Wave behaviour - light

Question 1

How are colours on an oil film created?

Answer 1

Thin film interference

Question 2

Briefly describe how thin film interference works

Answer 2

Light falls on the film and some is reflected from the top boundary while some travels into the film to be reflected from the bottom boundary. The light which has gone through the film traveled slower in this medium so is delayed, causing a phase difference. When both rays emerge this can cause constructive or destructive interference for different wavelengths, creating colours.

Question 3

What can happen at the boundaries in thin film interference?

Answer 3

Phase change (ie the waves are inverted)

Question 4

What are Newton’s rings?

Answer 4

When two pieces of glass are put together there is a very thin layer of air between them which acts like a thin film. An interference pattern can be seen and on curved surfaces they look like contour lines - Newton’s rings

Question 5

What can Newton’s rings be used for?

Answer 5

Glass-making

If the interference pattern is stable then the surface is smooth

Question 6

What happens in thin film interference with monochromatic light?

Answer 6

Dark and light fringes (no colour)

Question 7

Why are pearls iridescent?

Answer 7

They have lots of thin layers of different thicknesses, causing thin film interference

Question 8

What are anti-reflection coatings?

Answer 8

They are used in cameras to let light enter by stopping it all from being reflected. They are made with a certain thickness to cause destructive interference in a specific colour/wavelength of light, preventing reflection of that colour

Question 9

What were Ole Romer’s observations about light?

Answer 9

He noticed that the moons of Jupiter would fall behind for six months and then catch up in the next six. Earth and Jupiter’s orbits take each other further and further away for half the year. If light took a finite amount of time to reach the earth then that would explain this. He did not know the diameter of Earth’c orbit but he knew it had to be about 22 minutes of light time

Question 10

What did Isaac Newton think about light?

Answer 10

Wave and particle ideas of light were mixed together. He talked about wavelengths and phases but thought light could work by particles setting up vibrations in matter.
he also thought light traveled faster in glass to try and explain refraction.

Question 11

What were Huygens thoughts on light?

Answer 11

• Light does no hinder other light so it is a motion which can pass through other motions (like wave superposition)
• An longitudinal wave passing through small hard particles (since proven incorrect)

Question 12

What is the theory of Huygen’s wavelets?

Answer 12

• Light spreads out spherically from a source, like ripples
• Each point on a wavefront acts as a new source of wavelets
• Wavefront are formed where wavelets are in phase and so interfere constructively

Question 13

What is a wavefront?

Answer 13

Formed where wavelets superimpose constructuvely ( a plane can be created). Everywhere else wavelets are out of phase and interfere destructively, making zero

Question 14

What are wavelets?

Answer 14

Small, sperical waves (like ripples) which spread out from different points on a light source

Question 15

What rules did Huygen’s light theories explain?

Answer 15

• angle of incidence = angle of reflection
• sini/sinr = refractive index
• light travels more slowly in glass and water than in air

Question 16

What is path difference?

Answer 16

The extra distance a second wave has to travel to reach the same point as the first wave

Question 17

Why can rays emerging from one slit in one direction be considered parallel?

Answer 17

The angle between them is so small compared to the distance of several metres they travel to reach the screen.

Question 18

What is d in a double slit experiment?

Answer 18

The distance between the slits

Question 19

What is L in a double slit experiment?

Answer 19

The distance from slits to screen

Question 20

What is x in a double slit experiment?

Answer 20

The distance from fringe to centre fringe

Question 21

What is the formula for path difference in a double slit experiment?

Answer 21

dsinӨ OR d(x/L)

Question 22

What is the formula for distance between fringes in a double slit experiment?

Answer 22

Δx=(λL)/d

Question 23

What is sinӨ equivalent to in a double slit experiment?

Answer 23

x/L

Question 24

What is the formula for distance from the centre fringe to the nth fringe in a double slit experiment?

Answer 24

x = (nλL)/d

Question 25

What are the condition for constructive interference in a double slit experiment?

Answer 25

The path difference is a multiple of one wavelength

dsinӨ = nλ

Question 26

What are the condition for destructive interference in a double slit experiment?

Answer 26

The path difference is a multiple of half a wavelength (must be an odd number)
dsinӨ = (n +1/2)λ

Question 27

What is diffraction grating?

Answer 27

A grid of multiple slits which light is diffracted through

follows the same set of rules as double slit

Question 28

What affect does adding more slits have on the diffraction pattern?

Answer 28

The fringes become further apart

Question 29

How are diffraction patterns on a screen created with a single slit?

Answer 29

Every part of the aperture is imagined to send out wavelets. Where these are received (the screen) the wavelets are added up using their phasors. Where they add up constructively there is a bright fringe and where they add up destructively is a dark fringe

Question 30

What happens directly in front of the aperture in single slit diffraction?

Answer 30

All the wavelets travel in parallel and so have travelled equally far. They are all in phase so their phasors add up to make a very large amplitude and an intense bright fringe in the centre

Question 31

What happens at an angle to the aperture in single slit diffraction?

Answer 31

As you go further and further to the side, wavelets travel increasingly different distances. This causes lag and so phasors point in different directions. When they are added they could make zero or a smaller positive amplitude than the central bright fringe.

Question 32

How is a dark fringe created in single slit diffraction?

Answer 32

Phasors of the wavelengths add up to a complete circle when added tip to tail, making zero resultant. This is a path difference of one wavelength.

Question 33

What are the conditions for constructive and destructive interference in single slit diffraction experiments?

Answer 33

destructive = path difference a multiple of one wavelength
dsinθ = nλ
constructive = half way between destructive

Question 34

What is the central fringe called?

Answer 34

Zero order fringe

Question 35

What is d in a single slit diffraction experiment?

Answer 35

The width of the slit

Question 36

What is L in a single slit diffraction experiment?

Answer 36

The distance from slit to screen

Question 37

What is W in a single slit diffraction experiment?

Answer 37

The beam width (how wide the diffraction pattern is from horizontal) in radians

Question 38

What is the formula for beam angle in radians?

Answer 38

λ/d

Question 39

What happens in diffraction through a circular aperture?

Answer 39

Light is diffracted in two dimensions

Question 40

What is the equation for diffraction through a circular aperture?

Answer 40

θ = 1.22 λ/d 
(θ = angle of the first MINIMA in radians
d = diameter of aperture)

Question 41

What is the minimum angular distance apart at which instruments can resolve objects?
How can this problem be solved?

Answer 41

θ=1.22 λ/d

Two telescopes can be placed as far away as possible on Earth and then used as if they were a single instrument

Question 42

What is the Rayleigh criterion?

Answer 42

The criterion for minimum resolvable detail
Diffraction is limited when the first minimum of the image of one source coincides with the maximum of another image source