11.3 reflection refraction diffraction and polarisation

Question 1

When does reflection occur in waves ?

What important to remmeber
What doesn’t change

Answer 1

Reflection occurs when a wave changes direction at a boundary between two different media, BUT REMAINS IN SAME MEDIUM

Also wavelength frequency don’t change

Question 2

Angle of reflection

Answer 2

Law of reflection = angle of incidence = angle of reflection

Draw normal and it’s between the normal and incident ray, NOT THE MEDIUM

Question 3

Wave front basics

Answer 3

“Peak” of Esch wave represented,

• all in phase
• distance between waves is equal to wavelength

Question 4

Refraction - what is it

What always happens when a wave refracts

Why does it happen
What does it mean for wavelength

?

Answer 4

Refraction occurs when a wave changes direction due to changing speed when it passes from one medium to another

• whenever a wave refracts, some of it will have PARTISLLY RELFECTED FIRST
  2) one medium to other changes speed, meaning wavelentgh will change and thus it moves. If it speeds = away , slows = towards

Question 5

Important mediums to know speed differences for sound, water and em

So how will refraction affect water from deep to shallow in wavefronts?

Answer 5

) deep water is faster than shallow , sound is faster in denser too. Other then thst EM WAVES slows in denser

When a wave slows down, wavelength decrease

2) here the wavelength will decrease as speed decreases too

Question 6

Why does resolution get affected at certain magnifications for light microscopes ?

(bio)

Answer 6

• at huge magnifications the image gets blurry due to SPREADING of light = diffraction
• this because light gets diffracted as it passes through apertures in the microscope
• cannot be avoided, so microscopes that dont need light were made

Question 7

what is diffraction

anything change during diffraction ?

Answer 7

Property where when waves pass through a gap or travel around an obstacle, they SPREAD out.

• All waves can be diffracted, SPEED WL or FREQUENCY DONT CHANGE when diffraction occurs

Question 8

What makes a wave diffract more?

Answer 8

how much a wave diffracts depends on the wavelength relative to the gap/ obstacle + narrower the gap the better

• sound can diffract around a door because wavelength of sound is about the same as the gap- meaning you can hear around the corner
• but light cant , need very small gap so diffraction is more.

Question 9

what is Polarisation ?
which type of progressive wave can do it?
In terms of EM waves what does it refer to

Answer 9

polarisation is to make particles oscillate in one direction only, so thst the wave is confined to single planes
X for EM, it’s the plane of E WAVE

• thus property of transverse waves : longtidunal waves cant be
  polarised as they already in same plane as energy transfer

Question 10

what is the plane of polarisation of an electromagnetic wave?

Answer 10

EM waves are polarised in the Y AXIS, so the plane in which electric fields vibrate in.

Question 11

how can waves be polarised then? (come back to this)

Answer 11

2
= through absorbtion (to do with polaroid filters)
= or reflection (glare and sunglasses), here partially polarisation happens, like off water, most becomes horizontally alligned

?

Question 12

how can a polaroid filter polarise waves?

->what is important about directions of railings

Answer 12

-Filament lamp output unpolarised light planes
- For a filter that polarises it in the Y axis, the alinement of railings MUST BE HORIZONTAL.
->
this because a vertical wave hitting vertical (or horizontal hitting horizontal) WILL BE ABSORBED, as the waves make electrons delocalised go up and down CREATING A PD, meaning it is absorbed and now the wave wont go through

• however if vertical wave hit horizontal aligned, then there is not eniough space for delocalised electrons to move and create a PD, thus wave CSN GO THROUGH

Question 13

But what does the allignment of a polaroid filter mean even considering previous knowledge about orientation of the railings?

Answer 13

if a polaroid filter is vertically alligned, it will allow vertical through, or polarise in the VERTICAL filter.

light is then polarised only in that plane

here the railings are still horizontal tho

Question 14

in a polaroid filter, does it only allow if its perfectly Alligned??

what happens to intensity from unpolarised in ALL directions to just one !

Answer 14

NO

• BUT THE COMPONENTS in the same plane as its ALLIGNMENT OF ALL THE WAVES
• therefore some of the waves will be polarised but with less intensity

inetnsity drops by half

Question 15

what determines the intensity when a wave gets polarised? (how much gets transmitted?)(check with sir)

Answer 15

Inetnsity is PROPORTIONAL TO cos ^2 (angle)

As intensity is proportional to amplitude squared

Question 16

what happens if you put two polarising filters (check with sir)

Answer 16

Say first wave is polarised to go straight, if second is at angle, then the straight gets split into components that make it allign,
- the component that doesnt allign is filtered and THE one does is let through…

Question 17

REFLECTION POLARISATION

what happens when transverse waves reflect off a surface?
- how does this vary for the refracted bit and reflected bit

• what happens for light for example and what do sunglasses do to counter (say if you at beach)

Answer 17

When waves reflect off surface they become PARTIALLY POLARISED

• > this means some waves will be POLARISED but others not (so some in refracted section scattered , but most in reflected same direction)
• for example light is mostly HORIZONTALLY polarised when reflecting off metal and water
  
  • > thus sunglasses will have vertically alligned polaroid filters so that this light doesnt reach eye.

Question 18

Remember the opposite direction filter is just for the mets, filters

Answer 18

Remember

Question 19

recap what are the conditions for light to pass through a filter

What condition means none gets filtered?

Answer 19

• 1) MUST HAVE A COMPONENT in the same plane in which that filter is aligned
• > does not mean incident light has to be unpolarised, can be polarised and even tho it doesnt line up straight away it can then split into further components which do. however each time this happens intensity drops
  2) plane of polarisation of incident light and filter CANNOT BE PERPENDICULAR BASICALLY,
• > because then it cant be split into components that can go through, not possible, according to Cartesan coordinates…

Question 20

What are all the intensity rules then?

Answer 20

Intensity proportional to amplitude ^2
Intensity proportional to cos x ^2
Intensity = p/a = p/4pi r 2, means intensity is proportional to 1/ distance 2 too

Question 21

Again how does polarisation work from reflection and why sunglasses work?

Answer 21

When refraction occurs you ALWAYS get some reflection
- this results into partial polarisation in refraction and reflection planes
So
- sunglasses will have vertically alligned filters that inky let vertical light through, intensity drips yes but the horizontally partial polarisation from reflection if surfaces like water etc will not interfere now

Question 22

What need to remember about laser light polarisation

Answer 22

Laser light is only one direction, meaning power is not spread out so intensity is constant .

This is POLARIsed light, light from filament is UNPOLARISED

Question 23

Again for intensity / amplitude what is malus law?

Answer 23

I= initial I x cos x ^2

A= A initial x cos x