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Chapter 30

Chapter 30. Lenses. Converging and Diverging Lenses. Converging lenses are convex, or curve outward. Diverging lenses are concave, or curve inward. Lenses: Vocabulary. Principal Axis – a line perpendicular to the lens

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Chapter 30

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  1. Chapter 30 Lenses

  2. Converging and Diverging Lenses • Converging lenses are convex, or curve outward. • Diverging lenses are concave, or curve inward.

  3. Lenses: Vocabulary • Principal Axis – a line perpendicular to the lens • Focal point – where on the principal axis light rays converge (for a converging lens) • Focal length – distance from the lens to the focal point.

  4. Constructing Images Through Ray Diagrams • Draw the lens. • Draw the principal Axis, labeling the focal point on both sides of the lens • Draw the object • Draw a ray from the top of the object parallel to the principal axis through the lens, then through the focal point. • Draw a ray from the object, through the focal point to the lens, then parallel to the principal axis. • Where they meet is the image.

  5. Image Object Relationships • f = focal length • di = distance to image • d0 = distance to object • hi = height of image • h0 = height of object • For diverging lenses f is negative

  6. Sample Problem • If an object which is 3 cm in diameter is placed 10 cm from a converging lens with a focal length of 5 cm, where will the image appear, how large will it be, and will it be upright or inverted?

  7. Solution The image appears on the opposite side of the lens, is the same size as the object and is upright.

  8. The Eye • The lens of a human eye focuses an image on the retina at the rear of the eye. • If the lens forms an image in front of the retina, then a person is said to be nearsighted • If the lens form an image at the rear of the retina (behind it) then the person is said to be farsighted.

  9. Corrective Lenses • Since a nearsighted person’s eye forms the image too soon, the lens of their eye has too much convergence, hence a diverging lens, (concave) is used to separate the rays of light coming from the object so that the image will converge on the retina. • Since a Farsighted person’s eye forms the image after the retina, a converging lens, (convex) is used to help the rays of light converge sooner, forming the image on the retina.

  10. Things to Know • Recognize a concave and convex lens • Know that a concave lens is diverging and a convex lens is converging • Use a ray diagram to determine where an image will be formed, whether the image is enlarged or reduced in size, whether the image is upright or inverted. • Use the lens equations to determine focal length, image size, and image location. • Know the root causes of near and far sightedness and which lenses are used to correct these conditions.

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