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Interference (AQA A Level Physics): Flashcards

Exam code: 7408

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  • Define superposition.

    Superposition is the effect that occurs when two or more waves overlap at a point; the resultant displacement equals the sum of the displacements of the individual waves.

  • Define coherence.

    Two waves are coherent if they have the same frequency and a constant phase difference.

  • Define path difference.

    Path difference is the difference in distance travelled by two waves from their sources to the point where they meet.

  • What is the condition on path difference for constructive interference?

    Path difference = n\lambda, where n = 0, 1, 2, 3... (an integer number of whole wavelengths)

  • What is the condition on path difference for destructive interference?

    Path difference = \left(n + \frac{1}{2}\right)\lambda (an integer number of whole wavelengths plus a half wavelength)

  • When two waves are in phase, their combined effect is called .......... interference, and the resultant wave has double the amplitude.

    When two waves are in phase, their combined effect is called constructive interference, and the resultant wave has double the amplitude.

  • What is the difference between path difference and phase difference?

    Path difference compares the progress made by waves along a path; phase difference compares the distance between the phases (peaks and troughs) of coherent waves travelling parallel to each other at a point.

  • True or False?

    Path difference is the distance between the two paths taken by the waves.

    False.

    Path difference is the difference in the length of the two paths travelled, not the distance between them.

  • Define monochromatic.

    Monochromatic light has a single, constant wavelength.

  • State two properties of laser light that make it ideal for demonstrating interference and diffraction patterns.

    • Coherent (constant phase difference and frequency)

    • Monochromatic (same wavelength)

  • What type of light is produced by non-coherent sources, such as a filament bulb or sodium lamp?

    White light.

  • State three safety precautions to follow when using a laser.

    • Never look directly at a laser or its reflection

    • Do not shine the laser towards a person

    • Do not allow the beam to reflect from shiny surfaces into someone's eyes

    • Wear laser safety goggles

    • Place a 'laser on' warning light outside the room

    • Stand behind the laser

  • What is heard when two coherent sound waves undergo constructive interference?

    The compressions and rarefactions of each wave line up, so the sound appears louder.

  • What happens when two coherent sound waves undergo destructive interference, and what technology makes use of this effect?

    A compression from one wave lines up with a rarefaction from the other, so the waves cancel and zero sound is heard; this is the effect used in noise-cancelling headphones.

  • The intensity of a wave is proportional to the square of its ...........

    The intensity of a wave is proportional to the square of its amplitude.

  • True or False?

    The intensity of a wave is directly proportional to its amplitude.

    False.

    Intensity is proportional to the square of the amplitude: I \propto A^{2}

  • Define the order, n, of a maximum or minimum.

    The order, n, represents the position of a maximum or minimum away from the central maximum; n = 0 is the central maximum, n = 1 is the first maximum on either side, and so on.

  • Describe the setup of Young's double-slit experiment before the light reaches the double slit.

    The laser light source is placed behind a single slit, which diffracts the light to produce two coherent sources at slits A and B.

  • What produces the bright fringes (maxima) and dark fringes (minima) in Young's double-slit interference pattern?

    Bright fringes (maxima) are formed by constructive interference; dark fringes (minima) are formed by destructive interference.

  • State the fringe spacing equation and define each term.

    w = \frac{\lambda D}{s}

    • w = fringe spacing (m)

    • λ = wavelength of light (m)

    • D = distance between the slits and the screen (m)

    • s = separation of the slits (m)

  • State three ways the fringe spacing, w, can be increased.

    • Increase the wavelength, λ

    • Increase the distance, D, between the screen and the slits

    • Decrease the separation, s, between the slits

  • In the white-light double-slit interference pattern, the central maximum is .......... because every wavelength interferes constructively there.

    In the white-light double-slit interference pattern, the central maximum is white because every wavelength interferes constructively there.

  • Which colour appears nearest to the central maximum in the white-light double-slit pattern, and why?

    Violet/blue, because it is diffracted the least; red appears furthest away because it is diffracted the most.

  • True or False?

    In the white-light double-slit interference pattern, every bright fringe is white.

    False.

    Only the central maximum is white; all other maxima are made up of a spectrum of colours.

  • What did Newton (1672) propose about the nature of light, and what could this theory not explain?

    Newton proposed that light is a stream of microscopic particles called corpuscles; this could not explain interference or diffraction effects.

  • Define Huygens' wavelets.

    Points on a wavefront that each act as a source of secondary waves, spreading out and travelling at the same speed as the source wave (Huygens' Wave Theory of Light, 1678).

  • What did Young's double-slit experiment (1801) prove about the nature of light?

    That light is a wave, since it can undergo constructive and destructive interference.

  • What did Maxwell (1862) show about electric and magnetic fields, and what did this reveal about light?

    Maxwell showed that electric and magnetic fields obey the wave equation, meaning light is a wave made up of perpendicular electric and magnetic fields; Maxwell and Hertz later discovered the full electromagnetic spectrum.

  • Define a photon.

    A photon is a packet of energy used by Einstein (1905) to describe light behaving as a particle, as demonstrated by the photoelectric effect.

  • Einstein's explanation of the photoelectric effect showed that light can behave as both a wave and a particle; this is known as ...........

    Einstein's explanation of the photoelectric effect showed that light can behave as both a wave and a particle; this is known as wave-particle duality.

  • True or False?

    Newton's corpuscular theory of light could successfully explain interference and diffraction.

    False.

    Newton's corpuscular theory could not explain interference or diffraction; this is why the wave theory of light was adopted instead.

  • In the Young's double-slit required practical, state the independent, dependent and control variables.

    • Independent variable: distance between the slits and the screen, D

    • Dependent variable: fringe width, w

    • Control variables: wavelength of laser light, λ, and slit separation, s

  • In the diffraction grating required practical, state the independent, dependent and control variables.

    • Independent variable: distance between maxima, h

    • Dependent variable: angle between the normal and each order, θn

    • Control variables: grating-to-screen distance D, laser wavelength λ, and slit separation d

  • State the diffraction grating equation and define each term.

    n\lambda = d\sin\theta

    • n = order of the diffraction pattern

    • λ = wavelength of the laser light (m)

    • d = separation between the slits (m)

    • θ = angle between the normal and the maxima

  • How is the slit separation, d, of a diffraction grating calculated from the number of slits per metre, N?

    d = \frac{1}{N}

  • The resolution of a metre ruler is .........., and the resolution of Vernier callipers is ...........

    The resolution of a metre ruler is ±1 mm, and the resolution of Vernier callipers is ±0.01 mm.

  • State three ways to reduce random error when measuring fringe width, w, or maxima separation, h, in these practicals.

    • Take multiple measurements (three to eight) and calculate the mean

    • Use a Vernier scale to reduce percentage uncertainty

    • Measure across all visible fringes/maxima and divide by the number of them

    • Increase D to increase the fringe/maxima separation

    • Conduct the experiment in a darkened room so the fringes are clear

  • Why does using a diffraction grating with more lines per millimetre improve the experiment?

    It produces greater values of h (distance between maxima), which lowers the percentage uncertainty in the measurement.

  • True or False?

    In the diffraction grating practical, the angle θ is small enough to use the approximation sin θ ≈ θ.

    False.

    The angle is not small, so θ must be calculated using trigonometry from the measured values of h and D.

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