Light & Sound

We study light and sound together because they are both forms of energy and they have some physical properties in common.

But what exactly are those properties?

Let’s have a look….

•Both light and sound travel to us as waves. The waves transmit the energy to make the light we can see and the sound we can hear.

•When we modify the kinds of waves that make light, we see different colours.

•When we modify sound waves we make different kinds of sounds such as louder or softer sounds or ones with a higher or lower pitch.What do the light and sound waves look like? Are they

the same as waves that are formed in the water?

Light & Sound Waves

Waves

Wave Length

Amplitude

WavesWave Length

Wave length Wave length

What else can I learn about waves?

Amplitude

Peak

Trough

Light & Sound Waves

Light & Sound Waves

•Even though the types of waves are different, they have some properties that are similar.

•Some of those properties are how the waves travel and how they react when they hit a surface.

•Let’s have a look at those properties now...

Light & SoundConsider this straight line, it could represent a mirror or a wall…..•Now what happens if we shine a light, roll a ball, send a

water wave or a sound wave into it……... where will the light or the ball go or the sound go?

But what do I call all

this?

Angle of incidence

Angle of reflection

Normal

I see. But does the angle of incidence

always equal the angle of reflection?

Plane

Cool. That must be the law of reflection.

MMmmm it appears that the incidence ray and the reflected

ray always travel in straight lines and the angle of

incidence always equals the angle of reflection.

Incidence rays

Reflected rays

Normal

Light & Sound

According to my calculation the law of

reflection should also exist if the

surface of reflection is curved instead of plane. Let’s see….

Concave surface

Focal length

Focal pointYes, but what

is it all called?

Light & Sound

Light

MMMmmm that reminds me of how a lens works…..like the ones in my

groovy glasses.

Not exactly my friend. My calculations show that light bends when it travels through lenses like the ones in your groovy glasses.

Cool! That must be the law of refraction (not to be confused with the law of

reflection).

Light

Water or glass

Air

Angle of incidence

Angle of refraction

MMmmm so the angle of refraction appears to be smaller than the angle of

incidence because it bends towards the

normal. But how does that relate to my groovy

glasses??

Normal

Light

And what do you call those two different

types of lenses?

Well my calculations state that

converging lenses refract

the light together (like

in the top lens) and diverging lenses refract

the light outwards.

Converging/convex

Diverging/concave

Light

Hi! I’m Mr. BIV and I want to know when are we going

to do the prism trick?

In a minute ROY, let me just tell you about my last set of

calculations…it is amazing. Did you know that light travels at about 300 million meters per

second? Through glass it travels at about 1.9 million meters per

second so it really does slow down quite a bit.

LightListen Pete, all this talk

about light has made me wonder? What actually is it and where does it come

from?

Don’t know man, I’ll call Dave he knows

everything.

Well Pete I am not sure. I know light

comes from the sun but that’s all. Let me

phone a friend. I’ll call John.

Look Dave I have found some information you

need. It is all very interesting indeed.

Hey get me outta here, I’m too hip

for this slide show!!

LightMy calculations are completely wrong this time, but I did read in a book somewhere

that light from the sun is a type of electromagnetic radiation and belongs to

something called the electromagnetic spectrum. On this spectrum there are

various kinds of electromagnetic waves with different wave lengths. These waves are known as Transverse waves. Look at

this…

Cosmic rays X-rays Visible light Microwaves TV

Gamma rays Ultraviolet Infra-red Radar Radio

Short wavelength (carry more energy)

Long wavelength

Light

Visible light

Now are we going to do the prism

trick?

Okay, cool Roy.G.Biv. Here it is just as Sir

Isaac Newton

discovered in 1672.

That means that white

light contains all the

colours of the spectrum!!

It looks like a

rainbow

When sunlight passes through a raindrop it bends or refracts and splits into colours

Have you ever seen a rainbow without rain?

Light

All those colours are very lovely darling but why do we see them at all and not

just white light?

According to my calculations all colours hit different surfaces yet only some colours are reflected. We see that reflected colour or wavelength. The colours we don’t see are absorbed.

Light

If white light contains all colours, what do you see if there is no colour

reflected

That’s a tricky one my dear, but the answer is easy to remember as it is the opposite of

white…which is… black, like the colour of your groovy suit.

If I look at this leaf through a red filter, all the colour is absorbed and the leaf appears black. No colour is reflected. Cool!!

Light

We can mix colours 2 ways. If we mixed coloured light we

are adding the colours together so this is called

addition. If you shine red, blue and green light together you

can make white light.

If we mix coloured dyes or paints, we make a mixture that absorbs all the colours, this is called subtraction. If

I put enough different coloured paint on this

canvas I will end up with black.

LightMmm, that’s all very

good, but what about light that

doesn’t come from the sun?Good question. Well anything that gives off its own light is called luminous. If it emits light because it is hot it is said to be incandescent. If it doesn’t give off

heat when it emits light it is said to be fluorescent. How about some examples:

These are all incandescent other than the moon that reflects light to earth rather than produce it.

Fluorescence can be seen in paint, the hand of watches, fire flies, deep sea creatures and glow

worms.

Sound

Sound waves have some qualities different to light:

•Sound waves are made as vibrations in objects around us.

•The waves need to travel from place to place in solids, liquids or gases and will not be transmitted in a vacuum.

•Sound generally travels faster through a solid than a liquid, and faster through a liquid than a gas!

But how does sound work?

Sound

Take these trumpeters for example. They blow into the trumpet causing a vibration

along the hollow of the trumpet. The vibration escapes out the end of the trumpet and continues to vibrate in the air until it reaches our ear. Our ears are designed to pick up

the vibration and convert it to a sound message our brain can

understand.

Sound

MMmmmm that means when we talk, our voices send

vibrations across the air to another person.

Cool!

According to my calculations, sound waves travel much slower than light waves. In

fact sound waves only travel at 330 meters per second in air, that is why we see the

lightning before we hear the thunder!

Sound

One wavelength

Sound waves are called compression (or longitudinal waves). You can think of sound waves like a big slinky!

Compression

Rarefaction

Sound

What I want to know is how

music is made, how does this guitar work?

You may have noticed the strings on a guitar are thick and thin. The thin string gives a much higher pitched note because it vibrates quicker. A

string will also vibrate quicker if it is pulled tight. Combining all the

different pitched sounds the strings can make gives the guitar its sound.

The same with a woman’s voice…her vocal cords are shorter and vibrate quicker giving her a higher pitched

voice than a man.

Sound

But why do we have two ears?

According to my calculations, we need two ears to be able

to know which direction sound is coming from…each ear compares the incoming

sound and shares the information.

Light & Sound Waves

Okay so let’s summarise the properties of light waves and sound waves so I can get this straight

in my head...

Sound waves

Light waves

•Are compression (or longitudinal) waves

•Need a medium (liquid, solid or gas) to travel in, won’t travel in a vacuum

•Travel in air at speeds of about 330 m/s (depends on temperature)

•Are transverse waves

•Unable to travel through some substances, but can travel through a vacuum

•Travel in air at speeds of about 300,000,000 m/s

See you in the lab!

Mmm I think that is all for

now.

Yes my calculations state we have run out

of time this lesson

Bye. Can we do the

prism thing

again?

Cool!