Theme C — Wave Behaviour
C.3.7 Young’s Double-Slit Experiment — IB Physics HL Revision Notes & Topical Questions
5 min read 10 parts Lesson 23 of 37 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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
Section 1 of 10Set-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
Section 2 of 10Interference 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
Section 3 of 10The 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.
Section 4 of 10White 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
Section 5 of 10Intensity 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
Section 6 of 10Practical points
1
Set up the laser, double slit and a screen at a measured distance D.
2
Measure across several fringes on the screen.
3
Divide the total by the number of fringe spacings to find s.
4
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
Section 7 of 10Significance
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
Section 8 of 10More from the specification
More from the specification
Young's double-slit interference equation
- Superposition of coherent waves passing through two narrow slits creates an interference fringe pattern on a screen, with fringe separation s = lambda D / d.
Path difference and interference conditions
- Constructive interference occurs where path difference equals an integer multiple of wavelengths (n lambda);
- destructive interference occurs at (n + 0.5) lambda.
Refraction and Snell's law at optical boundaries
- When light passes between media of different refractive indices, Snell's law dictates that n1 sin(theta1) = n2 sin(theta2).
Critical angle
- Incline angle at which sliding just begins.
Optically dense
- Having a high refractive index
Refraction
- Change in direction of light when it changes speed between media.
Boundary
- Surface between two media at which waves are partially reflected
Medium
- Material through which light travels.
Reflection
- Light bouncing off a surface.
Normal
- Line at 90° to a surface.
Superposition
- Resultant displacement is the sum of the displacements of the individual waves.
Transmission
- Sending electricity along power cables from the power station to consumers.
Refractive index
- Ratio of the speed of light in a vacuum to its speed in a material
Section 9 of 10Key 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. |
Section 10 of 10Exam technique
1
Using the slit width instead of the slit separation for d.
2
Measuring s from the screen edge rather than between adjacent fringe centres.
3
Forgetting unit conversions between mm, nm and m.
4
Believing the formula holds for large angles — it assumes small angles.
5
Saying the central fringe is coloured with white light — it is white.
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