Electromagnetic waves- transverse waves consisting of changing electric and magnetic fields

Carry ENERGY from place to place Different from mechanical waves…

how?

An electric field exerts forces on charged particlesElectric fields are produced by

charged particles changing magnetic fields

A magnetic field produces magnetic forcesMagnetic fields are produced by

magnets changing electric fields vibrating charges

As the two fields regenerate each other, their E travels in the form of a wave.

EM waves DON’T need a medium!!! Can travel through a vacuum (=empty

space) OR through matter EM radiation – the transfer of E by EM

waves traveling through matter or across space.

What is faster…speed of sound or light?

Light travels much faster than sound. For example:

What is faster…speed of sound or light?

1) Thunder and lightning start at the same time, but we will see the lightning first.

2) When a starting pistol is fired we see the smoke first and then hear the bang.

First accurate estimations were in 1926 when Albert Michelson completed his experiment in California.

Light (and all EM waves) travel the same speed in a vacuum…

c = 3.00 x 108 m/s Or 300,000,000 m/s(compared to sound @ ~

340 m/s)That’s 8 times around the

earth in 1 second!!!!

EM waves vary in and frequency

c = * frequency

Since the speed of light (c) is always 3.00 x 108 m/s, you can always calculate from frequency and vice versa.

EM radiation sometimes behaves like a wave, and sometimes like a stream of particles.

There is evidence for both theories... That’s called wave-particle duality

In some experiments, the wave model works best.

In other experiments, the particle model works best.

Thus, we use both

LightLightPhotonsPhotons

ElectromagneticElectromagneticRadiationRadiation

Double Slit ExperimentPass light through two slits and an

interference pattern is observed Interference is a property of waves!

Photoelectric EffectLight shown on a metal can cause electrons

to be emitted from the metal

Photons- particles of light The greater the frequency of an EM

wave, the more E each of its photons has

Intensity- the rate at which a wave’s energy flows through a given unit of area…basically, it is brightness of light.

As you leave the source of light, does intensity increase or decrease?

Farther from the source, the photons spread out over a larger area, and intensity decreases.

What happens when you put a prism in front of a window?

EM spectrum- the full range of frequencies of EM radiation

How many different types of EM waves can you think of?

EM Spectrum includes:Radio wavesInfrared raysVisible lightUV raysX-raysGamma Rays

TelevisionTelevision

Shorter than radio, also used to carry messages (pictures & sound) to our TV sets.

We can sense the TV waves around us with our televisions.

MicrowaveMicrowave

Emitted by:Gas clouds collapsing

into starsMicrowave OvensRadar StationsCell Phones

Detected byMicrowave

TelescopesFood (heated)Cell phonesRadar (systems)

InfraredInfrared(Heat or Thermal)Are you a source of infrared? YES you are!

Emitted bySun and stars

(Near)TV Remote ControlsFood Warming

Lights (Thermal)*Everything at

room temperature or above,=HEAT

Detected by Infrared CamerasTVs, VCRs,Your skin

VisibleVisibleEach color is a different size wave.Red the longest & violet the shortest

Emitted byThe sun and other

astronomical objects

Laser pointersLight bulbs

Detected byCamerasHuman eyesPlants (red light)Telescopes

UltravioletSunburn / black light

Emitted byTanning booths (A)The sun (A)Black light bulbs (B)UV lamps

Detected bySpace based UV

detectorsUV CamerasFlying insects (flies)

X-rayX-ray Emitted by

Astronomical objects

X-ray machinesCAT scan machinesRadioactive

mineralsAirport luggage

scanners

Detected bySpace based X-ray

detectorsX-ray film

Gamma RayGamma Ray(Short electromagnetic waves but more energetic)

Emitted byRadioactive materialsExploding nuclear

weaponsSolar flares

Detected by Geiger countersGamma detectors and

astronomical satellitesMedical imaging

detectors

COSMIC RaysCOSMIC Rays(The highest energy waves and the deadliest)

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

When you are looking at things, anything, what you are really seeing is light.

We can’t see through walls because light doesn’t pass through walls.

We see things because they reflect light into our eyes:

Homework

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

Materials can be translucent, transparent, or opaque.

Transparent objects- No scatteringColor transmitted is color you see

and all other colors are absorbed Translucent-

Light is scattered and transmitted some

Opaque- Light is either totally reflected or

absorbed Color of opaque objects is color it

reflects

When light strikes a new medium, the light can be reflected, absorbed, or transmitted.

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

ReflectionReflection Reflection from a

mirror:

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

The Law of ReflectionThe Law of Reflection

Angle of incidence = Angle of reflection

The same !

!!

Clear vs. Diffuse Clear vs. Diffuse ReflectionReflection Smooth, shiny

surfaces have a specular reflection:

Rough, dull surfaces have a diffuse reflection

Diffuse reflection is when light is scattered in different directions

Using mirrorsUsing mirrors Two examples:

1) A periscope

2) A car headlight

The bending of light waves as they pass from one medium to another

Results in mirages, which are false or distorted images

Inferior Mirages:formed when the air near the ground is very warm compared to the air just above it.

Polarized light is light that all vibrate in the same plane (or direction!)

Light is redirected as it passes through a medium.

This is responsible for our red sunsets!

White light- not a single color; it is made up of a mixture of the seven colors of the rainbow We can demonstrate

this by splitting white light with a prism

This is how rainbows are formed: sunlight is “split up” by raindrops

.

Red Orang

e Yellow Green Blue

Indigo Violet

Adding ColorsAdding Colors White light can be split up to make

separate colors. These colours can be added together again

The primary colors of light are red, blue and green:

Adding blue and red makes magenta (purple)

Adding blue and green makes cyan

(light blue)

Adding all three makes white again

Adding red and green makes yellow

Seeing ColorSeeing Color The color an object appears depends on the colors of

light it reflects For example, a red book only reflects red light:

Whitelight

Only red light is

reflected

A white hat would reflect all seven colors:

A pair of purple trousers would reflect purple light (and red and blue, as purple is made

up of red and blue):

Purple light

White

light

Color Blind Tests

If you continue to focus on the sign in the centre of the image you will notice that the circle of violet circles will soon disappear completely, and you will see only the green spot (which is actually violet)

LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses Convex lenses

Thicker in the center than edges. Lens that converges (brings together)

light rays. Forms real images and virtual images

depending on position of the objectThe images formed are upside down

LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses Convex Lenses Ray Tracing

Two rays usually define an image Ray #1: Light ray comes from top of

object; travels parallel to optic axis; bends thru focal point.

Focal Point

Lens

Object

© 2000 D. L. Power

LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses

Convex Lenses Ray Tracing

Two rays define an image Ray 2: Light ray comes from top of

object & travels through center of lens.

Ray #1

Ray #2

© 2000 D. L. Power

LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses

Concave lenses – Lens that is thicker at the edges and

thinner in the center. Diverges light rays All images are upright and reduced

© 2000 D. L. Power

Vision The eye is a convex lens

Retina Lens refracts light to

converge on the retina and then nerves transmit the image

Rods Nerve cells in the retina.

Very sensitive to light & dark

Cones Nerve cells help to see

light/color

Rods – responsible for black and white vision and detection of motion.

Cones – Seeing in color and visual acuity. We have three types of cones: cones that see red, cones that see blue, and cones that see green.

How You See

Near Sighted – Eyeball is too long and image focuses in front of the retina

Far Sighted – Eyeball is too short so image is focused behind the retina.

LIGHT & USES: Optical InstrumentsLIGHT & USES: Optical Instruments

LASERSHolography – Use of Lasers to create 3-

D imagesFiber Optics – Light energy transferred

through long, flexible fibers of glass/plastic

Uses – Communications, medicine, t.v. transmission, data processing.