond the second lens. • f 2 = 0. 50 m, so 2 = -0. 55 m • This is a real image

Touching Lenses ] Two lenses are often brought into close contact. • Very short distance between optical centers ] The combined focal length can be approximated. • Let so 1 approach infinity • Applies to convex and concave lenses

Farsighted ] Some eyes have weak muscles. • Near image focuses in front of the retina • Hyperopia and presbyopia ] A compound converging lens compensates for the eye’s lens. • Forces the image forward in the eye.

Nearsighted ] Some eyes have eyes too long or bent corneas. • Distant image focuses in front of the retina • Myopia ] A compound diverging lens forces the image back in the eye. • Pinhole iris or squinting also works

Diopters ] Optometrics works on corrective lenses for eyes. ] Diopters measure the inverse of the focal length of the lens. • 1 D = 1 m-1 • Positive converging • Negative diverging

Reading Glasses ] A person sees blurred print at 25 cm, but is fine at 125 cm. ] Find the diopter correction needed. ] The diopter formula can be applied to object and image distances. • Object is at 0. 25 m • Virtual image is at 1. 25 m • Correction needed is +3. 2 D next