L12 waves , diffraction and interference

Question 1

Whats wrong with geometrical optics?

Answer 1

it will prove that it is not good enough to explain some phenomenon of light that takes place in optical system.
-light does not travel in straight lines
-light can interact with itself not just with matter.
As it is simpler , can start to find the location or size of image.
With wave optics you add an extra layer.

Question 2

What is light?

Answer 2

a wave which oscillates up and down with the result that energy travels forward from the source to somewhere else.

Question 3

What is a simple wave representation of light?

Answer 3

• A source of light emitting light
• A wave oscillates up and down travelling in some direction
• can extend this to show light rays travelling in other directions

Question 4

What is the direction of the rays?

Answer 4

the direction of travel of one of the waves

Question 5

What is a wavefront?

Answer 5

its is a collection of points that have been travelling for the same amount of time since being emitted by the source. note that this must mean that all points on a wavefront are such that all waves are at the same point in their cycle
-wavefront intersects at the same point on the cycle of the wave- at peak of oscillation
–the points on the wavefront are all such that the wave is at its maximum point.
-We say that these points are in phase since phase is the term used to indicate which part of the cycle a wave is in, that is the peak of the wave, the trough or somewhere in between.

Question 6

What are wavelengths ?

Answer 6

difference between the different types of waves/colours in the EM spectrum.
to refer to different types of light that are perceived as different colours by the human eye.

Question 7

What are the difference between the light of the different wavelengths?

Answer 7

the key difference between light of different wavelengths is the length of the wave, measured between two successive points which are in phase on a wave (say from one peak of a wave till the next)
- each wave of the EM has a specific wavelength

Question 8

What is different about the different colours of the EM ?

Answer 8

is the wave LENGTH of the wave

e. g red is longer than green.
- also has implications on how much energy is carried

Question 9

define wavelength

Answer 9

distance between 2 successive points that represent the same part of the wave cycle

Question 10

What is amplitude?

Answer 10

height between peak and trough

Question 11

What does the amplitude represent?

Answer 11

represents how much light (energy) is present in the wave and therefore we want a way to distinguish between waves representing different amounts of light (intensity or brightness

Question 12

What does it mean the larger the value of amplitude ? (A)

Answer 12

the more intense/bright the light is

Question 13

What is phase? ø

Answer 13

describes the position of the wave if 2 waves with the same wavelength are displaced with respect too each other.
The location of x can represent 2 different positons of the wave cycle at the same time that are displaced sideways

Question 14

What is the wave equation?

Answer 14

E (light wave) = y (x) A sin (2πx/wavlength +ø(phase))

Question 15

What does the wave equation describe?

Answer 15

to describe the value of e this equation tells us that the value of x a certain point along the path of travel

Question 16

What does the sine function show

Answer 16

an oscillating function

Question 17

What is osculating to create a a water wave?

Answer 17

With water waves, water is oscillating up and down to give a wave

• e,g throw a pebble in pond, you create a wavefront that is expanding outwards
• the wave appears to move forward but it is the water that oscillates up and down

Question 18

What is osculating to create a sound wave?

Answer 18

air molecules oscillate to make a sound wave
air molecules move up and down
sound wave carries energy forward
sound cannot travel in space as it needs air

Question 19

What oscillates to create a Mexican wave?

Answer 19

people who oscillate up and down.

Question 20

What oscillate to create a light wave?

Answer 20

light travels in space
doesn’t need a physical medium
needs an electro magnetic field

Question 21

What creates an electro-magnetic field?

Answer 21

anything that emits light is creating an electric and magnetic field that are coupled together l( in sync with each other), carrying energy which we call light.

Question 22

What is a light wave?

Answer 22

an electromagnetic wave that is visible to the human eye

Question 23

What do the light tubes do in the example of EM wave?- in powerpoint

Answer 23

emitting light that generates an electric and magnetic field that are oscillating perpendicular to each other and direction of travel

Question 24

What happens in an ideal case of wave propagation from a source of light ?

Answer 24

-we have spherical wavefronts diverging outwards expanding and expanding and maintaing its spherical shape

Question 25

What happens when a wavefront hits a block of glass?

Answer 25

• we have a plane wavefront
• we know that a wavefront represents a surface that has been travelling for the same amount of time since being emitted
• As light goes through a medium that is denser it slows down as it distorts the wavefront, light going though the block of glass is being slowed down with respect to the rest of the light.
• whereas the remaining part keeps travelling at the same speed. The effect of this is that the wavefront after the block of glass now has a distorted part due to the block of glass. This therefore represents a deviation from the ideal plane wavefront.

Question 26

How can we show that all the points represent the same point in time? - for block of glass

Answer 26

only way to keep them connected is by showing a distorted wavefront.

Question 27

How does a simple optical system look like in wave optics instead of geometrical optics?

Answer 27

-light diverges from from a. source and the centre of the wavefront will slow down first.
-The edges will still move forward, they will be travelling at a higher speed for a longer amount of time
-Also the thickness of the lens means that through the centre the wavefront travels more time in a denser material.
Hence the wavefront changes orientation and emerges as aa converging wavefront.
-For a negative lens the opposite happens, thinner at centre of lens

Question 28

What happens when the plane wavefront are coming from infinity ? - as a diagram in wave optics

Answer 28

this is equivalent to light rays being parallel to each other
it converges to a point

Question 29

What are the superposition of waves that are converging?

Answer 29

consider to waves that are converging - they are in phase

• peak and peak or trough and trough
• and when they meet they add together and w e get a wave that is larger which means more light

Question 30

What is the superposition of waves when they produce no light at all?

Answer 30

the waves are 180 degrees out of phase
a positive part of the wave matches with the negative part of the wave
-reusltantn is no light at all when added up together

Question 31

What is polarisation ?

Answer 31

the electric field oscillates in only one direction along this transmission axis of the polarising filter to produce polarised light

Question 32

How can unpolarised light be converted to plane polarised light ?

Answer 32

by putting in a polarising filter/ sheet

Question 33

What does a polarising sheet consist ofd?

Answer 33

molecules arranged in columns that are parallel to each other

• the arrangement of these molecules allows the electric field components that are aligned to the molecules of the polariser to go through
• however all the other light with different orientations will be absorbed
• this is how unpolarised light is transformed to polarised light.

Question 34

What does a polarising sheet consist ofd?

Answer 34

molecules arranged in columns that are parallel to each other

• the arrangement of these molecules allows the electric field components that are aligned to the molecules of the polariser to go through
• however all the other light with different orientations will be absorbed
• this is how unpolarised light is transformed to polarised light.

Question 35

What happens when we add a 2nd polarising filter at 90 degrees?

Answer 35

when you have 2 linear polarising filters perpendicular to each other at 90 degrees - the amount of light emerging will be minimal = virtually 0 intesnistiy

Question 36

How can the intensity of. light be controlled in polarisation?

Answer 36

by adjusting the angle between the 2 polarisers

Question 37

What happens when the 2nd polariser is rotated at a different angle ?

Answer 37

-the intensity of the light transmitted can be described by malus’s law.

Question 38

What does malus law explain?

Answer 38

allows to calculate the intensity of light transmitted through a polariser by knowing the initial intensity and the angle between the transmission axis of the polariser.

Question 39

What are applications of polarisation ?

Answer 39

polarised sunglasses - help reduce intensity and glare from reflections by horizontal surfaces

• Poalrised 3D glasses
• Polarised light used in Photo Elastic stress analysis
• Polarised light microscopy- examine biological tissue

Question 40

What is the example for using polarised sunglasses?

Answer 40

for e.g the surface of the sea
-the reflected light off the surface is polarised parallel to the surface
By using vertically polaroid sunglasses we are preventing the plane polarised glare to be transmitted

Question 41

What is polarisation encoding used for?

Answer 41

delivering separate image to the left and right eye in stereoscopic displays for 3d movies

Question 42

What does circular polarisation show ?

Answer 42

each eye receives only the intended image insensitive to head tilts

Question 43

What is interference ?

Answer 43

it will cancel reflection

-Constructive and destructive

Question 44

What is interference for anti reflection coating on spectacle lenses?

Answer 44

-some light will reflect when it arrives at the anti reflection coating and some will reflect at the surface of the glass
-One incident ray has given us 2 reflected rays, one has travelled a long distance
the bottom ray, if thickness is right, will end up with 2 waves that are out of phase peak coincinding with a trough and will cancel eachother out
-waves will cancel out and the observer will not see the relflelction , they meet at observers retina, there will be no retina at all.
-However this will work for one wavelength not all - so not ideal
-need an anti-relective coating that will work for many wavelengths

Question 45

What are applications of interference ?

Answer 45

• uses interference of light to view patients retina in very high detail
• Anti reflective coating glasses
• interference filters

Question 46

How can a interference filter be produced ?

Answer 46

if a thin transparent spacer is placed between 2 semi reflective coating multiple reflections and interference can be used to select an narrow frequency band
-if the spacer is a half wavelength for the desired wavelength, then the other wavelength ill be attenuated by destructive interference