interference

Question 1

at what point results in interference occurring?

Answer 1

interference occurs when the waves overlap.
their resultant displacement is the sum of the displacement of the individual waves

Question 2

principle derivation

Answer 2

the principle of superposition and the resultant waves may be smaller or larger compared to either of the individual waves

Question 3

if waves are neither in phase nor antiphase what happens?

Answer 3

the resultant amplitude is between the two extremes

Question 4

qualities required for waves to be coherent

Answer 4

the waves are coherent if they have the same amplitude, wavelength and have a constant phase difference

Question 5

why is coherence important for waves?

Answer 5

to produce an observable, or hearing pattern for interference

Question 6

define monochromatic

Answer 6

singular wavelength

Question 7

why is laser light useful for interference?

Answer 7

it is monochromatic and coherent, unlike filament lamps due to them producing incoherent light waves

Question 8

happens when coherent sound waves are in phase

Answer 8

get louder due to constructive interference

Question 9

define path difference

Answer 9

The difference in distance travelled by two waves from their sources to the point where they meet

Question 10

how is path difference expressed?

Answer 10

as wavelengths

Question 11

how can interference and diffraction of lasers be demonstrated?

Answer 11

using slits or diffraction gratings
e.g. light rays require laser light shining through two slits and an interference pattern can be observed

Question 12

explain how you can use two-source interference to see light

Answer 12

use two separate coherent light sources or shine laser through the two slits
the slits should be the exact same size as the wavelength of the laser light so it is diffracted. the light coming from the therefore slits acts similarly to two coherent sources
you end up getting a pattern of light and dark fringes

Question 13

the types of interference as a result

Answer 13

constructive interference is shown as bright fringes on the screen, path difference is a whole number integer (n lambda). the highest intensity is found in the middle as the central maximum.
destructive interference is shown as darker fringes on the screen zero intensity, therefore the path difference is a whole number and half of wavelength (n+1/2)lambda.

Question 14

why can lasers often be dangerous?

Answer 14

produce really high energy beams of light and the intense beam causes permanent eye therefore damage or even results in blindness

Question 15

explain how shining a laser in someones eye can result in the eye damaging therefore

Answer 15

The laser light is focused in a very direct powerful beam of monochromatic lights. if you looked at the laser beams directly, your eyes lenses would focus it on your retina.

Question 16

in schools which laser number is allowed?

Answer 16

laser - class 2
its power output is essentially less compared to 1mW however lasers having an overall greater power (powerful) can reach outputs greater compared to about 500mW
are class 4 lasers and are so powerful it makes a person become instantly blind and additionally damage skin so.

Question 17

precautions taken when handling lasers

Answer 17

It is important to use the lasers safely and follow the guidelines:
- Do not look directly at the laser or its reflection.
- Do not shine the laser towards a person.
- Do not allow the beams of laser to reflect from shiny surfaces and into another person’s eye.
- Laser safety goggles
- Placing on display to inform others (warning sign therefore)
- Standing behind laser.
- Turning laser off if it is not being used so.

Question 18

why is it simple to demonstrate two-source interference for sound or water?

Answer 18

their wavelengths are of a handy size that can be measured

Question 19

how can two source interference be used perfectly?

Answer 19

using coherent source meaning the wavelength and frequency should be same. The trick is to use the exact same oscillator to drive both sources.
For water, single vibrator can therefore drive about two dippers. for sound, an oscillator is connected to two so loudspeakers.

Question 20

sound and two source interference use

Answer 20

equipment; two speakers
- sound waves are longitudinal waves so therefore consist of compressions and rarefactions.
- constructive interference occurs whenever the compressions therefore lines up together along w a rarefaction and the sound appears louder
- destructive interference occurs whenever the compressions therefore lines up with a rarefaction and the sound is therefore quieter and vicer versa.
- the waves interfere therefore causing areas of constructive and destructive interference
- the lines of maximum displacement can occur when all the peaks and troughs are lining up w those on another wave so
the technology used in noise-cancelling headphones

Question 21

the use of noise cancelling headphones

Answer 21

a noise cancellation speaker emits a sound wave with the so exact same amplitude but has inverted phase (antiphase) relative to the original sound.
the waves undergo destructive interference where the waves displacements combine hence forming the wave with no amplitude.

Question 22

the use of microwaves to detect two source interference

Answer 22

• equipment; microwave detector
  if you move the probe along the path of the metal plates it means you will get an alternating pattern of strong/weak signals therefore and is similar to light/dark fringes on screen.
• constructive interference: regions resulting in the detector picking up the maximum amplitude
• destructive interference: regions resulting in the detector picking up no signal overall.

Question 23

calculation

Answer 23

1: calculate wavelength
v=fxl
Number 2: Write the condition required for destructive interference.
paths difference = (n+1/2)lambda
Parts 3: calculate the smallest of the so path difference

Question 24

however what is necessary for two source interference fringes to be observed?

Answer 24

coherent and monochromatic sources therefore
whenever the two waves interfere the resultant wave depends on the phase difference between the two waves. it is proportional to the overall defined path difference between the two waves and can be written in terms of the wavelength of waves therefore

Question 25

how is constructive interference impacted by it?

Answer 25

For constructive interference (maximas) the difference in wavelengths are an integer number of the whole wavelengths therefore (nlambda)

Question 26

how is destructive interference impacted by it?

Answer 26

However for destructive interference or the minimas, the difference in wavelengths is an integer number plus half of wavelength (n+1/2)lambda
- n is order of the maximas/etc pattern since there is usually about more one of the patterns produced by interference

Question 27

briefly explain the demonstration of Young’s double slit so

Answer 27

• Young’s double slit experiment demonstrates how light waves can produce an interference pattern
• The setup of the experiment is shown below:
• A monochromatic light source is placed behind a single slit resulting in the light being diffracted producing light sources present at the therefore double slits A and B.
• Since the both light source originate from the same primary source (single slit), theyre coherent and therefore create an observable interference pattern.
• It is both of the diffracted light coming from the double slits creating an interference pattern made up of both bright and darker fringes due to constructive + destructive interference.

Question 28

the equation used to calculate fringe spacing

Answer 28

W = lambda x D / s (distance between centre of slits so)
W = fringe width (the distance between successive bright fringes
D = distance between the screen and slits therefore

Question 29

explain the interference pattern for constructive interference

Answer 29

shown as bright fringes w varying intensity and has the greatest intensity in the middle therefore

Question 30

explain the interference pattern for destructive interference

Answer 30

darker fringes therefore no light observed

Question 31

why is the distance between fringes small somewhat?

Answer 31

due to the short wavelength of visible light and a monochromatic light source makes it easier to observe

Question 32

a laser is placed in front of a double slit as shown in the diagram below
the laser emits light of frequeny 750Hz. The separation of the maximas P and Q observed on the screen is 15mm. the distance between the therefore double slit and the screen is therefore 4.5m.
calculate the separation of the so two slits instead

Answer 32

1 Calculate the wavelength of the light
v = f x lambda
2 REARRANGE FOR LAMBDA SUBSTITUTE IN VALUES
v = f x lambda
lambda = v / f
= 5 x10^8 / 750 x 10 ^12
= 4 x 10^-7m
= 400nm
3 THE SO FRINGE SPACING EQUATION
W = lambda D / S
s = lambda D / W
s = 4 x 10^-7 x 4.5 / (15 x 10^-3) / 9
s = 1.08 x 10^-7mm
s = 1.1mm (2.s.f)
s = 1.1 x 10^3m

Question 33

explain what happens if monochromatic light is replaced by a white light source

Answer 33

• interference pattern observed is slightly different.
• white light is composed of all the colour of visible light, so therefore each wavelength of white light produces its own interference pattern.
• the central fringe is white because at that position the path difference for all wavelengths present is zero, therefore all wavelengths arrive in phase so.
  the central fringe is the exact same colour as the source ie. white
• however the first maximum occurs whenever the path difference is lambda
• since the blue light has a shorter wavelength compared to the red light, the paths difference is smaller, so the blues maximum appears closer to the centre
• eachs colour produces a maximum in a slightly different position and so spread out into spectrum of colours

Question 34

explain what Newton proposed as a theory supporting of electromagnetic radiation

Answer 34

Newton proposed that visible light is a stream of microscopic particles called corpuscles
However, the corpuscles could not explain interference or diffraction effects, therefore, the views of light as a wave was adopted instead

Question 35

explain Huygen’s proposition

Answer 35

Huygens comes up with the original Wave Theory of Light to explain the phenomena of diffraction and refraction
This theory describes light as a series of wavefronts on which every point is a source of waves that spread out and travel at the same speed as the source waves
These are known as therefore beingHuygens’ wavelets

Question 36

explain Young’s proposition

Answer 36

Devised double-slit experiment the famous phenomenon
This provided experimental proof that light is a wave that can undergo constructive and destructive interference

Question 37

explain Maxwell’s proposition

Answer 37

Maxwell showed that electric and magnetic fields obeyed the wave equation. This means that light was simply waves made up of electric and magnetic fields travelling perpendicular to one another
Later, Maxwell and Hertz discovered the full electromagnetic spectrum

Question 38

explain what Einstein proposed as a theory supporting of electromagnetic radiation

Answer 38

Einstein discovered that light behaves as a particle, as demonstrated by the photoelectric effect
He described light in terms of packets of energy called photons
Later the scientific community came to understand that light behaves both like a waves and a particle
This is known as waves-particle duality