CHAPTER 17: ELECTROMAGNETIC

WAVES

17.1: Electromagnetic waves have unique traits

• Electromagnetic waves:– A disturbance that transfers energy through a

field. – Also called EM waves– Most are invisible

• Field:– An area around an object where the object

can apply a force (push or pull) to another object without touching it

Field example

Electric Force Fields

How EM waves form

• Occur when electrically charged atomic particles move

• The charged particles exert an electric force on each other (electric field)

• A moving charged particle also creates a magnetic force thus creating a magnetic field

• Quickly moving charged particles vibrate at right angles to each other

Sources of EM waves

• Most of our EM waves come from the sun• Technology has given us the ability to

create EM waves on our own for various uses

EM waves can travel in a vacuum

• Radiation:– Energy that moves in the form of EM waves

• Different from mechanical waves:– EM waves can travel through a vacuum or

space with no matter which means it does not lose energy as it moves

– Mechanical waves must vibrate the medium they travel through using some of the wave’s energy

Speed of EM waves

• In a vacuum EM waves:– Travel at a constant speed– 186,000 mi/s (300,000 km/s) AKA speed of

light – At this speed, rays from the sun take

approximately 8 minutes to reach the earth

EM waves can interact with a material medium

• Energy can be transferred, reflected, refracted, or diffracted which causes a change in direction of the EM wave.

• Transferring energy:– Moves potential energy from one place to

another– Converts potential energy into kinetic energy

by moving the medium back and forth

17.2: Electromagnetic waves have many uses

• Frequencies:– Determines the wave’s characteristics and

uses– Higher frequency EM waves have more

vibrations per second and more energy– Lower frequency EM waves have longer

wavelengths and less energy

Electromagnetic spectrum

• The range of all EM frequencies• Broken up to represent 7 parts of the

spectrum by frequency from low to high (right to left)

• Some areas of the spectrum overlap• Frequency is measured in hertz (Hz)

– 1 Hz = 1 cycle per second

• Electromagnetic Spectrum - Introduction NASA

Radio waves

• EM waves that have the longest wavelength, lowest frequency, and lowest energy

• Easily pass through the atmosphere and other materials

• AM: amplitude modulation• FM: frequency modulation

Microwaves

• Shorter wavelength, higher frequencies, and higher energy than radio waves

• Radar– Radio detection and ranging– Used to detect the position of an object

• Cell phone:– Radio transmitter and receiver that uses

microwaves

Visible Light

• What the human eye can see• Range is between 1014 Hz and 1015 Hz

• Longest wavelength is seen as red

• Shortest wavelength is seen as violet

Infrared Light

• Found between microwaves and visible light

• Generally associated with heat (heat rays)• Can not see infrared rays but you can feel

them– Heat from the sun, fire, heat lamps

• Some animals can see infrared light (pit viper snakes)

Ultraviolet Light

• Frequencies above those of visible light and partially below x-ray

• Carries more energy than visible light• Causes damage to your skin and eyes• Causes your skin to produce vitamin D• Can be used to sterilize equipment and food• Bees and other insects can see higher

frequencies allowing them to see “nectar guides” on flowers

X-rays

• Very high frequency and energy• Produced by the sun and technology• Can pass easily through the soft tissues of

the body but are absorbed by bone• Blocked by lead

Gamma rays

• Highest frequency and energy—lowest wavelength

• Produced by radioactive substances, the sun, other stars

• Can penetrate the soft and hard tissues of the body-kills normal cells—causes cancer to develop

• Used as radiation treatment of cancer in some patients

17.3: The Sun is the source of most visible light.

• Incandescence:– The production of light by materials at high

temperatures– Glows or bursts into flames– Lightning – Light bulbs– fire

• Luminescence:– Production of light without the high

temperature• Bioluminescence:

– Production of light by living organisms– Produce light by chemical reactions inside an

organism’s tissues– firefly

Incandescent and Fluorescent Lighting

• Light bulbs:– Sealed glass tube with a thin tungsten wire

(filament)– Tungsten gives off bright light when heated– Wasteful in terms of energy by giving off a lot

of heat in the form of infrared radiation

• Fluorescence:– When a material absorbs EM radiation of

one wavelength and gives off EM radiation of another

– Bulb filled with a mixture of mercury vapor and other gases that give off UV light when electric current passes through

– Inside of the bulb is coated with phosphor powder

– More energy efficient and economical

• LEDs– Light emitting diode– Semiconductor which regulates the

electric current by converting electric energy directly into visible light

– Produce very bright light with little energy-produce little heat

17.4: Light waves interact with materials

• Transmission:– Passage of EM waves through a medium– If light was not reflected by objects, we would

not see most of the objects.• Absorption:

– Disappearance of EM waves into the medium– Affects how things look by limiting the light

available to be reflected or transmitted

How materials transmit light• Transparent:

– Allows most of the light that strikes them to pass through

– You can see objects through a transparent material

– Air, glass, sandwich bags• Translucent:

– Materials transmit some light– Cause light to spread out in all directions– You can see light through translucent

materials but not objects (clearly)– Notebook paper

• Opaque:– Material that does not allow any light to

pass through– They either reflect light, absorb light, or

do both– Wood, rocks, ceramic mugs

• Scattering:– The spreading out of light rays in all

directions– Particles reflect and absorb the light– Creates glare– Making the light brighter causes more

scattering

• Scattering cont.• Scattering is what makes the sky blue:

– The blue part of the visible spectrum is scattered more easily than the other colors.

– When the sun is setting, the blues and greens have been scattered leaving the reds and oranges in the sunset

• Polarization:– Quality of light where all wavelengths

vibrate in the same direction– Waves traveling in other directions are

filtered out

Wavelengths determine color

• When all wavelengths are present together the light appears white.

• Prism:– A tool that uses refraction to spread out the

different wavelengths that make up white light.

Color reflection and absorption• The color of an object or material is

determined by the wavelengths absorbed and reflected

• An object has the color of the wavelengths it reflects:– White objects reflect ALL wavelengths– Black objects absorb ALL wavelengths– Green limes absorb all wavelengths

except green which is reflected

Primary colors:

• Three colors of light which can be mixed to produce all possible colors– Red– Blue– Green

• These three color bands are the only ones the human eye can detect

• When equally mixed, white (colorless) is produced

Primary pigments

• Pigment:– A material that is used to produce colors

• Primary pigments:– Cyan– Yellow– Magenta

• When mixed together in equal amounts, black is produced