Light: Lenses and. Mirrors. Test Date: Name 1ÿ-ÿ. Physics. Light: Lenses and Mirrors

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1 Name 1ÿ-ÿ Physics Light: Lenses and Mirrors i Test Date: "Shadows cannot see themselves in the mirror of the sun." -Evita Peron

2 What are lenses? Lenses are made from transparent glass or plastice and refract light. They are used to magnify images or project images onto a screen. There are two types: diverging and converging. Diverging lenses are thinner at the center and thicker at the edges. Concave Biconcave Piano-concave Negative meniscus Bend light outward and create only virtual images.

3 O )b ÿ ',,l'bv- ÿ \ AA ÿ(._. Vk ÿ.z,,z J" -_.3 are thicker at the center and thinner at the edges. Convex Biconvex Piano-convex Positive meniscus Bend light lÿ/wq rays. and JÿO e..g ÿzÿ light. Can produce a y4ÿ ÿ or ÿ% \]\ 'ÿ! image.

4 Virtual and real images Image formation by a lens can be virtual or real. The image is ÿ \ÿ('ÿ ÿj OL\ when it is right-side up. For example, a diverging lens is often used for the viewfinder on a camera. "You look through the lens in order to see it" A ÿ( 20vÿ ÿ ÿlÿ ÿ2 is upside down or inverted. FOr example, converging lenses are used for projecting pictures on a screen. ASLE

5 Refraction Waves, like light, can sometimes bend as they travel from one medium to another. This is called refraction. Index of refraction Refraction can be explained as the result of a change in wave speed and can be calculated using this equation: m c Where n is the index of refraction, c is the speed of light in a vacuum and v is the speed of light in the medium (i.e. water, glass).

6 Ray diagrams The location of an image can be predicted using a ray diagram. In such a diagram, do is the distance from the object to the mirrob and dÿ is the distance from the image to the mirror. do di We must follow three rules of refraction when drawing ray diagrams: 1. Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens. 2. Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. 3. An incident ray that passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

7 Lens Ray Diagrams Using the three rules of refraction above, we can draw a diagram to show exactly how light is refracted through a lens. These are called lens ray diagrams. '- ÿocal...!:"!:',ÿiÿ?i:ÿ mcal /..,.ÿÿ POINT PRÿNC PAL AXIS / Principal axis - the line that passes through the center of the lens Focal length (F) - the distance from the lens to the focal point Focal point- rays of light that enter a lens parallel to the principal axis refract along lines that intersect at a common point ÿ,ÿ

8 11... do... We can trace at least three of the rays of light that leave a single point on our object. Rephrasing the rules of refraction into simple steps: 1. Rays parallel to the principle axis refract along a line containing the focal point, and vice versa. 2. Rays that go in through the center of the lens pass straight through. 3. Rays that enter the lens through the focal point, go out parallel to the principal axis. do is the distance from the object to the lens and dÿ is the distance from the lens to the image. Following these steps will allow us to locate an image from a lens! However, an image will not form if the lines do not focus or intersect at a point.

9 Thin Lens Equation f do! - Wheref is the focal length, do is the distance between the object and the lens and dÿ is the distance from the lens and the image. Using The Thin Lens Equation The front of the lens is the side that light enters from. do is positive if it is in front of the lens. di is positive if it is in back of the lens, negative if it is in front of the lens. f is positive for converging lenses, negative for diverging lenses. Practice When an object is placed 3 cm in front of a converging lens, a real image is formed 6 cm in back of the lens. Find the focal length.! _...,! O'Y I/

10 Magnification How can you tell if the image of the object will be larger, smaller or inverted? Use this equation! If m is negative, the image is inverted. The object and images distances will also tell us the size of the reflected images. Common Optical Instruments What are some common items that demonstrate the rules of refraction? Camera Telescope Projector Compound microscope The eye!

11 The eye The eye is one of the most important lenses we use each day and follows the rules of refraction we just learned about. Cross section of Human Eye If you don't have 20/20 vision, you have a refractive error that needs corrected with lenses, either glasses or contacts. Nearsighted Also called myopia, the eye focuses on nearby objects but cannot focus on objects located farther away. The images focuses in front of the retina. Corrective diverging lenses focus the image on the retina.

12 Farsighted Also called hyperopia, the eye cannot focus on nearby objects. The lens focuses the light behind the retina. This condition is corrected with a converging lens..ÿ".4" ÿ "ÿ ÿ ÿ Mirrors Unlike lenses which transmit and refract light, a mirror reflects light. There are a few types of mirrors.

13 Plane Mirror A plane mirror is flat and has no magnification. The image distance is the same as the object distance. A plane mirror produces a virtual image that is laterally inverted and cannot be focused on a screen. light source plane mirror apparent source., 5 -" tÿ2-* j / / J Light that is reflected must follow the Laws of Reflection, where the angle of incidence, 0ÿ, must equal and be in the same plane as the angle of reflection Or. i I I I I interface

14 Curved Mirror A curved mirror causes image distortions. The distortions are due to light rays obeying the Laws of Reflection. There are two types of curved mirrors we will cover: Concave Mirror- looks like the inside of a sphere. Parallel rays conveiÿe when reflected and create a real image, \ / Convex Mirror-looks like the outside of a sphere. Parallel rays diverge when reflected. Also called a diverging mirror.

15 The point that is equidistant from all points of a curved reflecting surface is called the center of curvature (C). The principal axis is a straight line passing through the vertex and the center of curvature. All optical measurements are made in reference to this axis. The vertex is the geometric center of a curved surface and the point where the mirror crosses the principal axis. Radius of curvature J C Principal axis -iÿ.m vertex / C - center of curvature