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CHAPTER 14 REFRACTION Ms. Hanan 14-2 Thin Lenses Objectives: • Use ray diagram to find the position of an image produced by a converging or diverging lens, and identify the image as real or virtual. • Solve problems using the thin-lens equation. • Calculate the magnification of lenses. • Describe the positioning of lenses in compound microscopes and refracting telescopes. 14-2 Thin Lenses Vocabulary: • • • • • • Converging lens Convex lens Diverging lens Concave lens Ray diagram Focal point • • • • • Focal length Centre of the lens Real image Virtual image Magnification 14-2 Thin Lenses The first telescope, designed and built by Galileo, used lenses to focus light from faraway objects, into Galileo’s eye. His telescope consisted of a concave lens and a convex lens. light from far away object convex lens concave lens Light rays are always refracted (bent) towards the thickest part of the lens. Concave (Diverging) Lenses Concave lenses are thin in the middle and make light rays diverge (spread out). • F Principal axis If the rays of light are traced back (dotted sight lines), they all intersect at the focal point (F) behind the lens. Concave (Diverging) Lenses • F Principal axis Light Therays light that rayscome behave in parallel the same to the wayoptical if we ignore axis diverge the thickness from the offocal the lens. point. Concave (Diverging) Lenses • F Principal axis Light rays that come in parallel to the optical axis still diverge from the focal point. Concave (diverging) Lens (example) • F Principal axis The first ray comes in parallel to the optical axis and refracts from the focal point. Concave (diverging) Lens (example) • F principal axis The first ray comes in parallel to the optical axis and refracts from the focal point. The second ray goes straight through the center of the lens. Concave (Diverging) Lens (example) • F Principal axis The first ray comes in parallel to the optical axis and refracts from the focal point. The second ray goes straight through the center of the lens. The light rays don’t converge, but the sight lines do. Concave (Diverging) Lens (example) • F Principal axis The first ray comes in parallel to the optical axis and refracts from the focal point. The second ray goes straight through the center of the lens. The light rays don’t converge, but the sight lines do. A virtual image forms where the sight lines converge. Your Turn (Concave (Diverging) Lens) object • F Principal axis concave lens • Note: lenses are thin enough that you just draw a line to represent the lens. • Locate the image of the arrow. Your Turn (Concave (Diverging) Lens) object • Fimage Principal axis concave lens • Note: lenses are thin enough that you just draw a line to represent the lens. • Locate the image of the arrow. Convex (Converging) Lenses Convex lenses are thicker in the middle and focus light rays to a focal point in front of the lens. The focal length of the lens is the distance between the center of the lens and the point where the light rays are focused. Convex (converging) Lenses • F Principal axis Convex (converging) Lenses • F Principal axis Light rays that come in parallel to the optical axis converge at the focal point. Convex (converging) Lens (example) • F Principal axis The first ray comes in parallel to the optical axis and refracts through the focal point. Convex (converging) Lens (example) • F Principal axis The first ray comes in parallel to the optical axis and refracts through the focal point. The second ray goes straight through the center of the lens. Convex (converging) Lens (example) • F Principal axis The first ray comes in parallel to the optical axis and refracts through the focal point. The second ray goes straight through the center of the lens. The light rays don’t converge, but the sight lines do. Convex (converging) Lens (example) • F Principal axis The first ray comes in parallel to the optical axis and refracts through the focal point. The second ray goes straight through the center of the lens. The light rays don’t converge, but the sight lines do. A virtual image forms where the sight lines converge. Your Turn (Convex (converging) Lens) Principal axis • F object convex lens • Note: lenses are thin enough that you just draw a line to represent the lens. • Locate the image of the arrow. Your Turn (Convex (converging) Lens) • F object Principal axis image convex lens • Note: lenses are thin enough that you just draw a line to represent the lens. • Locate the image of the arrow. Rules for Drawing Reference Rays Ray From object to lens From converging lens to image Parallel ray (P ray) Parallel to principal axis Passes through focal point, F Central ray (M ray) To the center of the lens From the center of the lens Focal ray (F ray) Passes through focal point, F Parallel to principal axis Ray From object to lens From diverging lens to image Parallel ray (P ray) Parallel to principal axis Directed away from focal point, F Central ray (M ray) To the center of the lens From the center of the lens Focal ray (F ray) Proceeding toward back focal point, F Parallel to principal axis Ray Tracing for Lenses These diagrams show the principal rays for both types of lenses: Lens & Mirror Equation 1 1 1 f p q ƒ = focal length p = object distance q = image distance f is negative for diverging mirrors and lenses di is negative when the image is behind the lens or mirror Magnification Equation h q M h p ' M = magnification h’= image height h = object height If height is negative the image is upside down if the magnification is negative the image is inverted (upside down) The Thin-Lens Equation Sign conventions for thin lenses: M M q q p p Assignments • Class-work: Practice B, page 501, odd questions. • Homework: Practice B, page 501, even questions. Homework due next class Eyeglasses and Contact Lenses Vocabulary: • • • • • • Myopia Short Sightedness Hyperopia Far sightedness Objective Lense Eye Piece • Compound Microscope • Refracting Telescope Eyeglasses and Contact Lenses Leads to the occipital cortex at the posterior (back) of the brain Anatomy of the Human Eye Normal Vision The process in which the lens changes its focal length to focus on objects at different distances is called accommodation Myopia, Hyperopia If the incoming light from a far away object focuses before it gets to the back of the eye, that eye’s refractive error is called “myopia” (nearsightedness). If incoming light from something far away has not focused by the time it reaches the back of the eye, that eye’s refractive error is “hyperopia” (farsightedness). Myopia - Nearsightedness Hyperopia - Farsightedness Combination of Thin Lenses In lens combinations, the image formed by the first lens becomes the object for the second lens (this is where object distances may be negative). The Compound Microscope • In the basic compound microscope, the object to be magnified is placed under the lower lens (objective lens) with a focal length of less than 1 cm, and the magnified image is viewed through the upper lens (eyepiece lens) with a focal length of few centimeters. • The magnification of the image can be calculated by multiplying the magnifying power of the objective lens times the magnifying power of the eyepiece lens. • The microscope is composed of a mechanical system which supports the microscope, and an optical system which illuminates the object under investigation and passes light through a series of lens to form an image of the specimen. The Compound Microscope The principle of the compound microscope. The passage of light through two lenses forms the virtual image of the object seen by the eye. The Refracting Telescope Assignments • Homework: Section review, page 505, questions: 1, 2, 3, 4, 5, and 6. Homework due next class