The Light Fantastic

What is the speed of light?

a) 1, 000, 000 Kilometres per second

b) 300, 000 Kilometres per second

c) 299,792.458 Kilometres per second

d) 657,456,345 Kilometres per second

How long does it take for light from the sun to reach the Earth?

a) 10.5 minutes

b) 10.5 seconds

c) 8.5 minutes

d) 8.5 seconds

Hint: Sunlight Travels 150 million Kilometres to get to earth!

Light….

• Is a form of energy

• Travels in straight lines

• Travels at 299,792.458 Kilometres a second

• Sunlight Travels 150 million Kilometres to get to earth

• This takes 8.5 Minutes!

Light Sources

Make a List of different ways that we can produce light?

Star Light

• It takes 4.3 years for light from Alpha Centauri to reach earth

• So you are seeing light that is 4.3 years old• If Alpha Centauri was to blow up right now

you wouldn’t see the effect for 4.3 years.

Science in Action

• Herald Sun 5/3/08

• A Star approx 8000 light years away might explode

• If it did it could destroy earth!

• We have plenty of time to prepare!

Luminous or Non-Luminous?• Luminous: Emit their own light

• Non-Luminous: Reflect Light

• Most objects we see are non-luminous

• If an object gives off light as well as heat it is called an Incandescent object.

Bioluminescent

• Bioluminescent refers to a living organism that is able to emit light.

• The mouse on the right has had the gene for a bioluminescent protein cloned into it.

• Can you think of any other bioluminescent organisms?

Light and Objects

• Light can be reflected or refracted depending on the object it strikes.

• Some objects are Transparent and allow light to pass through them.

• Translucent objects allow light to pass through but will scatter light.

• Opaque objects do not allow light to pass through.

Translucent Concrete Transparent Toaster

Shadows

• Light Travels in a straight line• If something gets in its path the light is blocked• When this happens a shadow is formed

Sundial

• Uses the movement of the sun in the sky and the formation of shadows to tell time

• Depending on where the sun is, shadows are cast on the appropriate hour line, thus displaying the "time."

Reflection

Reflection

• When light strikes an object some of it is absorbed and some bounces off.

• This is called Reflection

• Reflection creates the images you see in mirrors

Law of reflection

• Reflected light allows us to see an object

• Law of reflection:Angle of incidence= Angle of reflection

• This is measured from the normal (90o)

Refraction

Which way does it bend?

• Transparent and Translucent substances allow light to pass through them.

• Examples:– Water, air, glass, Perspex, Diamond

• However when Light passes through different substances it alters its speed.

The changing speed of light

• Light travels fastest in a vacuum– Slower in gases

• Slower in Liquids– Slowest in Solids

Solids Liquids Gases Vacuum

SLOW FAST

Refractive Index

• The speed of light as it travels through a substance depends on its Optical Density or Refractive Index

Refraction

Car Refraction Light Refraction

Activity: Cup & Coin

• Do this in pairs• Place a coin or small

metallic object in a plastic cup

• One of you will walk backwards from the cup till you can just see the coin

• Take one more step back

• Now get your partner to fill the cup ¾ of the way with water

Refraction

• Used to describe the bending of light

• When light passes from one medium to another.

• We use this principle to build telescopes and magnifying glasses

Total Internal Refraction

Total Internal Reflection

• When a light travels from a optically dense substance to a less dense one sometime it can reflect instead of refract.

• This is known as Total Internal Reflection

Real Internal Reflection

• If the Incidence Ray > The critical angle, Total Internal Reflection occurs.

Using Refraction

Refraction Illusions• Light from the end of the

ruler bends away from the normal

• What you see is not the actual location of the ruler but the image your brain constructs based on where the light is coming from.

Mirages

A mirage in the Egyptian Desert. What do you think is happening here?

Mirages explained• Cool and warm air

have different optical densities.

• Light therefore bends as it passes from one to the other

• If the variation in density is gradual total internal reflection can occur.

Refraction: Spear Fishing

• Where would the hunter have to aim his spear to Fish?

• Why?

Activity: Bent spoon

• Different liquids with different consistencies will bend light differently

• Using the different liquids provided predict and observe what happens to the spoon or pencil when you place it in various liquids

Light wave

Traverse Waves• Light Travels as a Transverse Wave

• We know this because of the way light behaves

Activity: Creating Waves

• You can create a replica of a transverse wave using a piece of rope or chain.

The Visible spectrum

The Visible spectrum

Spectral Hues

• Another way to look at the spectrum is too look at the different wavelengths of Spectral Hues.

• Spectral Hues (Colours) differ in their wave properties.

• This is because they refract at different wavelengths

White Light

• Light coming from the Sun and most light globes is referred to as ‘white’ light

• But white light can be spilt into different colours

• This is called dispersion

• We will talk more about colour later

Activity: The colour of Light

• Focus the light into a narrow beam• Place the Prism in the direct path of the

beam • Rotate the beam until you get a rainbow

pattern

This might look familiar

Rainbows

• A beam of white light (in this case the sun) has spilt into the colours of the visible spectrum.

• The effect is called dispersion

• Small droplets of water acting like tiny prisms.

Two types of Rainbow Primary Rainbow Secondary Rainbow

Normal Internal Refraction Two Internal RefractionsColours are reversed

Activity: Remember that spectrum

• We need to remember the colours of the rainbow and also the visible spectrum.

• One way to remember it is ROY G BIV• Or Red, Orange, Yellow, Green, Blue, Indigo

and Violet. • Now we need to come up with a phrase for

ROY G BIV (as seen above)

Colour

White Light

• Red + Green + Blue = White

• Secondary Colours:– Red + Green = Yellow – Green + Blue =

Turquoise– Red + Blue = Magenta

Reflection of Colour

White light is made of all the other wavelengths of colour

When White light hits an object the colour you see is the colour that is reflected back

The object will absorb all other wavelengths of colour

Activity: Object Light test

• Place a few light boxes close together

• Place a single primary colour slide in the light box

• Combine the primary light colours to create secondary colours

Colour Addition

How the TV works

• If you look closely at a TV you might notice lots of tiny spots of Red, Blue and Green

• These are made from special chemicals called phosphors

Addition on TV

• Phosphors glow when they are hit by beams of electrons

• Your eyes add together the spots to form pictures

Addition of Lights

• Colours can also be added together using coloured lights.

• White light is produced when all the primary colours are added together.

• If only two primary colours are added secondary colours of Cyan, magenta and yellow are produced

Colour Subtraction

Filter Subtraction• A coloured filter such

as a piece of cellophane subtracts colour from the light passing through it.

• The filter absorbs all the other colour light.

Seeing Colours

• When you look at something you see the light reflected back to you

• All other colours have been absorbed .

• Black objects absorb all light while White objects reflect is all !

Mixing Pigments

• Pigments are small particles found in substances like paint or ink.

• Mixtures of pigments behave differently to light!

Chameleons

• Chameleons have the ability to change colour.

• They do this in response to temperate, stress or to blend in.

• They do this with special pigment cells, called chromatophores

Printing• The secondary colours

of light (yellow, cyan and magenta) .

• The are used because each subtracts a primary colours.

• Black is used in contrast and shading

• This is known as CYMK (K standing for Black)

Primary colours of pigments

• Cyan, magenta and yellow are sometimes referred to the primary colours of pigment.

• This is again useful in order to print a full range of colours

Colour separation files

• Computer image programs can create separate files for each printing colour (CYMK)

• These are called Colour Separation files

Mirror Mirror

Plane Mirrors

• Plane mirrors let you see an exact image of yourself

• This image is called a virtual image

• Light does not actually come from the image

• The law of reflection explains how you see virtual images

Convex mirrors

• Bulge outwards at the centre • Used in shops (like above), but are also

in rear-vision mirrors in cars to you give a wider view of what is behind you

Concave mirrors….

• Are sunken in the middle• close up, they produce magnified virtual

images• If an object is far way they produce an

inverted real image

Lenses

Types of Lenses

• Two most common types of lenses are: – Convex lenses which are fatter in the

middle. – Concave lenses which are thinner in the

middle

Terms for lenses

• Principal Axis: A line drawn directly through the centre of the lens.

Principal Axis

• Principal Focus: The point where light that enters in parallel to the principal axis is focused.

Principal Axis

Principal Focus

Light Ray

• Focal Length: The distance from the centre of the lens to the principal focus (F)

Principal Axis

Principal Focus

Centre

Focal Length

Lens shape and focal length

• A weak (thin) lens will create a long focal length

• A strong (fat) lens will create a short focal length

Image formation

Two types of images

Real Images

• Formed when light rays cross.

• Can be captured on film• There is Light energy in a

real image

Virtual Images• Formed when light rays • Can’t be captured on film• There is no Light energy

in the image

Activity: Images and lenses

• We can draw diagrams to explain how images have formed

• These are called ray tracing diagrams. • Now we will draw some diagrams of

images we create.

Images and convex

Convex Lense

• A convex lens will produce: – A real upside-down image whenever

the object is closer to the lens then the focus.

– No image when the object is placed exactly on the focus

– A virtual image whenever the object is further away from the lens then its focus

Images and concave lenses

• A concave lens will always produce a virtual image

Terms for describing images• Real : light passes through image• Virtual : Light does not pass through the

image. • Upright: Same way as original object• Inverted: opposite way to original object • Enlarged: bigger than original object• Diminished: smaller than orginal object

Calculating Magnification

• An image can be larger or smaller then the object it originated from.

• Its magnification can be calculated by dividing the size of the image by the size of the original object.

• Original Object = 2 cm• Image size = 10cm • 10/2 = 5 • Then the images magnification is 5• If the object was 8 cm tall and the image 4 cm

then the magnification would be 0.5.

Polarisation

What is Polarisation ?

What you might see

Traverse Waves• Light Travels as a Transverse

Wave

Polarisation

How Polarising Lenses work

How the Eye Works

The Human Eye

Rods and Cones

• The retina contains two types of photoreceptors, rods and cones • The rods are more numerousand are more sensitive than the

cones • However cones allow you to see color• There are three types of cones, Red sensitive, Blue sensitive and

Green

Colour Blindness• The term “Colour blind” misconception is that people who are colour blind

only see in black in white. • This is in fact extremely rare and is called Monochromasy • The other Misconception is that they can’t tell the difference between Red

& Greens.• Most however suffer from varying degrees in deficiency from the extreme

of Monochromasy to Normal Vision • Statistically at least one of the guys in classroom may suffer from some

level of colour deficiency. • This is because colour blindness is more common in men (7 – 10%) than

women (.4%).