ELECTROMAGNETIC WAVES

CHAPTER 18

Brief review:

Water and sound waves transfer energy from one place to another- they require a medium through which to travel. They are mechanical waves.

• They are Transverse waves without a medium. (They can travel through empty space)

• They travel as vibrations in electrical and magnetic fields.

• produced by the motion of electrically charged particles

• When an electric field changes, so does the magnetic field. The changing magnetic field causes the electric field to change. When one field vibrates—so does the other.

• RESULT-An electromagnetic wave.

Speed of EMR

–All emr waves travel at the speed of light.

–300,000 km/s or 3.00 x 108 m/s

–C symbol for speed of light

– speed in a vacuum

• All EMR has same speed but differ in frequencies and wavelengths

• EMR can behave as a wave or as a stream of particles.

• Wave model – shows that EMR acts as waves because of interference.

Particle Model

When directed at a substance light can knock electrons off of a substance

(Photoelectric effect)-emission of electrons from light striking metal

Photon – packets of electromagnetic energy

Brightness dims as photons travel away from source

Waves of the Electromagnetic Spectrum• Electromagnetic Spectrum—name for the range of

electromagnetic waves when placed in order of increasing frequency

RADIO WAVES

MICROWAVES

INFRARED RAYS

VISIBLE LIGHT

ULTRAVIOLET RAYS

X-RAYS

GAMMA RAYS

RADIO WAVES

• Have the longest wavelengths and lowest frequencies of all the electromagnetic waves.

• Each radio station in an area broadcasts at a different frequency. # on radio dial tells frequency.

MRI (MAGNETIC RESONACE IMAGING)

Uses Short wave radio waves with a magnet to create an image

• Modulation- adding data (like music or news) to radio waves

FM- frequency modulated

AM- amplitude modulated

AM=Amplitude modulation—waves bounce off ionosphere can pick up stations from different cities.

(535kHz-1605kHz= vibrate at 535 to 1605 thousand times/second)

+

FM=Frequency modulation—waves travel in a straight line & through the ionosphere--lose reception when you travel out of range.

(88MHz-108MHz = vibrate at 88million to 108million times/second)

+

Bands of Radio/TV/Microwaves

MICROWAVES a type of radio wave

• Microwaves— have the shortest wavelengths and the highest frequency of the radio waves.– Used in microwave ovens.

• Waves transfer energy to the water in the food causing them to vibrate which in turn transfers energy in the form of heat to the food.

– Used by cell phones and pagers.– RADAR (Radio Detection and Ranging)

• Used to find the speed of an object by sending out radio waves and measuring the time it takes them to return.

Bands of Radio/TV/Microwaves

INFRARED RAYS• (below red)• Shorter wavelength and higher frequency than microwaves.• You can feel these as warmth on your skin• Heat lamps give off infrared waves.• Warm objects give off more heat energy than cool objects.

• Thermogram—a picture that shows regions of different temperatures in the body. Temperatures are calculated by the amount of infrared radiation given off. Therefore people give off infrared rays.

Let’s take a look at Herschel’s Experiment

• Herschel’s Experiment

– Discovered Invisible Light

– In 1800, Herschel places his control thermometer just outside the red end of the spectrum

– Result: The outside thermometer registered the highest temperature

Let’s set up Herschel’s Experiment

• Set up of Box Design for Conducting the Herschel Experiment.

Conducting Hershel’s Experiment• Place a sheet of white paper

inside a cardboard box

• Tape three thermometers together and place inside box

• Cut a small notch in the top of the box and position a glass prism so that the spectrum is projected inside the box

• Arrange the thermometers so that one is just outside the red end of the spectrum, with no visible light falling on it

VISIBLE LIGHT

• Electromagnetic waves we can see.

• Longest wavelength= red light

• Shortest wavelength= violet (purple) light

• When light enters a new medium it bends (refracts). Each wavelength bends a different amount allowing white light to separate into it’s various colors ROYGBIV.

• Red, orange, yellow, green, blue, indigo, violet

ULTRAVIOLET RAYS• Shorter wavelength and higher frequency than

visible light• Carry more energy than visible light• Used to kill bacteria. (Sterilization of equipment)• Causes your skin to produce vitamin D (good for

teeth and bones)• Used to treat jaundice ( in some new born babies.• Too much can cause skin cancer. • Use sun block to protect against (UV rays)

X- RAYS• Shorter wavelength and higher frequency than UV-rays• Carry a great amount of energy• Can penetrate most matter.• Bones and teeth absorb x-rays. (The light part of an x-

ray image indicates a place where the x-ray was absorbed)• Too much exposure can cause cancer

– (lead vest at dentist protects organs from unnecessary exposure)

• Used by engineers to check for tiny cracks in structures.– The rays pass through the cracks and the cracks appear dark on

film.

GAMMA RAYS

• Shorter wavelength and higher frequency than X-rays

• Carry the greatest amount of energy and penetrate the most.

• Used in radiation treatment to kill cancer cells.

• Can be very harmful if not used correctly.

• Brief SUMMARY• A. All electromagnetic waves travel at the

same speed. (300,000,000 meters/second in a vacuum.

• B. They all have different wavelength and different frequencies.– Long wavelength-lowest frequency– Short wavelength highest frequency– The higher the frequency the higher the energy.

COSMIC Rays(The highest energy waves and the deadliest)

• Cosmic rays come from deep space and can pass through the Earth.

• A great question!• Radio waves= (Buildings to human size);• Microwaves

(Humans-beetles);• Infrared waves (Eye of a needle);Visible waves (microscopic size)!

WOW! All the rest are the size of molecules, atoms, atomic nuclei and smaller..

Speed of Mechanical

Waves

Section 17.2

Speed of Mechanical

Waves

Section 17.2

Speed of Mechanical

Waves

Section 17.2

Speed of Mechanical

Waves

Section 17.2

Calculating Wave Speed

Section 18.1

Calculating Wave Speed

Section 18.1

Calculating Wave Speed

Section 18.1

Calculating Wave Speed

Section 18.1

Interest GrabberWhat Can You See Through?Just by looking around, you know that light can travel through air. A

transparent material, such as air, transmits light. An opaque material, such as metal, blocks light. Look around your classroom. What are some transparent and opaque materials that you see?

Can an object block light and also allow some light to pass through it? Hold a book up toward a light. Put your hand between the light and the book. Can you see your hand through the book? Now hold a piece of white paper up toward the light, and put your hand behind it. Can you see your hand through the paper? 1. Could light pass through the book? 2. Was light able to pass through the paper? 3. List some transparent and opaque objects that you saw in

the classroom.

Behavior of LightChapter 18.3

• Transparent – transmits light

• Translucent – Scatters light

• Opaque – reflects or absorbs light

Interactions of Light

• When light strikes an object it is either reflected, absorbed, or transmitted.

• When light is transmitted, it can be refracted, polarized, or scattered.

Reflection• Light that bounces off of a medium.

• Regular reflection – parallel light waves strike a surface and reflect all in the same direction

• Diffuse reflection – light waves hit surface and reflect unevenly.

Refraction

• The bending of light as it travels from air to water.

• Mirages

Polarization• Light vibrates only in one plane

Scattering

• The redirection of light.

• Earth’s atmosphere is blue due to scattering.

• Sunrise and sunsets appear red due to scattering

• Dispersion – white light being separated into colors.

Color of Objects

• Depends on the material and what color light is striking it.

• Red car absorbs all but the red. The red is reflected.

• Page 552

Mixing colors of light

• Primary colors of light

• Red, Blue, and Green

• Can be mixed to form all other colors.

• Page 552

• Primary colors bounce off objects, mix, and appear as the color we see.

• Page 552

Secondary colors

• Combination of two primary colors

• Cyan, yellow, and magenta

• Page 552

Venn Diagram

Section 18.4

a. blue

b.magenta

c. cyan

d. yellow

Complimentary colors of light

• One primary color and one secondary color equals white light

• All three secondary colors make black

Pigments

• A material that absorbs some colors of light and reflects all others.

• Primary pigment colors

–Cyan

–Yellow

–Magenta

–Page 553

Complimentary colors of pigments – any two pigments that combine to form black.

Page 553

Sources of Light• Incandescent – light is produced

when object gets hot enough to glow.

• Electrons flow through filament.• Uses nitrogen and argon gas.

• Fluorescent – Mercury vapor gas inside emits UV rays which make Phosphors glow.

Use less energy

• Laser – light waves with the same wavelengths line up crest to crest.

• Coherent light – beam of light does not spread out with distance.

• Neon – excited electrons move through a gas inside of glass tubing.

• Sodium Vapor – a mix of solid sodium, neon gas, and argon gas heat up to form light.

• Ex. Street lights and parking lot lights.

• Tungsten – Halogen light – just like incandescent but uses halogen gas inside.

• Very hot. Bulb is made of quartz instead of glass.

E. Cool Applications!

• Fiber Optics– Total Internal Reflection

• when all light is reflected back into the denser medium

E. Cool Applications!

• The “Broken Pencil”– refraction

View animation and explanation of the “Broken Pencil.”

E. Cool Applications!

• Rainbows– refraction-reflection-refraction

E. Cool Applications!

• Diffraction Gratings– glass or plastic made up of

many tiny parallel slits– may also be reflective– spectroscopes, reflective

rainbow stickers, CD surfaces

E. Cool Applications!

• Thin Films - Bubbles & Oil Slicks– interference results from double reflection

E. Cool Applications!

• Blue Sky & Red Sunsets

NOON• less atmosphere• less scattering• blue sky, yellow

sun

SUNSET• more atmosphere• more scattering• orange-red sky & sun

• Molecules in atmosphere scatter light rays.• Shorter wavelengths (blue, violet) are scattered more easily.

B. Seeing Colors

• The retina contains…– Rods - dim light, black & white– Cones - color

• red - absorb red & yellow

• green - absorb yellow & green

• blue - absorb blue & violet

Stimulates red & green cones

Stimulates all cones

B. Seeing Colors

• Color Blindness– one or more sets of

cones does not function properly

Test for red-green color blindness.

C. Mixing Colors

• Primary light colors– red, green, blue

– additive colors

– combine to form white light

View Java Applet on primary light colors.

– EX: computer RGBs

C. Mixing Colors

• Filter– transparent material

that absorbs all light colors except the filter color

View Java Applet on filters.

C. Mixing Colors

• Pigment– colored material that absorbs

and reflects different colors

• Primary pigment colors– cyan, magenta, yellow– subtractive colors– combine to form black

– EX: color ink cartridges

C. Mixing Colors

Light Pigment

When mixing pigments, the color of the mixture is the color of light that both

pigments reflect.

Negative Afterimage - One set of cones gets tired, and the remaining cones produce an image in the

complimentary color.