What path difference gives constructive interference for two in-phase sources?

A whole number of wavelengths: nλ.

What path difference gives destructive interference for two in-phase sources?

An odd number of half wavelengths: (n + ½)λ.

What does coherent mean?

Same frequency and a constant phase difference.

Why must the sources be coherent to see a stable interference pattern?

Otherwise the phase difference changes randomly, so the positions of maxima and minima keep moving.

How is coherence achieved in Young's experiment?

One source illuminates both slits, so the light from the slits has a constant phase difference.

State the double-slit fringe spacing formula.

w = λD/s, where s is the slit separation and D the distance from slits to screen.

A student increases the screen distance D. What happens to the fringes?

The fringe spacing increases in proportion to D.

What is the pattern with white light in a double-slit set-up?

A white central fringe with coloured spectra either side, violet nearest the centre and red furthest.

Why is the red end of each white-light spectrum furthest from the centre?

Red has the longest wavelength, and w ∝ λ.

State two laser safety precautions.

Never look along the beam or at reflections; use a low-power laser; end the beam at a screen; display a warning sign.

Why is a laser used in a double-slit experiment?

It gives intense, monochromatic, coherent light, so a clear fringe pattern forms.

What did Young's experiment show about light?

That light behaves as a wave, since it produces interference fringes.

Which later observation showed light also behaves as particles?

The photoelectric effect, explained by photons.

Interference and Young's double-slit experimentAQA A-Level Physics: Flashcards

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Define path difference.

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Define path difference.: The difference in the distances travelled by two waves from their sources to the point.
What path difference gives constructive interference for two in-phase sources?: A whole number of wavelengths: nλ.
What path difference gives destructive interference for two in-phase sources?: An odd number of half wavelengths: (n + ½)λ.
What does coherent mean?: Same frequency and a constant phase difference.
Why must the sources be coherent to see a stable interference pattern?: Otherwise the phase difference changes randomly, so the positions of maxima and minima keep moving.
How is coherence achieved in Young's experiment?: One source illuminates both slits, so the light from the slits has a constant phase difference.
State the double-slit fringe spacing formula.: w = λD/s, where s is the slit separation and D the distance from slits to screen.
A student increases the screen distance D. What happens to the fringes?: The fringe spacing increases in proportion to D.
What is the pattern with white light in a double-slit set-up?: A white central fringe with coloured spectra either side, violet nearest the centre and red furthest.
Why is the red end of each white-light spectrum furthest from the centre?: Red has the longest wavelength, and w ∝ λ.
State two laser safety precautions.: Never look along the beam or at reflections; use a low-power laser; end the beam at a screen; display a warning sign.
Why is a laser used in a double-slit experiment?: It gives intense, monochromatic, coherent light, so a clear fringe pattern forms.
What did Young's experiment show about light?: That light behaves as a wave, since it produces interference fringes.
Which later observation showed light also behaves as particles?: The photoelectric effect, explained by photons.

Exam questions on Interference and Young's double-slit experiment

Two loudspeakers are connected to the same signal generator, so they emit sound of the same wavelength, 0.50 m, in phase with each other. A listener stands at a point P that is 4.00 m from one loudspeaker and 4.75 m from the other.
The two loudspeakers are now connected to two separate signal generators, each set to emit nominally the same frequency. Explain why the listener no longer hears a stable pattern of loud and quiet positions.2 marks
A student shines light from a laser of wavelength 650 nm at a pair of narrow parallel slits whose centres are 0.40 mm apart. A fringe pattern forms on a screen 2.5 m from the slits.
Explain why a laser must never be viewed directly along the beam, and state one other precaution the student should take.2 marks
A student measures the wavelength of the light from a laser pointer using a double slit with a slit separation of 0.30 mm. The screen is 2.00 m from the slits. She measures the distance across ten fringe spacings on the screen as 46 mm.
Calculate the wavelength of the laser light.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).