interference Flashcards
at what point results in interference occurring?
interference occurs when the waves overlap.
their resultant displacement is the sum of the displacement of the individual waves
principle derivation
the principle of superposition and the resultant waves may be smaller or larger compared to either of the individual waves
if waves are neither in phase nor antiphase what happens?
the resultant amplitude is between the two extremes
qualities required for waves to be coherent
the waves are coherent if they have the same amplitude, wavelength and have a constant phase difference
why is coherence important for waves?
to produce an observable, or hearing pattern for interference
define monochromatic
singular wavelength
why is laser light useful for interference?
it is monochromatic and coherent, unlike filament lamps due to them producing incoherent light waves
happens when coherent sound waves are in phase
get louder due to constructive interference
define path difference
The difference in distance travelled by two waves from their sources to the point where they meet
how is path difference expressed?
as wavelengths
how can interference and diffraction of lasers be demonstrated?
using slits or diffraction gratings
e.g. light rays require laser light shining through two slits and an interference pattern can be observed
explain how you can use two-source interference to see light
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
the types of interference as a result
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.
why can lasers often be dangerous?
produce really high energy beams of light and the intense beam causes permanent eye therefore damage or even results in blindness
explain how shining a laser in someones eye can result in the eye damaging therefore
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.
in schools which laser number is allowed?
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.
precautions taken when handling lasers
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.
why is it simple to demonstrate two-source interference for sound or water?
their wavelengths are of a handy size that can be measured
how can two source interference be used perfectly?
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.
sound and two source interference use
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
the use of noise cancelling headphones
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.
the use of microwaves to detect two source interference
- 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.
calculation
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
however what is necessary for two source interference fringes to be observed?
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
how is constructive interference impacted by it?
For constructive interference (maximas) the difference in wavelengths are an integer number of the whole wavelengths therefore (nlambda)
how is destructive interference impacted by it?
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
briefly explain the demonstration of Young’s double slit so
- 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.
the equation used to calculate fringe spacing
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
explain the interference pattern for constructive interference
shown as bright fringes w varying intensity and has the greatest intensity in the middle therefore
explain the interference pattern for destructive interference
darker fringes therefore no light observed
why is the distance between fringes small somewhat?
due to the short wavelength of visible light and a monochromatic light source makes it easier to observe
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
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
explain what happens if monochromatic light is replaced by a white light source
- 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
explain what Newton proposed as a theory supporting of electromagnetic radiation
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
explain Huygen’s proposition
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
explain Young’s proposition
Devised double-slit experiment the famous phenomenon
This provided experimental proof that light is a wave that can undergo constructive and destructive interference
explain Maxwell’s proposition
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
explain what Einstein proposed as a theory supporting of electromagnetic radiation
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
