12. Superposition Flashcards
explain what is meant by coherent waves
they have a constant phase difference between waves
[1. same type 2.same wavelength 3. same frequency]
explain how interference fringes are formed (4 points)
- waves from a double slit are COHERENT
- waves from each slit OVERLAP/superpose (NOT interfere)
- maximum/bright fringe where PATH difference is nλ
- waves reach PHASE difference of n360° / 2π n rad
- maximum/bright fringe where PATH difference is nλ
- minimum/dark fringe is where PATH difference is (n+1/2) λ
- waves reach PHASE difference of (2n+1) 180° / (2n+1) π rad
- minimum/dark fringe is where PATH difference is (n+1/2) λ
state conditions required for maxima to be formed in an interference pattern produced by 2 sources of microwaves
- waves overlap/meet/superpose
- coherence/constant phase difference (NOT constant λ or freq)
- path difference = 0, λ, 2λ
phase difference = 0, 2π, 4π - same direction of polarisation or unpolarised
describe and explain high intensity regions due to 2 source interference
path difference = nλ
phase difference = 0 or waves reach in phase
constructive interference, hence it is an intensity maximum
intensity of light incident on the double slit is now increased without altering its frequency.
compare the appearance of fringes after this change with their appearance before this change
no change to fringe separation/ fringe width/ number of fringes
bright fringes are brighter
dark fringes are unchanged
CONSTRAST between fringes increases
the intensity of light passing through 2 slits was initially the same
the intensity of light through ONE of the slits is now reduced
compare the appearance of fringes before and after the change of intensity
same fringe separation
bright fringes less bright
dark fringes brighter
CONTRAST between fringes decreases
explain why pattern of fringes on the screen is seen over a limited area on the screen
pattern seen is due to diffraction (at each slit)
for large amount of diffraction, wavelength is about slit width
slit width is much bigger than wavelength, so very little diffraction
monochromatic light is incident on a diffraction grating
describe the diffraction of light waves as they pass through the grating
as wave passes through the slit, it spreads into a geometric shadow
light of single wavelength is incident on diffraction grating.
explain the part played by interference in the production of the first order maximum by diffraction grating
[define] interference: overlapping of waves (from coherent sources at each element
path difference λ
waves reach with phase difference of 360° / 2π rad
produces first order
state 2 differences between first order spectrum and second order
- lines are further apart in second order
2. lines fainter in second order
suggest why fringe pattern produced by light passing through a diffraction grating is BRIGHTER than that produced from the same source with a double slit
more slits for light to pass through
state the conditions required for formation of a stationary wave (2)
- two waves of SAME TYPE travelling in opposite directions overlap
- waves have same speed, frequency, wavelength
state the principle of superposition
when 2 or more waves overlap at a point, the resultant displacement is the sum of individual displacements of each wave at that point
using the principle of superposition, explain the formation of a stationary wave
- 2 waves of the same type travelling in opposite directions overlap/meet
- same speed, frequency, wavelength
- resultant displacement is sum of displacements from each wave
- produces nodes and antinodes
explain how a stationary wave is formed on a string attached to a wall
- wave moves along the string and reflects at the FIXED boundary/point/end/wall
- incident and reflected waves interfere/superpose
- waves same speed, freq, wavelength
incident sinusoidal sound wave of single frequency travels up a tube
explain how a stationary wave is formed from the incident sound wave
- incident sound wave reflects at top/end of tube
- incident wave and reflected waves interfere/superpose
- waves same speed, freq, wavelength
microwave detector is placed between source of microwaves S and metal reflector R
describe how stationary waves are formed between R and S
- microwave from sound S reflects at reflector R
- reflected and incident waves superpose
- waves same speed, freq, wavelength
state the features of a stationary wave that distinguishes it from a progressive wave (4)
- no energy transfer
- amplitude varies along its length
- nodes and antinodes
- neighbouring points within inter-nodal loop vibrate in phase
the wave on the string is a stationary wave
explain, by reference to the formation of a stationary wave, what is meant by the speed of wave on the string
wave is reflected at the end
incident and reflected waves travelling in opposite directions interfere/superpose
speed is speed of incident and reflected waves
Explain how heaps of powder are formed in the loud speaker placed near open pipe
progressive sound waves produced by the speaker reflects at the piston, overlaps with incoming wave
2 waves are of the same type, same speed, same frequency, forming a stationary wave
piles of powder gather at nodes as air particles remain stationary
A linear polarizer is placed in front of each slit (of a double slit), with individual axes of polarisation at right angles to each other
screen is illuminated with no observable pattern
Note: intensity is not zero! light can still pass through
State and explain changes to observed diffraction pattern when visible light of longer intensity is used in a single slit
sinθ=λ/b
λ increases, θ larger for angular positions of the first minima
When white light is incident on a single slit, the central fringe is coloured at the edges and has a central white region.
Explain this observation
white light spectrum consists of wavelength spanning 400-700nm
sinθ=λ/b, amt of spreading depends on λ of incident light
longer wavelengths at the red end spreads more compared to shorter, hence edges are red
at central region, different wavelengths overlap/ interfere to produce white
state what happens to intensity of light when width of single slit is doubled
double wave energy pass through
(sinθ=λ/b) since b is small, θ∝1/b
½ θ when bx2, so half the area
twice amount of energy over half the area, Intensity x4
