C.3 · IB Physics
Single-slit diffraction
Monochromatic light passes through one narrow slit and spreads out, giving a bright, wide central maximum with fainter maxima either side. The narrower the slit, the wider the spread.
Predict first
Start from the default settings (b = 80 μm, λ = 600 nm). If you double the slit width b to 160 μm, what happens to the width of the central maximum?
The central maximum has width 2λD ÷ b, so doubling b halves it: 30.0 mm becomes 15.0 mm.
Check it in the model: Move the Slit width, b slider from 80 to 160 μm and compare the Central maximum width readout.
pattern now frozen pattern first minima
screen position y = D tanθ
central maximum width = 2λD ÷ b
I = I₀ sinc²(β), β = π b sinθ ÷ λ
same source: I₀ ∝ b²
Try this
- Freeze the pattern at b = 120 μm, then drag b down to 60 μm. Compare the two curves for width and for peak height with the same-source option on.
- Use Red vs blue. Check that the blue central maximum is 420 ÷ 700 = 0.6 of the width of the red one.
- Pick Challenge 3 and find settings that put the first minimum 10.0 mm from the centre on the 1.50 m screen.
Common error: using θ = λ ÷ b as the angular width of the whole central maximum. It is the angle from the centre to the first minimum, so the full width is 2θ, and θ must be in radians with b and λ both in metres.