Theme C — Wave Behaviour

C.3.7 Young’s Double-Slit Experiment — IB Physics HL Revision Notes & Topical Questions

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Lesson summary

C.3.7 Young’s Double-Slit Experiment — IB PHYSICS SL notes: Describe Young's double-slit experiment and use s = λD/d to find fringe separation, wavelength or slit separation.Origin — in 1801 Thomas Young showed that light undergoes interference and therefore behaves as a waveSour…

Double slit
Diffraction
Bright fringes
Dark fringes
Path difference
Central fringe

Part 1 of 8Set-up

Origin

Origin in 1801 Thomas Young showed that light undergoes interference and therefore behaves as a wave

Source

Source monochromatic, coherent light is needed, and a laser is the most common source
Light may first pass through a single narrow slit, or a laser may be used directly.

Double slit

Double slit two narrow parallel slits separated by distance d are fed by the same wavefront, so they act as coherent sources

Diffraction

Diffraction each slit diffracts the light into a wide beam so that the two beams overlap

Screen

Screen it is placed a distance D from the slits, far from them compared with d

Pattern

Pattern equally spaced bright fringes (maxima) and dark fringes (minima) appear

Part 2 of 8Interference pattern

Bright fringes

Bright fringes constructive interference forms strips of equal maximum intensity

Dark fringes

Dark fringes destructive interference forms strips of minimum, nearly zero, intensity

Equality

Equality each bright fringe has the same width and intensity for very narrow slits

Path difference

Path difference the wave from slit S₂ travels slightly further than the wave from S₁ to the same point
Bright fringes lie where the path difference is nλ.
Dark fringes lie where it is (n + ½)λ.

Order n

Order n it counts the fringes from the centre
n = 0 is the central maximum.
n = 1 is the first maximum on either side.

Central fringe

Central fringe the path difference is zero, so the central point is bright

Small angles

Small angles at an angle θ from the axis the path difference is d sin θ ≈ dθ, so nλ = d sin θ

Part 3 of 8The fringe separation formula

Equation

Equation s = λD/d
s is the separation of adjacent bright (or dark) fringes in m.
λ is the wavelength, D the slit-to-screen distance and d the slit separation.

Derivation idea

Derivation idea the fringe at distance y from the centre has θ ≈ y/D, so adjacent fringes are equally spaced

Fringes get wider when

Fringes get wider when the wavelength increases, the screen is further away, or the slits are closer together

Label

Label s and d are in the numerator with D below, so label the diagram clearly

Assumptions

Assumptions narrow slits, d much smaller than D and small angles, so sin θ ≈ tan θ ≈ θ
School values are D of 1 to 3 m and d a fraction of a millimetre, so the approximation is excellent.

Part 4 of 8White light

Different spacings

Different spacings each colour has its own fringe spacing

Central fringe

Central fringe it is white because all colours are in phase there

Outer fringes

Outer fringes they are coloured, with blue on the inside edge and red on the outside, since red spreads more

Part 5 of 8Intensity pattern

Intensity pattern

Envelope

Envelope real slits have finite width,
- and their single-slit diffraction pattern forms an envelope that reduces the intensity of fringes away from the centre
A fringe can be missing where an interference maximum falls on a diffraction minimum.

Graph features

Graph features the central maximum is the highest, fringes are spaced s apart and minima reach nearly zero

Changing the set-up

Changing the set-up doubling d halves the fringe spacing and doubling λ doubles it

Part 6 of 8Practical points

Set up the laser, double slit and a screen at a measured distance D.

Measure across several fringes on the screen.

Divide the total by the number of fringe spacings to find s.

Use λ = sd/D or d = λD/s.

Uncertainty

Uncertainty measuring across many fringes lowers the percentage uncertainty in s

Measuring s

Measuring s measure between adjacent fringe centres and not from the screen edge

Safety

Safety a laser is brighter and safer than a lamp, but never look into the beam

Part 7 of 8Significance

Significance

Wave evidence

Wave evidence the experiment gave strong evidence for the wave nature of light against the particle view

Wavelength measurement

Wavelength measurement a measurable s allows the wavelength to be found

Particles

Particles single photons and electrons also build up the same pattern one at a time, evidence of wave–particle duality

Part 8 of 8More from the specification

Key terms

Term	Meaning
Coherent sources	Two slits illuminated by the same wavefront.
Fringe separation, s	Distance between adjacent bright fringes.
Slit separation, d	Distance between the centres of the two slits.
Monochromatic	Light of a single wavelength.
Envelope	Single-slit diffraction pattern limiting the double-slit intensities.

Avoid these mistakes

Using the slit width instead of the slit separation for d.

Measuring s from the screen edge rather than between adjacent fringe centres.

Forgetting unit conversions between mm, nm and m.

Believing the formula holds for large angles — it assumes small angles.

Saying the central fringe is coloured with white light — it is white.

Last updated: 15 October 2026