Diffraction

Overview

Diffraction is the spreading of waves when they pass through a gap or around an edge. This lesson also applies diffraction and interference to gratings, where many closely spaced slits produce sharp maxima that can be used to find wavelength.

What You Need to Know

• Diffraction is most noticeable when the gap width is similar to the wavelength.
• If the gap is much wider than the wavelength, the wave spreads less and most of the wave continues forward.
• Ripple tanks provide a clear model for the qualitative effect of gap width on diffraction.
• A diffraction grating has many equally spaced lines or slits. The spacing between adjacent slits is d.
• Bright maxima occur where waves from adjacent slits arrive in phase.
• For a diffraction grating, use d sin(theta) = n lambda, where n is the order number.
• The central maximum is order n = 0. Higher orders appear at larger angles for the same wavelength and grating spacing.

How to Work Through It

1. Start with ripple-tank diagrams showing wide gaps, narrow gaps, and different wavelengths.
2. Define diffraction and compare it with interference without separating the two ideas completely.
3. Introduce diffraction gratings as many-slit interference patterns with sharp maxima.
4. Practise using d sin(theta) = n lambda, including finding d from lines per metre.

Check Your Understanding

• When is diffraction most noticeable?
• What does the grating spacing d represent?
• Why is the central maximum labelled order zero?
• What measurements are needed to determine the wavelength of monochromatic light with a grating?

Common Mistakes

• Saying diffraction only happens when a gap is smaller than the wavelength.
• Confusing slit width in single-gap diffraction with grating spacing in a diffraction grating.
• Using degrees directly in algebra without checking calculator angle mode.
• Forgetting that the order number n must be a whole number.

Next Steps

• Practise grating calculations with careful unit conversion.
• Review how diffraction and interference both depend on wavelength and phase.