Waves 2 Flashcards
Define path difference
The difference in distance travelled by 2 waves from their sources to the point where they meet
Define what a coherent light source is, giving an example
The waves have a constant phase difference, the same frequency and same wavelength, e.g. monochromatic laser light
Define monochromatic
Light of 1 wavelength i.e. colour
Define diffraction
The spreading out of a wave when it passes through a gap or around an object
What is meant by diffraction occurring around an object
- When light meets an object, diffraction occurs at the edges of the object
- The wider the obstacle compared to the wavelength, the less diffraction
What is the purpose of young’s double slit experiment
To investigate the interference of a coherent light source through 2 slits
Why does the light in young’s double slit experiment need to be coherent
So that there is a clear, consistent interference pattern
What is the alternative if a coherent light source is unavailable
Use a filter to make the light monochromatic. Then use a single slit which causes the light to come from the same point source so the path from the source to to the slits is the constant for both slits, so constant phase difference
How can you maximise diffraction, by adjusting the size of each slit
The slit size should be similar to the wavelength of the light because it the slit is too small, light will be reflected instead and if too big, light will pass straight through without being diffracted
Explain how bright and dark fringes are formed
- When the light meets in phase, when the path difference is nλ, constructive interference occurs, then bright fringes (maxima) form
- When the light meets out of phase, when the path difference is (n+1/2)λ, destructive interference occurs, then dark fringes (minima) form
Describe the interference pattern from young’s double slit experiment
Equally spaced bright and dark fringes. The intensity decreases slowly further from the central maxima but the maxima still have similar intensity
Describe the formula showing the relationship between fringe spacing, distance to the screen and slit separation
W=λD/s
(Slit sep measured from the centre of each slit)
What is the effect of using white light (e.g. a bulb) instead of monochromatic laser light in Young’s double slit experiment
- Wider, less intense diffraction pattern
- Every maxima is the same width
- The central maxima is white
- The other maxima are spectrums where red diffracts the most (on the outside of the spectrum) and blue diffracts the least (inside of the spectrum)
Describe the safety precautions needed when using lasers
- Safety goggles
- Don’t shine at reflective surfaces
- Don’t shine directly in eyes
- Display a warning sign outside the room
Describe how we can see the interference pattern of sound waves instead of light
Use 2 loudspeakers connected to the same signal generator instead of a double slit
Use a microphone to measure intensity at different points (to find minima and maxima)
How does constructive and destructive interference of sound waves occur
- superposition of 2 points in phase (2 compression points or 2 rarefaction points) leads to constructive interference
- superposition of 2 points completely out of phase (rarefaction and compression) leads to destructive interference
Explain how young’s double slit experiment provides evidence for the wave nature of light
Diffraction and interference are wave-like properties. This experiment therefore showed that light has both wave and particle properties, not just particle
Describe the interference pattern created during the single slit diffraction experiment
- Wide bright central maxima
- Alternating bright and dark fringes either side with half the width
- The intensity of the maxima decreases as you move further away from the central maxima
Describe the diffraction pattern caused by white light being diffracted through a single slit
- Central maxima is the same intensity and width as with monochromatic light, but it is now white
- Non-central maxima are spectrums with red on outside and violet on inside. They are also wider than with monochromatic as red diffracts a lot and blue doesn’t diffract much so the range of wavelengths is higher so the range of the maxima is higher.
Explain how slit width and wavelength affect the central maxima qualitatively (without w=λD/s)
- Increasing slit width decreases diffraction so the central maxima is narrower and more intense
- Increasing the wavelength increases diffraction as the slit is closer in size to the wavelength. Therefore the central maxima is wider and less intense
Explain why a diffraction grating may be used instead of a double slit
A diffraction grating is a glass slide with many equally spaced slits close together
Therefore more light gets through, causing more light to reinforce the diffraction pattern, making it sharper and brighter. (the general pattern is the same)
What is the diffraction grating equation
dsinθ=nλ
where d is spacing between adjacent slits, θ is the angle between the normal and the order of a maxima, n is the order number of that maxima and λ is the wavelength of the source
How would you calculate the maximum number of orders
- put sinθ = 1 (max value)
- rearrange to get n=d/λ
- n is the maximum order number
- to get maximum number of orders, x2 +1
Make sure you know the derivation of the diffraction grating pattern. (too much info to write on a flashcard)
Go to pmt or savemyexams to read over it
What are 2 uses of diffraction gratings
- Splitting the light in stars to analyse the wavelength of the light
- X - ray crystallography and atomic spacing - a crystal sheet (ie a diffraction grating) is used to measure the wavelength of x-rays which is similar to the spacing between atoms
Define the refractive index of a material
A measure of how much light changes speed when passing into that material
How would you calculate the n of a material by knowing the speed of light in that material
n=speed of light in a vacuum/ speed of light in that material
Describe what would happen to light when it moves to a material with a higher refractive index
- The light slows down
- The light bends towards the normal (refraction)
- This means the new medium has a higher optical density
- The wavelength decreases
Describe Snell’s law
n1sinθ1=n2sinθ2
Define the critical angle
The angle of incidence which causes an angle of refraction of 90 degrees
What is the formula for the critical angle
SinC=n2/n1
What are the conditions for TIR
- Angle of incidence is greater than the critical angle
- Light is moving from a more to less optically dense medium
This causes all of the light to be reflected t the boundary
What is an optical fibre
Narrow glass tubes which carry information in the form of light, using TIR
What are the parts of an optical fibre
Optically dense core, surrounded by cladding of lower density.
What is the purpose of the cladding
- It has a lower optical density than the core, allowing TIR to occur
- Protects the core from damage
- Prevents light escaping the core, which would cause a loss of information ie signal degradation
What is pulse absorption
When the signals energy is absorbed by the fibre, causing signal degradation
What is pulse broadening
When the received signal is broader than the original signal, causing overlap and loss of information
The 2 types are modal and material dispersion
What is modal dispersion
When light enters the fibre at different angles, causing multiple paths of the light. This causes the light to take different amounts of time to travel along the fibre, causing overlap and pulse broadening
What is material dispersion
When light of different wavelengths are travelling through the core, they are travelling at different speeds, causing overlap and pulse broadening
How can modal dispersion be solved
Using a narrower core so there are less possible paths
How can material dispersion be solved
Using monochromatic light
