Combining Waves Flashcards
What is a Stationary Wave
A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant
How are stationary waves produced
A stationary wave is produced from the superposition fo 2 progressive waves travelling in opposite directions
What is Interference
The superposition of waves from two coherent sources of waves
What are the conditions for a stationary wave to be produced
- The waves must be of the same frequency, wavelength and amplitude (cohernet)
- They must be travelling in opposite directions
- This normally happens when a wave is reflected back onto itself
What does Coherent mean
When two waves have a fixed phase difference and same frequency
What are the Different Types of Interference
- Constructive interference - when 2 waves have displacement in the same direction
- Destructive interference - when one wave has positive displacement and the other has negative displacement
Describe Destructive and Constructive Interference Diagram
What is Total Destructive Interference
If the waves have equal but opposite displacements so they cancel each other out
Describe Young’s Double Slit Experiment
- Shine a coherent light source through 2 slits about the same size as the wavelength of the laser light so the light diffracts
- Each slit acts as a coherent point source making a pattern of light and dark fringes
- Light fringes are formed where the lights interfere constructively, this occurs where the path difference between waves is a whole number of wavelengths (nλ, where n is an integer)
- Dark fringes are formed where the light interferes destructively, this occurs where the path difference is a whole number and a half wavelengths ((n+½)λ)
Describe Young’s Double Slit Experiment Diagram
Describe what happens using White Light rather than Monochromatic in Young’s Double Slit
Gives wider maxima with a central white fringe with alternating bright fringes (violet is closest to the central maximum and red furthest)
Explain what happens to the amplitude of two wave superposing when the frequency of them decreases
- If the frequency decreases then their wavelengths will decrease
- So the path difference will get closer and closer to one wavelength
- Therefore the ampltiude will get bigger as the waves become more in phase
How would you increase the accurancy of finding the wavelength of Young’s Double Slit Experiment
- Increase the distance from the screen to the slits - when the distance is greater the fringe spacing is greater so the maximas are easier to read off
- Measure across 2 maximas and taking the mean - reduces percentage error
What is the FringeSpacing Equation
w = λD/s
Fringe Space (m) = Wavelength(m) x Distance from slits to screen(m) / Slit Spacing (m)
Describe the Safety Procuations of Using Lasers
- Wear laser safety goggles
- Don’t shine the laser at reflective surfaces
- Never shine the laser at a person
Why do we use a lazer to show iterference and diffraction
Lazers produce monochromatic light so diffraction and interference patterns are more defined
How was the knowledge and understanding of the nature of electromagnetic radiation changed over time
- Young’s double slit experiment showed that light can diffract and have interference
- These are wave properties so proved that electromagnetic radiation must act as a wave (at least some of the time).
- At the time some theories suggested light was formed of tiny particles, however this experiment disproved that theory
Describe the Different Orders Diagram
What is a Node and an Antinode
- Node - a point on a stationary wave where the displacement is 0
- Antinode - a point on a stationary wave with maximum displacement
What is a Harmonic
A mode of vibration that is the multiple of the first harmonic
Describe Three Lowest Harmonics in a Diagram
Describe the Three Lowest Harmonics of a Stationary Wave
- First Harmonic- two nodes at either end and an antinode in the middle, wavelength is 2L
- Second Harmonic - three nodes and two antinodes, wavelength is L
- Third Harmonic - four nodes and three antinodes, wavelength is 2L/ 3
What is the Fundamental Mode of a Stationary Wave
The Lowest Harmonic
What is First Harmonic Equation
f = 1/2 x L x square root T/u
Frequency (Hz) = 1/ 2 x Length (m) x Tension (N) / Mass per unit length (kg m-1)
What is Path difference
The difference in the distance travelled by two waves
What is Diffraction
When waves spread around an obstacle or through a gap
When is there Maximum Diffraction
When the gap is the same as the wavelength
Describe the Single Slit Diffraction Graph
A bright central fringe, which is double the width of all other fringes, with alternating dark and bright fringes on either side
Describe how can you vary the width of the central maximum
- Increasing the slit width - decreases the amount of diffraction so the central maximum becomes narrower and its intensity increases
- Increasing the light wavelength - increases the amount diffraction as the slit is closer in size to the light’s wavelength, so the central maximum becomes wider and its intensity decreases
What is a Diffraction Grating
A slide containing many equally spaced slits
Why would you use a diffraction grating
The interference pattern is much sharper and brighter than it would be in Young’s Double Slit Experiment this is because there are many more rays of light reinforcing the pattern
Describe the Single Slit Diffraction Graph with 50 slits compared to 20 slits
What is the Diffraction Grating Equation
d sin(𝛳) = n λ
Distance between Slits(m) x Sin (angle between 0th and n order maxima) = Order of the Maxima x Wavelength (m)
Explain where the formual dsinθ = nλ is derived from
- For the first order maximum, the path difference between two adjacent rays of light is 1λ and the angle between the normal to the grating and the right ray is θ
- A right angled triangle is formed, with side lengths d and λ, the upper angle is θ
- For the first maxima sinθ = λ/d this rearranges to dsin = λ
- For other maxima the path difference between the two rays of light is nλ, where n is the order of maxima
How would you calculate the distance between the slits using the number of lines per metre
You divide the number of lines per metre by 1 (for example if you know there are 300 lines per metre the distance between the slits would be 1/300 m)
What are the Applications of Diffraction Grating
- Line Absorption Spectra - splits up light from stars and is used to identify elements present in stars
- X-ray Crystallography - crystal sheet acts as diffraction grating the X-ray passes through, used to find the spacing between atoms
