Introduction to Light Electro-Magnetic Radiation

Introduction to LightIntroduction to Light

Electro-Magnetic RadiationElectro-Magnetic Radiation

Simon Heppenstall, Simon Heppenstall, Park Lane College, LeedsPark Lane College, Leeds

Today, we will look at…Today, we will look at…

What is light?What is light? From where does light come?From where does light come? How does it get here?How does it get here? How do materials affect light?How do materials affect light? For what do we use light?For what do we use light? How do we detect light?How do we detect light?


What is light?What is light?

A form of energyA form of energy Electromagnetic RadiationElectromagnetic Radiation SometimesSometimes

• A particleA particle• A waveA wave

PhotonPhoton


A form of energyA form of energy

There are many forms of energy.There are many forms of energy. Energy can neither be created nor Energy can neither be created nor

destroyed: just changed from one destroyed: just changed from one form to another.form to another.

Another form of energy associated Another form of energy associated with light is heat.with light is heat.

Sound is also a form of energy.Sound is also a form of energy. We use energy to do things.We use energy to do things.


Electromagnetic RadiationElectromagnetic Radiation

For us, this comes from the sun.For us, this comes from the sun. Short wave (ie <400nm)Short wave (ie <400nm)

• Ultraviolet, X-rays, Gamma raysUltraviolet, X-rays, Gamma rays Visible (400nm>x>750nm)Visible (400nm>x>750nm)

• Colours from Violet to RedColours from Violet to Red Long wave (>750nm)Long wave (>750nm)

• Infrared, microwave and radio wavesInfrared, microwave and radio waves


SometimesSometimes

A ParticleA Particle• Newton believed thisNewton believed this• No-one dare argueNo-one dare argue

A WaveA Wave• James Clerk Maxwell believed thisJames Clerk Maxwell believed this

Discovered that electricity and magnetism Discovered that electricity and magnetism travelled at the speed of lighttravelled at the speed of light

The decided that light must be an electro-The decided that light must be an electro-magnetic wave.magnetic wave.


A photonA photon

A photon behaves like a wave in A photon behaves like a wave in spacespace

A photon behaves like a particle near A photon behaves like a particle near mattermatter

Light is not waves, nor is it Light is not waves, nor is it particulate. It is photonic.particulate. It is photonic.


From where does light come?From where does light come?

The SunThe Sun The StarsThe Stars Artificial light sourcesArtificial light sources


The SunThe Sun

Source of most of our natural lightSource of most of our natural light 15 million K at centre15 million K at centre Diameter of 864,000 milesDiameter of 864,000 miles 99% total mass of solar system99% total mass of solar system Runs in 11 year cyclesRuns in 11 year cycles ““Day” of between 25 and 27 daysDay” of between 25 and 27 days Further Information:Further Information:

• N.A.S.A. Site


The StarsThe Stars

A minor source of light for us A minor source of light for us because of their distance awaybecause of their distance away

Help with navigation around the Help with navigation around the globeglobe

Light takes between 4 years and 15 Light takes between 4 years and 15 billion years to get to Earth from the billion years to get to Earth from the starsstars

They could have all gone out 3They could have all gone out 3½ ½ years agoyears ago


Artificial Light SourcesArtificial Light Sources

Second most important source to us Second most important source to us nownow

Light pollution stops us from seeing Light pollution stops us from seeing the starsthe stars

Usually a conversion of electrical Usually a conversion of electrical energy to light and heat energyenergy to light and heat energy

Sometimes a conversion of chemical Sometimes a conversion of chemical energy to light and heat energyenergy to light and heat energy


How does light get here?How does light get here?

Light travels in straight linesLight travels in straight lines It travels about 186,000 miles per It travels about 186,000 miles per

secondsecond Light speed is not relative, ie the Light speed is not relative, ie the

light travelling from the front of a light travelling from the front of a train is travelling at the same speed train is travelling at the same speed as light travelling from a treeas light travelling from a tree

It takes 8 minutes for light to get It takes 8 minutes for light to get from Earth to the sunfrom Earth to the sun


How do materials affect light?How do materials affect light?

RefractionRefraction ReflectionReflection DiffractionDiffraction TransparentTransparent TranslucentTranslucent OpaqueOpaque


What is Refraction?What is Refraction?

Materials have a property called the Materials have a property called the refractive indexrefractive index

If light moves from one material to If light moves from one material to another with a different refractive another with a different refractive index, it bendsindex, it bends

This bend is called refraction.This bend is called refraction. Different frequencies of light bend by Different frequencies of light bend by

different amounts. White light is split different amounts. White light is split into coloursinto colours


What is Reflection?What is Reflection?

Some materials are shiny (metals)Some materials are shiny (metals) Shiny materials make some light Shiny materials make some light

bounce backbounce back This is called reflectionThis is called reflection The light from the moon is reflected The light from the moon is reflected

lightlight


What is Diffraction?What is Diffraction?

This is complicated, but when light This is complicated, but when light hits an edge or a corner, a tiny hits an edge or a corner, a tiny amount is bent round the corner.amount is bent round the corner.

The only place you are likely to see The only place you are likely to see this effect is in the science lab.this effect is in the science lab.

You need a LASER and a mesh called You need a LASER and a mesh called a diffraction grating.a diffraction grating.


What is a Transparent Material?What is a Transparent Material?

A transparent material allows light to A transparent material allows light to pass through itpass through it

Transparent means see-throughTransparent means see-through Glass and water are transparent Glass and water are transparent

materialsmaterials Even coloured glass is transparentEven coloured glass is transparent


What is a Translucent Material?What is a Translucent Material?

A translucent material allows some A translucent material allows some light to pass through itlight to pass through it

Usually, the view through a Usually, the view through a translucent material is not very clear, translucent material is not very clear, but you can make out shapesbut you can make out shapes

Tracing paper is translucentTracing paper is translucent


What is an Opaque Material?What is an Opaque Material?

An opaque material does not allow An opaque material does not allow light to travel through itlight to travel through it

If you placed a light source next to If you placed a light source next to an opaque material, you would get a an opaque material, you would get a shadow at the opposite side of the shadow at the opposite side of the object to the lightobject to the light

Wood is an opaque materialWood is an opaque material


How is a shadow formed?How is a shadow formed?

A shadow is formed when light is A shadow is formed when light is blockedblocked

Umbra – full shadow (darkest bit)Umbra – full shadow (darkest bit) Penumbra – half shadow (lighter)Penumbra – half shadow (lighter) Penumbra is formed when the light Penumbra is formed when the light

source is larger than the object or source is larger than the object or there are two light sourcesthere are two light sources


For what do we use light?For what do we use light?

To seeTo see To measureTo measure To cutTo cut To entertainTo entertain


To seeTo see

Our eyes have adapted to pick up Our eyes have adapted to pick up nearly half the wavelengths that the nearly half the wavelengths that the sun emitssun emits

Light hits the retina in our eyesLight hits the retina in our eyes• Rods respond to dim light: no colourRods respond to dim light: no colour• Cones respond to coloured lightCones respond to coloured light

We see in shades of red, green and We see in shades of red, green and blueblue


To measureTo measure

LASER stands for Light Amplification LASER stands for Light Amplification by Stimulated Emission of Radiationby Stimulated Emission of Radiation

Allows for measurement within a Allows for measurement within a fraction of a millimetrefraction of a millimetre


To cutTo cut

Again, Lasers allow us to cut things Again, Lasers allow us to cut things very accuratelyvery accurately


To entertainTo entertain

Fireworks started light entertainmentFireworks started light entertainment Light Shows set to musicLight Shows set to music Lasers drawing images in smoke at a Lasers drawing images in smoke at a

night clubnight club Entertainers have to remember that Entertainers have to remember that

light itself is invisible. You can only light itself is invisible. You can only see the effect of light bouncing off see the effect of light bouncing off something, eg smoke particles.something, eg smoke particles.


How do we detect light?How do we detect light?

Our EyesOur Eyes• Shape and ShadeShape and Shade• ColourColour

InstrumentsInstruments• CamerasCameras• SensorsSensors• Optical InstrumentsOptical Instruments


Our Eyes – Shape and ShadeOur Eyes – Shape and Shade

Shape and Shade is detected by the Shape and Shade is detected by the rods in our retinarods in our retina

Only part of the brain visible from Only part of the brain visible from outside the bodyoutside the body

Seeing: CorneaSeeing: Cornea Lens Lens Retina Retina Rods and Cones Rods and Cones Optic Nerve Optic Nerve BrainBrain


Our Eyes – ColourOur Eyes – Colour

Colour is detected by conesColour is detected by cones A lot of light is needed for cones to A lot of light is needed for cones to

start to workstart to work Three types of cone detectThree types of cone detect

• RedRed• GreenGreen• Blue light. Blue light.


Instruments – CamerasInstruments – Cameras

Cameras behave in a similar manner Cameras behave in a similar manner to the eyeto the eye

Lens Lens Retina (in the eye) Retina (in the eye) Lens Lens Film (in the camera) Film (in the camera) Light can affect the mood of a Light can affect the mood of a

picturepicture Photographers often use filters to Photographers often use filters to

enhance this affectenhance this affect


Instruments – SensorsInstruments – Sensors

Electronics are getting more complexElectronics are getting more complex Light Dependant Resistors (LDR) are Light Dependant Resistors (LDR) are

often used as sensor/switch devicesoften used as sensor/switch devices Made of Cadmium SulphideMade of Cadmium Sulphide Resistance alters with light amountResistance alters with light amount


Optical InstrumentsOptical Instruments

Galileo invented the telescopeGalileo invented the telescope Infrared instruments allow us to see Infrared instruments allow us to see

in the darkin the dark


Links to other sources of Links to other sources of information on Lightinformation on Light

Treating Cancer Solar System Simulator Optics Kodak Info on Light The National Geographic Presentatio

n on Sight and Sound


Safety With LightSafety With Light

Never look directly at the sunNever look directly at the sun Never look into a LASERNever look into a LASER Use your common senseUse your common sense


Quiz InstructionsQuiz Instructions

You will be set a series of questions about You will be set a series of questions about the subject, Light. You have to choose the the subject, Light. You have to choose the best answer available.best answer available.

Click on the next to the answer you Click on the next to the answer you think is correct. If you are correct, you will think is correct. If you are correct, you will score two points. score two points.

To find your current score, click on To find your current score, click on To get help, click on To get help, click on (If you click help, you only score 1 point)(If you click help, you only score 1 point) Click here to start:Click here to start:


What is the main source of light on What is the main source of light on Earth?Earth?

Get Help (Score Max = 1) Tell me my score

The Sun

The stars

Artificial light


What are the three primary colours What are the three primary colours of light?of light?


Red, Yellow, Green

Red, Green, Blue

Red, Yellow, Blue


For what does LASER stand?For what does LASER stand?


Light Amplification by Stimulated Electromagnetic Radiation

Light Amplification by Stimulated Emission of Radiation

Light Amplitude and Stimulated Electromagnetic Radiation


From what is an LDR made?From what is an LDR made?


Silicon Oxide

Cadmium Sulphide

Silicon Hydroxide


Where in the eye would you find Where in the eye would you find cones?cones?


The Cornea

The Lens

The Retina


What were the first light shows?What were the first light shows?


Light and Sound Shows

Laser Shows

Fireworks


Which of these best describes the Which of these best describes the energy conversion in a light bulb?energy conversion in a light bulb?


Chemical Light

Electrical Light

Electrical Light & Heat


What detects shade and shape in What detects shade and shape in our eye?our eye?


The Lens

The Cones

The Rods


How fast does light travel?How fast does light travel?


186,000 miles per hour

186,000 miles per minute

186,000 miles per second


How does light travel?How does light travel?


Generally, in straight lines

Into the eye

By splitting into single frequencies


Help OneHelp One

There are three basic sources of light There are three basic sources of light that we receivethat we receive• Artificial light (bulbs etc)Artificial light (bulbs etc)• Stars (millions of light years away)Stars (millions of light years away)• Sun (or Sol)Sun (or Sol)

Which of these affects us most?Which of these affects us most?


Help TwoHelp Two

Primary colours are there, not Primary colours are there, not because of light, but because of our because of light, but because of our eyes.eyes.

Our cones have a peak sensitivity to Our cones have a peak sensitivity to red, green and blue light. (These are red, green and blue light. (These are the colours they detect best)the colours they detect best)

Paint uses red, yellow and blue to Paint uses red, yellow and blue to mix to make all other coloursmix to make all other colours


Help ThreeHelp Three LASERS were invented by Charles Townes and Arthur Schawlow in LASERS were invented by Charles Townes and Arthur Schawlow in

the 1950’sthe 1950’s When an electron changes from a high energy state to a low When an electron changes from a high energy state to a low

energy state, its atom emits a photon.energy state, its atom emits a photon. Take a crystal and shine a light on it. The electrons get excited Take a crystal and shine a light on it. The electrons get excited

and jump up a level.and jump up a level. As they calm down, they move down a level and emit a photon.As they calm down, they move down a level and emit a photon. This causes a chain reaction as these photons excite more This causes a chain reaction as these photons excite more

electrons, which in turn jump up a level and then calm down electrons, which in turn jump up a level and then calm down again. More photons are given off.again. More photons are given off.

The light is amplified as more light comes off than was put in.The light is amplified as more light comes off than was put in. That’s the theory. In practice, lasers are blasted with a small That’s the theory. In practice, lasers are blasted with a small

amount of radiation.amount of radiation.


Help FourHelp Four

Light Dependant Resistors are used Light Dependant Resistors are used for turning sections of circuitry on or for turning sections of circuitry on or off.off.

The chemical formula for the The chemical formula for the material from which they are made is material from which they are made is CdSCdS

The Cd has a RAM of 112.41 and The Cd has a RAM of 112.41 and boils at 1038 Kelvinboils at 1038 Kelvin


Help FiveHelp Five

The part of the eye that actually The part of the eye that actually detects the light passed back to it by detects the light passed back to it by the lens is called the retina.the lens is called the retina.

This has the light detection cells in itThis has the light detection cells in it The lens focuses the light coming The lens focuses the light coming

into the eyeinto the eye


Help SixHelp Six

The Chinese were the first people to The Chinese were the first people to witness an artificial light showwitness an artificial light show

LASERS were invented in the 1950’s LASERS were invented in the 1950’s byby Charles Townes and Arthur Charles Townes and Arthur SchawlowSchawlow

The electric light bulb was invented The electric light bulb was invented by Thomas Edisonby Thomas Edison


Help SevenHelp Seven

Light bulbs are usually powered by Light bulbs are usually powered by electricity nowadays, although some electricity nowadays, although some gas lights still existgas lights still exist

When a form of energy is changed When a form of energy is changed into light, heat is usually given off as into light, heat is usually given off as a by-producta by-product

This heat is a source of inefficiencyThis heat is a source of inefficiency


Help EightHelp Eight

Light entering the eye is focused by Light entering the eye is focused by the lensthe lens

It is focused onto the retina at the It is focused onto the retina at the back of the eyeback of the eye

The retina is made of about 125 The retina is made of about 125 million sensors called rods and conesmillion sensors called rods and cones

Cones come in three basic typesCones come in three basic types


Help NineHelp Nine

The sun is about 93 million miles The sun is about 93 million miles from the earthfrom the earth

It takes about 8 minutes for the light It takes about 8 minutes for the light to get here from the sunto get here from the sun

Which speed would light have to be Which speed would light have to be travelling in order to make that travelling in order to make that possible?possible?


Help TenHelp Ten

Can you see round a corner?Can you see round a corner?


Well DoneWell Done

The light we The light we receive from the receive from the sun makes up over sun makes up over 90% of the light 90% of the light that we use.that we use.

Only one billionth Only one billionth of the energy given of the energy given off by the sun off by the sun reaches the earth.reaches the earth.


Well doneWell done

The three primary colours of light are The three primary colours of light are red, green and blue.red, green and blue.

This is because the cones in our eyes This is because the cones in our eyes detect these three colours best.detect these three colours best.

The eyes in bees detect ultra violet The eyes in bees detect ultra violet lightlight


That is correct!!That is correct!!

Lasers are created by stimulating the Lasers are created by stimulating the atoms in crystals and this causes a atoms in crystals and this causes a chain reaction which leads to more chain reaction which leads to more light coming out than went in.light coming out than went in.


Fantastic. You got it right.Fantastic. You got it right.

LDR’s are made from Cadmium LDR’s are made from Cadmium Sulphide.Sulphide.

Silicon Oxide is sand and is used to in Silicon Oxide is sand and is used to in making transistors and diodesmaking transistors and diodes


That is correct.That is correct.

The retina is a carpet of over 125 The retina is a carpet of over 125 million rods and cones.million rods and cones.

The rods detect the shape and shade The rods detect the shape and shade while the cones detect the colour.while the cones detect the colour.

It is because the cones are best at It is because the cones are best at detecting red, green and blue that detecting red, green and blue that these are the primary colours of these are the primary colours of light.light.


Well done.Well done.

The Chinese invented the fireworks The Chinese invented the fireworks several thousand years ago.several thousand years ago.

Thomas Edison invented the electric Thomas Edison invented the electric light bulb at the end of the last light bulb at the end of the last centurycentury

LASERS were invented in the 1950’s LASERS were invented in the 1950’s byby Charles Townes and Arthur Charles Townes and Arthur SchawlowSchawlow


Correct!Correct!

Electrical energy goes into the bulb Electrical energy goes into the bulb when you turn it on at the switch.when you turn it on at the switch.

Light comes out because it brightens Light comes out because it brightens the room.the room.

Heat comes out because the bulb Heat comes out because the bulb gets hot.gets hot.


That is right.That is right.

The job of the lens is to focus the The job of the lens is to focus the light towards the retinalight towards the retina

The Cornea protects the eye and The Cornea protects the eye and starts to focus the light towards the starts to focus the light towards the retinaretina


Well DoneWell Done

Light travels 186,000 miles every Light travels 186,000 miles every secondsecond

That means that it takes 8 minutes That means that it takes 8 minutes for light to get here from the sunfor light to get here from the sun

It takes over four years for light to It takes over four years for light to get here from the next nearest starget here from the next nearest star


Correct!!!Correct!!!

Generally, light travels in straight Generally, light travels in straight lines, although a diffraction grating lines, although a diffraction grating and laser can be used to and laser can be used to demonstrate that a minute amount demonstrate that a minute amount of light does go round cornersof light does go round corners


Sorry, that’s incorrectSorry, that’s incorrect

Although the stars give off as much Although the stars give off as much or more light than the sun, because or more light than the sun, because of the distance from the stars to the of the distance from the stars to the earth, the light that reaches us is earth, the light that reaches us is insignificant.insignificant.


Sorry, that’s incorrectSorry, that’s incorrect

Although we make a lot of use of Although we make a lot of use of artificial light, the amount we artificial light, the amount we actually produce on earth is very actually produce on earth is very small compared to the amount of small compared to the amount of light we get from the sun.light we get from the sun.

Light pollution is becoming a problem Light pollution is becoming a problem in some of the larger towns and cities in some of the larger towns and cities though.though.


I’m afraid that is wrongI’m afraid that is wrong

Red, yellow and green are all primary Red, yellow and green are all primary colours for either light or pigment, colours for either light or pigment, but they are mixed together herebut they are mixed together here



Red, yellow and blue are the primary Red, yellow and blue are the primary colours for pigment (such as paint colours for pigment (such as paint and dye)and dye)

Light mixes together in a slightly Light mixes together in a slightly different way, which means that one different way, which means that one of these colours is different.of these colours is different.

If you mix yellow and blue pigment, If you mix yellow and blue pigment, you would make the missing primary you would make the missing primary colour of light.colour of light.


IncorrectIncorrect

What you have actually said there is What you have actually said there is that laser stands for light that laser stands for light amplification by stimulated light. (as amplification by stimulated light. (as electromagnetic radiation is light)electromagnetic radiation is light)

This isn’t too far from the truth, but This isn’t too far from the truth, but the answer is still wrong.the answer is still wrong.


IncorrectIncorrect

This is just a set of words that begin This is just a set of words that begin with the correct letters.with the correct letters.

Look at the help file when you go Look at the help file when you go back to the question.back to the question.


I’m afraid not.I’m afraid not.

You just have to remember that an You just have to remember that an LDR is made from Cadmium SulphideLDR is made from Cadmium Sulphide

Cadmium is now thought to be Cadmium is now thought to be carcinogeniccarcinogenic


I’m afraid not.I’m afraid not.

You just have to remember that an You just have to remember that an LDR is made from Cadmium SulphideLDR is made from Cadmium Sulphide

Cadmium is now thought to be Cadmium is now thought to be carcinogeniccarcinogenic


Sorry, that’s wrongSorry, that’s wrong

The cornea is like the eye’s The cornea is like the eye’s windscreenwindscreen

It stops foreign bodies from getting It stops foreign bodies from getting into the eye.into the eye.

It also starts to focus the light It also starts to focus the light towards the lenstowards the lens


Sorry, that’s wrongSorry, that’s wrong

The lens has the job of focusing The lens has the job of focusing incoming light at the back of the eyeincoming light at the back of the eye


That’s incorrectThat’s incorrect

Light and sound shows are a Light and sound shows are a comparatively recent phenomenacomparatively recent phenomena

They came about when Pop Music They came about when Pop Music took off in popularitytook off in popularity


That’s incorrectThat’s incorrect

The laser wasn’t invented until the The laser wasn’t invented until the 1950’s1950’s

It wasn’t until the 1980’s that the It wasn’t until the 1980’s that the lasers began to be used in night lasers began to be used in night clubs and other entertainment clubs and other entertainment venuesvenues



Although gas lights use the change Although gas lights use the change from chemical to light energy, these from chemical to light energy, these also give off heatalso give off heat



The change from electrical energy to The change from electrical energy to light energy also has a by-product: light energy also has a by-product: heat.heat.

As bulbs are becoming more As bulbs are becoming more efficient, this may change at some efficient, this may change at some stage, but we are not there yet.stage, but we are not there yet.


I’m afraid notI’m afraid not

The lens collects the light together The lens collects the light together and focuses it on the back of the eyeand focuses it on the back of the eye

The actual detection is done by the The actual detection is done by the retinaretina


I’m afraid notI’m afraid not

Even though the retina detects shape Even though the retina detects shape and shade, the cones are there to and shade, the cones are there to detect colourdetect colour

The cones detect the three primary The cones detect the three primary colours best: red, green and bluecolours best: red, green and blue


That is not the correct answerThat is not the correct answer

If light travelled at 186,000 miles per If light travelled at 186,000 miles per hour, it would take 20 days for the hour, it would take 20 days for the light from the sun to get to the Earthlight from the sun to get to the Earth

It takes about 8 minutesIt takes about 8 minutes


That is not the correct answerThat is not the correct answer

If light travelled at 186,000 miles per If light travelled at 186,000 miles per hour, it would take 8 hours for the hour, it would take 8 hours for the light from the sun to get to the Earthlight from the sun to get to the Earth

It takes about 8 minutesIt takes about 8 minutes


IncorrectIncorrect

Light does travel into the eye, but Light does travel into the eye, but the question related to how it the question related to how it travelled into the eye and travelled into the eye and everywhere elseeverywhere else

Look at the help and then have Look at the help and then have another goanother go


IncorrectIncorrect

Light splits into single frequencies Light splits into single frequencies when it moves from travelling when it moves from travelling through one material to travelling through one material to travelling through anotherthrough another

This sometimes causes the rainbow This sometimes causes the rainbow effect (or indeed a rainbow when effect (or indeed a rainbow when travelling from air travelling from air water) water)


You have finished the lessonYou have finished the lesson

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Introduction to Light Electro-Magnetic Radiation