LENSES

1. LENSES April 13

2. Uses of Lenses

3. What is a lens? • Curved, smooth transparent material that refracts light in a predictable (and useful) way • Usually glass or hard plastic, which can be shaped and polished to make light rays produce a certain type of image

4. 2 Types

5. sketch of a converging (convex) lens axis of symmetry secondary focal point optical centre of lens F F’ primary focal point principal axis • axis of symmetry – imaginary vertical line drawn through the optical centre of a lens

6. axis of symmetry secondary focal point optical centre of lens F F’ primary focal point principal axis • optical centre of lens - the point where the principal axis and the axis of symmetry meet

7. both concave and convex lenses have a focus in front of and behind the lens (because light can travel through the lens in either direction) • The focal point where light comes to a focus or appears to diverge from is F, the opposite focus is F’

8. The focal length is the distance from the axis of symmetry to the principal focus

9. Convex Lenses • convex lenses are also called converging lenses because when incident rays are parallel to the principal axis, the refracted rays all converge on the focal point behind the lens refracted ray incident ray F F’ • in a convex lens, refracted rays move toward each other after passing through the lens

10. Convex Lenses • Able to produce larger images so are often used in eyeglasses, magnifying glasses or microscopes as well as making far-away objects look closer (e.g. telescopes)

11. Concave Lenses • Concave lenses are also called diverging lenses because when incident rays are parallel to the principal axis, the light is refracted away from the principal axis and the light rays diverge (spread out)

12. Concave Lenses • Only produce smaller images, so are used for eyeglasses (for people who are nearsighted) and in door peepholes!