Light and SoundLight and Sound

In this unit:

1) Properties of light2) Reflection3) Refraction4) Colours

What is Light?What is Light?

• A form of ENERGY!

Scientific Definition:• A stream of photons that are massless

particles each travelling with wavelike properties at the speed of light. A photon is the smallest quantity of energy which can be transported.

Where does light come Where does light come from?from?

• The sun gives off light due to nuclear fission

• Other methods will be discussed later….

The methods of energy transfer:1. Conduction requires contact2. Convection requires a medium (like air)3. Radiation a method of energy transfer

that does not need a medium and travels at the speed of light

Properties of LightProperties of Light

Light travels in straight lines:

Light does not need a medium to travel

Laser

Light Travels as a waveLight Travels as a wave

What is wavelength?What is Amplitude?What is frequency?

Light travels VERY FAST – around 300,000 kilometres per second.

At this speed it can go around the world 8 times in one second.

Light travels much faster than sound. For example:

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.

Electromagnetic Electromagnetic SpectrumSpectrum

What is white light? How do we see colours?

It turns out that visible light is just a small part of a spectrum called Electromagnetic Radiation

The Wave Model of LightThe Wave Model of Light

White light is not a single colour; it is made up of a mixture of the seven colours 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.

Newton was the first to note that white Newton was the first to note that white light breaks into the colours of a rainbow light breaks into the colours of a rainbow when it passes through a prism. when it passes through a prism.

These colours are called a These colours are called a SpectrumSpectrum. .

When light from the Sun (White Light) is When light from the Sun (White Light) is broken down, itbroken down, it’’s Spectrum looks like this.s Spectrum looks like this.

•White Light is made up of shades of Red, Orange, Yellow, Green, Blue, and Violet. •A mnemonic for remembering the order of rainbow colours is the name Roy G BV.Roy G BV.

Newton noted that the individual coloured Newton noted that the individual coloured lines could not be separated further. lines could not be separated further.

These lines are pure colours and are These lines are pure colours and are identified by a identified by a frequency and wavelength frequency and wavelength numbernumber. .

The Conceptual definition of light is; Light is Light is produced when produced when electrons change electrons change energy levels.energy levels.

Our ears cannot hear some high sound Our ears cannot hear some high sound frequencies that dogs can. frequencies that dogs can.

Similarly, our eyes cannot Similarly, our eyes cannot ““seesee”” some of the some of the colours of light.colours of light.

For instance, you cannot see if an iron is For instance, you cannot see if an iron is hot or not. Our eyes are not sensitive to hot or not. Our eyes are not sensitive to the the ““colourcolour”” the iron emits but our hands the iron emits but our hands can feel it. can feel it.

The The ““colourcolour”” hot irons emit is called hot irons emit is called Infra Infra Red Radiation.Red Radiation.Some Some snakes can snakes can see Infra see Infra Red Red Radiation.Radiation.

The Visible Light Spectrum is a very The Visible Light Spectrum is a very small part of a much larger spectrum small part of a much larger spectrum called the Electromagnetic Spectrum.called the Electromagnetic Spectrum.

How do we see?How do we see?

We do not see light that does not enter our eyes.

How do we see?How do we see?

We don’t see light directed away from our eyes unless it is reflected into our Eye-Brain by something.

How do we see things?How do we see things?

We see things because they reflect light into our eyes:

Homework

Can you see the Eye-BrainCan you see the Eye-Brain’’s feet?s feet?

Since no ray of light can go straight from Since no ray of light can go straight from the Eye-Brain’s feet into your eyes, you the Eye-Brain’s feet into your eyes, you can’t see them.can’t see them.

Experience has trained our Eye-Brain to expect that things are located in the direction that the light comes from.

Luminous and non-luminous objects

A luminous object is one that produces light.

A non-luminous object is one that reflects light.

Luminous objects Reflectors

Atoms in Atoms in Luminous ObjectsLuminous Objects emit light rays in emit light rays in all directions produced from other energy all directions produced from other energy sources.sources.

Rays of light travel from the object to the Rays of light travel from the object to the observerobserver’’s eye through a Medium. A s eye through a Medium. A TransparentTransparent MediumMedium allows nearly all the allows nearly all the rays to pass straight through unaltered. Air rays to pass straight through unaltered. Air is a transparent medium. is a transparent medium.

An An Opaque MediumOpaque Medium absorbs or scatters all the absorbs or scatters all the rays. A text book is an opaque medium. rays. A text book is an opaque medium.

A A Translucent MediumTranslucent Medium transmits and transmits and scatters the rays. The medium indicates scatters the rays. The medium indicates whether it is being illuminated but the whether it is being illuminated but the object cannot be clearly seen. Wax paper is object cannot be clearly seen. Wax paper is a translucent medium. a translucent medium.

Light and MatterLight and Matter

TransparentTransparent• Transmit light freely - absorb and reflect very little light.• Eg. Clear glass, clear plastic, window

TranslucentTranslucent• Transmit some light but not enough to see clearly• Eg. Frosted window/glass

OpagueOpague• Objects that absorb and reflect light but do not transmit

it

Shadows

Shadows are places where light is “blocked”:

Rays of light

Properties of Light Properties of Light summarysummary

1) Light travels in straight lines2) Light travels much faster than sound3) We see things because they reflect

light into our eyes4) Shadows are formed when light is

blocked by an object

Part 2 - ReflectionPart 2 - ReflectionReflection 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 Angle of incidence = Angle of reflectionreflection

In other words, light gets reflected from a surface at ____ _____ angle it hits it.

The same !

!!

Clear vs. Diffuse ReflectionClear vs. Diffuse Reflection

Smooth, shiny surfaces have a clear reflection:

Rough, dull surfaces have a diffuse reflection.

Diffuse reflection is when light is scattered in different directions

Using mirrorsUsing mirrorsTwo examples:

1) A periscope

2) A car headlight

ColourColour

White light is not a single colour; it is made up of a mixture of the seven colours 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.

The colours of the rainbow:The colours of the rainbow:

RedOrangeYellowGreenBlue

IndigoViolet

Adding coloursAdding coloursWhite light can be split up to make separate

colours. These colours can be added together again.

The primary colours 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 colourSeeing colourThe colour an object appears depends on the

colours of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is

reflected

A white hat would reflect all seven colours:

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

Using coloured lightUsing coloured light

If we look at a coloured object in coloured light we see something different. For example, consider a football kit:

White

light

Shorts look blue

Shirt looks red

In different colours of light this kit would look different:

Red

lightShirt looks red

Shorts look black

Blue

light

Shirt looks black

Shorts look blue

Some further examples:

Object Colour of lightColour object seems to be

Red socks

Red Red

Blue Black

Green Black

Blue teddy

Red Black

Blue

Green

Green camel

Red

Blue

Green

Magenta book

Red

Blue

Green

Using filtersUsing filtersFilters can be used to “block” out different colours of

light:

Red Filte

r

Magenta

Filter

Investigating filtersInvestigating filters

Colour of filter Colours that could be “seen”

Red

Green

Blue

Cyan

Magenta

Yellow

Red

Magenta

White

Yellow

Blue Green

Cyan

RefractionRefraction

Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pen is placed in water it looks like this:

In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______.

Words – speed up, water, air, bent