Light Chapter 12

Light and Matter Objects can absorb, reflect, or transmit light Depends on what material the object is made up of Typically, “shiny” objects reflect a lot of light, while “dull” objects absorb a lot 3 types of materials: Opaque- Only absorb and reflect light. No light passes through them Translucent- Transmit light, but scatter it Somewhat see-through, but the image is blurry Transparent- Transmits light without scattering it The object can clearly be seen through

Reflection of Light The law of reflection Recall from several chapters ago? The angle of reflection is equal to the angle of incidence This is only true for perfectly smooth objects, which cannot exist in nature However, some objects can be close In order for it to occur, the rough spots must be smaller than the wavelengths of the light You can see reflections in these objects Light reflected from a smooth surface is called regular reflection Light that is reflected from a surface that is uneven is scattered in many directions Called diffuse reflection You cannot see an image in diffuse reflections

Refraction of Light Refraction occurs when the speed of a light wave decreases as it moves from one medium to another Index of refraction-property of a material that indicates how much light slows down when it passes through compared to the speed in a vacuum Refraction is used intentionally in objects such as eyeglasses, cameras, and microscopes Prisms also cause light to refract The light is arranged by wavelength into the visible light spectrum (rainbow) White light is made up of all of the colors of the rainbow Can lead to mirages Image of an object caused by refraction of light through air layers

Light and Color The color that an object appears to be is due to the color of light being reflected The other light wavelengths are either absorbed or not reflected as much White objects reflect all wavelengths Black objects absorb all wavelengths, and thus reflect none

How you see color Your eyes contain 2 types of cells: Rods and cones These cells collect light and convert them to nerve signals that travel to the brain Cones Allow you to distinguish colors and detailed shape 3 types found in the eye Each corresponds to a different wavelength of light Rods Sensitive to dim light and useful in night vision People who are color-blind have cones that are not as sensitive to green and red

Mixing Color Light has 3 primary colors: Red, Green, Blue All other colors are combinations of these colors in different quantities This is different from pigments, which are used in paint and printers Magenta, cyan, yellow

Producing Light Incandescent Bulbs Heating a piece of metal until it glows Used in homes (lamps, ceiling lights, etc) Fluorescent Bulbs Uses a phosphor to convert UV to visible light Phosphor= substance that converts UV to visible Electrodes emit electrons that collide with gas to release UV waves More efficient (1/5 of the energy consumption) Used in schools, and many other large buildings Recently, have been made compact for home use due to efficiency Neon Lights Glass tubes filled with gases Work similar to fluorescent lights, except the gas emits visible lights Different gases and coatings create the vibrant colors

Producing Light, Continued Sodium-Vapor Lights Often used in streetlights Argon gas is heated and cause a piece of sodium to become vaporous This emits a yellow-orange glow Tungsten-Halogen Lights Used to produce very bright light Car headlights Tungsten filament in a quartz bulb surrounded by a gas Filament gets very hot, and emits a lot of light Lasers Tubes containing gas absorb electrons from a flashtube (device that emits short bright flash of light) Mirrors cause the light to bounce back and forth The light then exits out of a partially reflective mirror

Coherent vs incoherent Light Coherent light Only one wavelength Constant distance between corresponding crests Constant wavelength and frequency Concentrated light that does not spread out Lasers Incoherent Light More than one wavelength Not a constant distance between corresponding crests Light spreads out Lightbulbs

Uses of Lasers Medicine Lasers can make incisions instead of scalpels Laser eye surgery Videodisc players CDs, DVDs, Blu-Rays, Etc