Waves Model Answer Flashcards
How is stationary wave formed?
progressive waves travel from centre to ends and reflect
two progressive waves travel in opposite directions across the string
waves have same frequency
waves have same amplitude
superposition
why do stationary waves have large amplitude at centre (when just one loop)?
waves travelling in opposite directions
waves in phase
interfere constructively
displacement varies from negative to positive
what is meant by the critical angle?
the angle of incidence at the more dense-less dense boundary producing an angle of refraction of 90 degrees
OR minimum angle for TIR to occur
define angle of incidence
angle between incoming ray and normal to the boundary
define angle of refraction
angle between outgoing ray and normal to boundary
Why is glass cladding normally used?
- protect core
- prevent leakage of light
- prevent cross-talk
explain what the term critical angle means for a transparent material in air
Angle to normal in material
Which results in 90 degree refraction in air
Describe double slit white light diffraction pattern
Central white fringe
Fringes either side showing range of colours/spectrum
With red furthest and blue/violet being closest to centre
Double slit with white light. Filter that transmits red light of 1.2lambda and green light of lambda placed in front of light source. Describe interference pattern.
central fringe is mix of red and green light
spacing of red fringes is 20% greater than spacing of green fringes
6th green fringe overlaps with 5th red fringe
student trying to determine wavelength of red light using double slit set up. Student changes the slit separation s and the distance from screen to slit D independently. Discuss the effects that each independent change has on the interference pattern, and whether this change is likely to reduce uncertainty in the wavelength.
The following statements may be present for decreasing slit separation s:
Fringe separation increases
Uncertainty in measuring fringe separation will decrease
and as this is needed to measure wavelength,
uncertainty in wavelength measurement will decrease
The following statements may be present for smaller D:
Uncertainty in measuring D would increase
Fringe separation would also decrease
so uncertainty in measuring fringe separation would increase
Both are required to find wavelength so uncertainty in finding wavelength would increase
FOR Middle Band one of these considered:
Decrease s
Larger fringe separation so smaller (%) uncertainty (in w)
Smaller s so higher (%) uncertainty in s
Decrease D
Smaller fringe separation so larger (%) uncertainty (in w)
Smaller D so higher (%) uncertainty in D
What happens to theta of first order fringe when you use smaller wavelength and why?
theta decreases
path difference gets smaller OR
d constant, lambda smaller so theta smaller
Explain the formation of fringes seen on the screen (when monochromatic red light is directed at double slit, parallel red and dark fringes can be seen).
Maxima/bright fringes are seen when rays from one slit arrive in phase, with a phase difference of 0 radians and a path difference of nlambda wavelengths with rays from the second slit. This causes constructive interference.
Minima/dark fringes are seen when rays from one slit arrive out of phase, have a phase difference of pi radians and a path difference of (n+1/2)lambda with rays from the second slit, causing destructive interference.
Applications of polarisation
TV signals transmittted as polarised radio waves. Aerials are aligned with electric field component
Satellite TV- one horizontally polarised and one vertically polarised channel on same frequency
Polaroid sunglasses- cut out glare
Principle of superposition
When two waves meet the displacement at that point is equal to the sum of the individual displacements at that point
Characteristic of double slit diffraction pattern
Equally spaced parallel light and dark fringes
