Chapter 18: Electromagnetic Spectrum & Light

18.1: Electromagnetic Waves

• Question: What do x-ray machines, microwave ovens, and heat lamps have in common with police radar, TV, and radiation therapy???

Electromagnetic Waves

• Answer: They all use WAVES to transport energy from one location to another!!!

Electromagnetic Waves

• Electromagnetic Waves- (EM) transverse waves consisting of changing electric fields and changing magnetic fields

Electromagnetic Waves• Can carry energy from one place to another

• Produced by constantly changing fields

• Magnetic and electric fields travel at right angles to each other

EM Waves

• Electromagnetic waves are produced when an electric charge vibrates or accelerates.

•  As fields regenerate, their energy travels in the form of a wave.

• Unlike mechanical waves, EM waves do not need a medium to travel through!– EM waves can travel through a vacuum (or

empty space) or matter.

EM Radiation

• Electromagnetic Radiation- the transfer of energy by electromagnetic waves traveling through matter or across space.

THE SPEED OF EM WAVES

• Question: Why do you see lightning before you hear thunder?

Speed of EM Waves

• Answer: Because light travels faster than sound!

 

• But how much faster is light???

How long would it take you to drive from San Francisco to New York?

Speed of Light Analogy

• Scientists have discovered that light and all electromagnetic waves travel at the same speed when in a vacuum 3 x 108 m/s!

• Consider driving non-stop at 60 mph from NYC to San Francisco.– This trip would take you ~50 hours– Light travels this distance in less than 0.02

second!!!

Speed of EM Wave

• Speed of EM wave = wavelength x frequency– Wavelength is inversely proportional to

frequency– As the wavelength increases, the frequency

decreases

Differences between EM Waves• Even though all EM waves travel at the

same speed, it does not mean they are all the same!– EM waves vary in wavelength and frequency

18-2: The EM Spectrum 

Prism Experiment• In 1800, William Herschel used a prism to

separate the wavelengths present in sunlight. He produced a band of color: red, orange, yellow, green, blue, indigo and violet.

EM Spectrum

• The full range of frequencies of electromagnetic radiation is called the electromagnetic spectrum

• Which includes the following parts: – radio waves, infrared rays, visible light,

ultraviolet rays, X-rays, and gamma rays.

EM Spectrum• Each kind of wave is characterized by a

range of wavelengths and frequencies.

Radio Waves

• Radio waves have the longest wavelengths in the EM spectrum, from 1mm to 1000’s of km. They also have the lowest frequencies, 300,000 mHz or less.– used in radio, TV, microwaves, and radar

 

Radio Waves• In a radio studio, music and voices are changed

into electronic signals that are carried by radio waves.– AM radio stations broadcast by amplitude

modulation, the amplitude of the wave is varied– FM radio stations broadcast by frequency

modulation, the frequency of the wave is varied• A station is lost when its signal becomes too weak

to detect, an FM station is more likely to be lost because FM signals do not travel as far

Difference between AM & FM

More Applications of Radio Waves

• Radio Waves also include the application of:– Television

• Radio waves also carry signals for TV, including the information for pictures

– Microwaves

– Radar (Radio Detection and Ranging)• Short bursts of radio waves that reflect off objects they

encounter and bounce back, being detected by a radio receiver

Infrared Radiation• Infrared waves have higher frequencies than radio

and wavelengths that vary from about 1mm to 750nm– Used as a source of heat and to discover areas of heat

differences

– Invisible to our eye

– Warmer objects give off more infrared than cooler, a device called a thermograph create thermograms (color-coded picture) that show temperature variation

– Thermograms can be used to find places where a building loses heat, search and rescue teams use infrared cameras to locate victims

Infrared Radiation

Visible Light• The visible part of the EM spectrum is light the

human eye can see.– Each color of the visible spectrum corresponds to a

specific frequency and wavelength (ROYGBIV)

Ultraviolet (UV) Rays

• The wavelengths of ultraviolet rays very from 400nm to about 4nm, and higher frequencies than violet light.– In moderation, UV rays help your skin produce vitamin

D

– Excessive exposure can cause sunburn, wrinkles, and skin cancer

– Used to kill microorganisms

– Plant nurseries use UV to help plants to grow during winter

UV Radiation

X-Rays

• X-rays have very short wavelengths from about 12nm to 0.005nm , and have higher frequencies than UV– Have high energy and can penetrate matter that light

cannot

– Used in medicine (pictures of bones), industry (test sealed lids), and transportation (contents of truck trailers)

Gamma Rays

• Gamma rays have the shortest wavelengths about 0.005nm or less, and have the highest frequencies and therefore the most energy and the greatest penetrating ability– Used in the medical field to kill cancer cells, in brain

scans, and in industrial situations such as inspecting pipelines for sign of damage

– Overexposure can be deadly

Gamma Rays & Radiotherapy

The normal cells receive a lower dose of gamma radiationthan the cancer cells, where all the rays meet. Radiotherapy aims to kill the cancer cells while doing as little damage as possible to healthy normal cells.

18.3 Behavior of Light

Question to Ponder…

• What would you see if you were snorkeling in warm ocean waters over a coral reef? You might see fish of bright colors, clown fish, sea stars, etc. Why can you see these animals SO CLEARLY??? Why can you see the reef through the water but not through the bottom of the boat that brought you to the reef???

Light & Materials

• Without light, nothing is visible! – When you look at the reef animals, what you

are really seeing is LIGHT – You can see the reef through the water, because

LIGHT passes through the water between the reef and your eyes.

– You can’t see the reef through the bottom of the boat because LIGHT doesn’t pass through the boat!

Behavior of Light

• How light behaves when it strikes an object depends on many factors…including the material it is made of.

• Materials can be:– Transparent– Translucent– Opaque

Transparent• Transparent: material through which you can see

clearly, transmits light– Most light is able to pass through

– Examples: water, windows

Translucent

• Translucent: you can see through the material, but the objects you see through it does not look clear or distinct. – Scatters Light– Examples: some types of jello, certain bars of

soap, frosty windows

Examples of Translucent

Opaque• Opaque: material either absorbs or reflects

all of the light that strikes it. – NO light is able to pass through– Examples: fruit, wooden table, metal desk

Interactions of Light

• When light encounters matter, some or all of the energy in the light can be transferred to the matter. And just as light can affect matter, matter can affect light.

• When light strikes a new medium, the light can be:– Reflected

– Absorbed

– Transmitted

Reflection• When you look in a mirror, you see a clear image

of yourself. – An image is a copy of an object formed by reflected (or

refracted) waves of light.

• Two types of reflection:– Regular Reflection

– Diffuse Reflection

Regular Reflection• Regular Reflection: occurs when parallel light

waves strike a surface and reflect all in the same direction – Occurs when light hits a smooth, polished surface

– Mirrors or surface of a still body of water (page 547, figure 18)

Diffuse Reflection• Diffuse Reflection: occurs when parallel light

waves strike a rough, uneven surface, and reflect in many different directions – Paper has a rough surface, (page 547, figure 18)

– Rough surfaces causes diffuse reflection of the light that shines on it

When Light is TRANSMITTED

• Reflection occurs because there is no transmission of light (light is not able to pass through to the new material)

• However, when light is transmitted different things can happen. Light can be:– Refracted– Polarized– Scattered

Refraction• Refraction: ability of light to refract, or bend

when it passes at an angle from one medium into another.

• Two easily observable examples that occur when light travels from air into water:– Underwater objects appear closer and larger than they

really are

– Can make an object such as a skewer (or pencil) appear to break at the surface of the water (page 548, figure 19)

Refraction

Refraction Can Create a Mirage

• Refraction can sometimes cause a mirage. • Mirage: a false or distorted image. • Mirages occur because light travels faster in hot

air than in cooler, dense air– On a sunny day, air tends to be hotter just

above the surface of a road than higher up– Mirages also form this way above the hot sand

in deserts

Examples of Mirages

What is Polarization?

Polarization

• Light is an EM Wave EM waves vibrate in TWO planes

• Light waves that vibrate in only one plane is called polarized light.– Polarizing filters transmit light waves that vibrate in

only one direction or plane (page 548, figure 20)

– Unpolarized light vibrates in ALL directions

Polarization

Scattering

• Earth’s atmosphere contains many molecules and other tiny particles. These particles can scatter light.

• Scattering: light is redirected as it passes through a medium (page 549, figure 21)

Scattering explains a red/pink sunset!• Scattering effect reddens the sun at sunset

and sunrise– Small particles in the atmosphere scatter shorter-

wavelength (blue light) more than light of longer wavelengths

– By the time the sunlight reaches your eyes, most of the blue and even some of the green and yellow have been scattered

– Most of what remains for your eyes to detect are the longer wavelengths of light, orange and red

Scattering of Light by Atmosphere