Jan 02, 2016
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
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