# Section 1: The Behavior of Light

• Light and Matter

• What you see depends on the amount of light in the room and the color of the objects. For you to see an object, it must reflect some light back to your eyes.

• Objects can absorb light, reflect light, and transmit light—allow light to pass through them.

• The type of matter in an object determines the amount of light it absorbs, reflects, and transmits.

• Opaque: only absorbs and reflects light—no light passes through it.

• Translucent: allow some light to pass through them

• Transparent: transmit almost all the light striking them, so you can see objects clearly through them.

• Reflection of Light

• Reflection occurs when a light wave strikes an object and bounces off.

• According to the law of reflection, light is reflected so that the angle of incidence always equals the angle of reflection.

• Reflection of light waves from a smooth surface is regular reflection.

• Reflection of light from a rough surface is diffuse reflection.

• Even a surface that appears to be smooth can be rough enough to cause diffuse reflection.

• To cause a regular reflection, the roughness of the surface must be less than the wavelengths it reflects.

• Refraction of Light

• Refraction is caused by a change in the speed of a wave when it passes from one material to another.

• The amount of bending that takes place depends on the speed of light in both materials.

• Index of Refraction: a property of the material that indicates how much the speed of light in the material is reduced.

• The larger the index of refraction, the more light is slowed down in the material.

• Wavelengths of visible light range from the longer red waves to the shorter violet waves. White light, such as sunlight, is made up of this whole range of wavelengths.

• Refraction causes a prism to separate a beam of white light into different colors.

• Because the longer wavelengths of light are refracted less than the shorter wavelengths are, red light is bent the least

• Like prisms, rain droplets also refract light. The refraction of the different wavelengths can cause white light from the Sun to separate into the individual colors of visible light

• Mirage: an image of a distant object produced by the refraction of light through air layers of different densities.

• Mirages result when the air at ground level is much warmer or cooler than the air above it

# Section 2: Light and Color

• Colors

• An object’s color depends on the wavelengths of light it reflects.

• Although some objects appear to be black, black isn’t a color that is present in visible light. Objects that appear black absorb all colors of light and reflect little or no light back to your eye.

• White objects appear to be white because they reflect all colors of visible light.

• A filter is a transparent material that transmits one or more colors of light but absorbs all others. The color of a filter is the color of the light that it transmits.

• Seeing Color

• In a healthy eye, light enters and is focused on the retina, an area on the inside of your eyeball.

• The retina is made up of two types of cells that absorb light

• When these cells absorb light energy, chemical reactions convert light energy into nerve impulses that are transmitted to the brain.

• One type of cell in the retina, called a cone, allows you to distinguish colors and detailed shapes of objects. Cones are most effective in daytime vision.

• Your eyes have three types of cones, each of which responds to a different range of wavelengths.

• Red cones respond to mostly red and yellow, green cones respond to mostly yellow and green, and blue cones respond to mostly blue and violet.

• The second type of cell, called a rod, is sensitive to dim light and is useful for night vision.

• If one or more of your sets of cones did not function properly, you would not be able to distinguish between certain colors.

• About eight percent of men and one-half percent of women have a form of color blind- ness.

• Color blindness is an inherited sex-linked condition in which certain cones do not function properly.

• Mixing Colors

• Pigment: a colored material that is used to change the color of other substances.

• The color of a pigment results from the different wavelengths of light that the pigment reflects.

• Paint pigments usually are made of chemical compounds such as titanium oxide, a bright white pigment, and lead chromate, which is used for painting yellow lines on highways.

• White light is produced when the three primary colors of light are mixed.

• You can make any pigment color by mixing different amounts of the three primary pigments.

• A primary pigment’s color depends on the color of light it reflects.

• Pigments both absorb and reflect a range of colors in sending a single color message to your eye.

• Because black results from the absence of reflected light, the primary pigments are called subtractive colors.

# Section 3: Producing Light

• Incandescent light: generated by heating a piece of metal until it glows.

• Inside an incandescent lightbulb is a small wire coil, called a filament, that usually is made of tungsten metal.

• About 90% of the energy given off by an incandescent bulb is in the form of thermal energy.

• Fluorescent Light: uses phosphors to convert ultraviolet radiation to visible light.

• The inside of the bulb is coated with phosphors that emit visible light when they absorb ultraviolet radiation.

• Fluorescent lights use as little as one fifth the electrical energy to produce the same amount of light as incandescent bulbs.

• Fluorescent bulbs also last much longer than incandescent bulbs. This higher efficiency can mean lower energy costs over the life of the bulb.

• Neon Lights

• The vivid, glowing colors of neon lights make them a popular choice for signs and eye- catching decorations on buildings.

• These lighting devices are glass tubes filled with gas, typically neon, and work similarly to fluorescent lights.

• Sodium-Vapor Lights

• Sodium-vapor lights often are used for streetlights and other outdoor lighting.

• Sodium-vapor lights emit mostly yellow light.

• Inside a sodium-vapor lamp is a tube that contains a mixture of neon gas, a small amount of argon gas, and a small amount of sodium metal.

• Tungsten-Halogen Lights

• Tungsten-halogen lights sometimes are used to create intensely bright light.

• These lights have a tungsten filament inside a quartz bulb or tube.

• The light is much brighter and also lasts longer.

• Lasers

• A laser’s light begins when a number of light waves are emitted at the same time.

• Lasers can be made with many different materials, including gases, liquids, and solids.

• One of the most common is the helium-neon laser, which produces a beam of red light.

• Beams of laser light do not spread out because laser light is coherent.

• Coherent Light: light of only one wavelength that travels with its crests and troughs aligned.

• Incoherent Light: can contain more than one wavelength, and its electromagnetic waves are not aligned

• The energy carried by the light waves is spread over a large area, so the intensity of the light is much less than that of the laser beam.

• Using Lasers

• A laser beam is narrow and does not spread out as it travels over long distances.

• Compact disc players, surgical tools, and many other useful devices take advantage of the unique properties of lasers.

• Lasers are routinely used to remove cataracts, reshape the cornea, and repair the retina.

# Section 4: Using Light

• Polarized Light: the waves vibrate in only one direction.

• If the light passes through a special polarizing filter, the light becomes polarized.

• A polarizing filter acts like a group of parallel slits.

• Polarized lenses are useful for reducing glare without interfering with your ability to see clearly.

• The lenses of polarizing sunglasses have vertical polarizing filters that block out the reflected light that has been polarized horizontally.

• Holography: a technique that produces a hologram—a complete three- dimensional photographic image of an object.

• Illuminating objects with laser light produces holograms.

• An ordinary photographic image captures only the brightness or intensity of light reflected from an object’s surface, but a hologram records the intensity as well as the direction.

• Holographic images are used on credit cards, identification cards, and on the labels of some products to help prevent counterfeiting.

• Optical Fibers

• When laser light must travel long distances or be sent into hard-to-reach places, optical fibers often are used.

• A light wave is bent away from the normal as it passes from water to air. At the critical angle, the refracted wave is traveling along the water surface. At angles greater than the critical angle, total internal reflection occurs.

• Total Internal Reflection: occurs when light traveling from one medium to another is completely reflected at the boundary between the two materials.

• For total internal reflection to occur, light must travel slower in the first medium, and must strike the boundary at an angle greater than the critical angle.

• Total internal reflection makes light transmission in optical fibers possible.

• Optical fibers are most often used in communications.

• Optical fibers also are used to explore the inside of the human body.

• Optical Scanners

• An optical scanner is a device that reads intensities of reflected light and converts the information to digital signals.