Notes: Light and Optics. Reflection. Refraction. Law of Reflection. Light goes straight 12/13/2012

Transcription

1 Notes: Light and Optics Light goes straight Light travels in a straight line unless it interacts with a medium. The material through which a wave travels is called a medium. Light can be reflected, refracted (bent), scattered, or absorbed. Reflection Reflection: when an object or wave bounces back off a surface through which it cannot pass. Law of Reflection The law of reflection states that the angle of incidence equals the angle of reflection. Video Two Types of Reflection Refraction Refraction: the bending of light waves due to change in speed. 1

2 Refraction Prisms Refraction: when light rays enter a medium at an angle, the change in speed causes the rays to bend, or change direction. Refraction is what causes white light to break into colors like in a rainbow. Rainbows Light & Matter Transparent objects let light through. Translucent objects scatter light. Opaque objects reflect or absorb light. Color of Light 16.2 How we see other colors Color of Objects White light is the presence of ALL the colors of the visible spectrum. Black objects absorb ALL the colors and no light is reflected back. The additive primary colors are red, green, and blue. We don t see everything white because the strength of the signal matters. All the different shades of color we can see are made by changing the proportions of red, green, and blue Microsoft Clip Gallery 2

3 16.2 How we see the color of things Colored fabrics and paints get color from a subtractive process. Chemicals, known as pigments, in the dyes and paints absorb some colors and allow the color you actually see to be reflected. Magenta, yellow, and cyan are the three subtractive primary colors Color and Vision 16.2 Color and Vision When all the colors of the rainbow are combined, we do not see any particular color. We see light without any color. We call this combination of all the colors of light "white light". Key Question: How do we see color? *Students read Section 16.2 AFTER Investigation How we see the color of things When we see an object, the light that reaches our eyes can come from two different processes: 1. The light can be emitted directly from the object, like a light bulb or glow stick. 2. The light can come from somewhere else, like the sun, and we see the objects by reflected light. 3

4 16.2 Why are plants green? Plants absorb energy from light and convert it to chemical energy in the form of sugar (food for the plant). Chlorophyll is an important molecule that absorbs blue and red light. How the Eye Works Retina Correcting Vision LASIK Surgery The process: After your eye has been numbed with "eye drop" anesthesia, an instrument known as an eyelid speculum will be positioned to hold your eyelids open. You will remain awake and comfortable throughout the procedure. A small suction ring will be placed around the cornea and serves as a platform for the microkeratome. The microkeratome separates the surface layers of the cornea, and the corneal flap is folded back. You will be asked to look at a target light while the Excimer laser reshapes the corneal tissue. A clicking sound can be heard as each microscopic layer of tissue is vaporized. This process will last from seconds to minutes, depending on the amount of correction necessary. The corneal flap is then placed back into its original position and allowed to dry for a few minutes. 4

5 How the human eye sees color Photoreceptors in the Eye optic nerve The retina in the back of the eye contains photoreceptors. These receptors release chemical signals. Chemical signals travel to the brain along the optic nerve. Cones respond to three colors: red, green and blue. Rods detect intensity of light: black, white, shades of gray. Brain interprets Fooling your brain 5

6 6

7 Fooling your brain Mirrors Mirror: a plane sheet of glass or similar material with a smooth, shiny coating on one side. Types of mirrors Plane mirror: a flat mirror that usually shows a correct virtual image. A virtual image is an upright image whose left and right sides are reversed. 7

8 Types of mirrors Concave mirrors: a mirror with a surface that curves inward, thinner in the middle. Concave mirrors can form virtual images or a real image depending on the bend of the mirror. Types of mirrors Concave mirrors rely on the focal point in order to produce images. Focal point: the point in which light rays meet to form a picture. Types of mirrors Convex mirrors: a mirror with a surface that curves outward, thicker in the middle. Convex mirrors form virtual images that are always smaller than the originals. Convex mirrors allow you to see a larger area. Lenses Lens: a curved piece of glass or other transparent material that refracts light. A lens forms an image by refracting light rays that pass through it. Convex Lens Convex lens: thicker in the center than at the edges that focuses rays of light (magnify). Convex lenses magnify things or make them look bigger. Examples are microscopes, magnifying glasses, and telescopes. Convex Lens Used in things like microscopes, magnifying lenses, and in some telescopes. 8

9 Concave Lens Concave lenses: thinner in the center and thicker at the edges (minimize). Concave lenses form virtual images that are usually smaller than the original. Used in eyeglasses to correct for nearsightedness. Concave lens Mainly used to correct vision in eye glasses and contact lenses How does a color TV work? Televisions give off light. To make color with a TV, you can use red, green, and blue (RGB) directly. Uses Lasers- produce very bright beams of light. The atoms all give out their light together The screen is made of tiny red, green, and blue dots. The dots are called pixels and each pixel gives off its own light. TV sets can mix the three colors to get millions of different colors. Uses of Laser (1) In medicine to break up gallstones and kidney stones, to weld broken tissue (e.g. detached retina) to destroy cancerous and precancerous cells; at the same time, the heat seal off capillaries, to remove plaque clogging human arteries. Uses of Laser (2) In industry to drill tiny holes in hard materials, for welding and machining, for lining up equipment precisely, especially in inaccessible places. 9

10 Uses of laser (3) CDs spiral In everyday life to be used as bar-code readers, to be used in compact disc players, to produce short pulses of light used in digital communications, to produce holograms. CDs use laser light and reflection- The surface has tiny pits and smooth bits in a spiral pattern the pits scatter light from a laser the smooth bits reflect the light so a coded message is read. Holography Holograms Holography is the production of holograms by the use of laser. A hologram is a 3D image recorded in a special photographic plate. The image appears to float in space and to move when the viewer moves. How holograms work Laser show Physics is in everything 10

11 Total Internal Reflection Involves both reflection and refraction When light bends (refracts) enough it is reflected rather than transmitted. Total Internal Reflection Fiber Optics Transmit signals as flashes of light. Since light has a higher frequency than radio waves, much more information can be carried on a light beam than on a radio waves Light carries information 16.1 Light carries information The fiber-optic networks you read about are pipelines for information carried by light. In some cities, a fiber-optic cable comes directly into homes and apartments carrying telephone, television, and Internet signals Electric Light 16.1 Electric Light The process of making light with heat is called incandescence. Incandescent bulbs generate light when electricity passes through a thin piece of metal wire called a filament. The filament heats up and gives off light. The other common kind of electric light is the fluorescent bulb. Fluorescent bulbs convert electricity directly to light without generating a lot of heat. Fluorescent bulbs use high-voltage electricity to energize atoms of gas that fill the bulb. 11

12 16.1 Light intensity The intensity of light from a small source follows an inverse square law because its intensity diminishes as the square of the distance The speed of light The speed at which light travels through air is approximately 300 million meters per second. Light travels almost a million times faster than sound The speed of light 16.1 Reflection and refraction The speed of light is so important in physics that it is given its own symbol, a lower case c. The best accepted experimental measurement for the speed of light in air is 299,792,500 m/sec. For most purposes, we do not need to be this accurate and may use a value for c of m/sec. Another example of refraction of light is the twinkling of a star in the night sky As starlight travels from space into the Earth s atmosphere, the rays are refracted. Since the atmosphere is constantly changing, the amount of refraction also changes Photons and Atoms Key Question: How does light fit into the atomic theory of matter? *Students read Section 16.3 AFTER Investigation Photons and intensity Intensity measures power per unit area. There are two ways to make light of high intensity. One way is to have high- energy photons. A second way is to have a lot of photons even if they are low-energy. The number and energy of photons determine the intensity of the light. 12

13 Application: Color Printing 13