Diffraction always happens when a wave passes a gap or an edge, but the amount of spreading varies enormously. The single rule that decides everything is the size of the gap compared to the wavelength.
The Key Idea
Diffraction is most dramatic when the gap width is roughly equal to the wavelength of the wave. When the gap is much bigger than the wavelength, the wave barely spreads at all. When the gap is much smaller, almost no wave gets through.
SECTION 01
Why gap size matters.
Think of each point in the gap as a tiny source of new circular wavelets. If the gap is wide, many wavelets line up and produce a mostly straight wavefront on the other side. If the gap is narrow, only a few wavelets exist, and the wave spreads out in a circular pattern.
The comparison that matters is the ratio of gap width to wavelength:
If gap is much greater than wavelength: minimal diffraction
If gap is roughly equal to wavelength: strong diffraction
If gap is much smaller than wavelength: most of the wave is blocked, what passes through diffracts strongly but with low intensity
SECTION 02
The three regimes.
Wide Gap
Gap much greater than wavelength.
Minimal diffraction. Waves pass through almost straight, with only slight curving at the edges of the gap.
Equal Gap
Gap approximately equal to wavelength.
Strong diffraction. The wave spreads out into clear circular wavefronts almost as if from a point source.
Narrow Gap
Gap smaller than wavelength.
Strong diffraction, but most of the wave is blocked. What passes through spreads in all directions but at low intensity.
The Examiner's Trap: The "Expanding" Wavelength
In Paper 4, you will often be asked to draw wavefronts diffracting through a gap. The single most common mistake is drawing the curved lines getting further and further apart.
Wavelength does not change during diffraction. If the lines are 1 cm apart before the gap, your curved lines MUST be exactly 1 cm apart after the gap. Use a ruler to check your own drawing.
SECTION 03
The same idea at an edge.
Diffraction does not need a gap. A single edge does it too. When a wave passes the edge of an obstacle, it bends into the region behind the obstacle, creating a shadow that is not perfectly sharp.
Plane waves travel downward and pass the edge of the obstacle. In the open region they continue straight. At the edge they bend around the corner into the shadow region behind the obstacle. The spacing between the wavefronts does not change.
This is why a hill or a tall building does not give a perfectly silent shadow for sound. The sound waves bend around the edge and leak into the region behind. The same rule applies: the longer the wavelength compared to the size of the obstacle, the more the wave bends into the shadow zone. Sound bends much more around a hill than light does, because sound wavelengths are typically much longer.
In the Real World
Why you can hear around corners but cannot see around them.
Sound in a classroom has wavelengths between roughly 2 cm and 17 m. Door-sized gaps and corner-sized edges are well-matched to these wavelengths, so sound diffracts heavily and reaches your ears even when the source is hidden.
Visible light has wavelengths of roughly 500 nanometres, about a million times shorter than door-sized gaps. The ratio is so extreme that diffraction at everyday openings is completely imperceptible, and light effectively travels in straight lines through doorways. To see diffraction of light, you need a slit narrow enough to compete with that wavelength: a fraction of a millimetre or less.
Worked Example
In a ripple tank, water waves of wavelength 2 cm approach a gap in a barrier. The gap is initially 8 cm wide. The student then narrows the gap to 2 cm. Compare the diffraction pattern in the two cases, and explain the difference.
Step 1 · Identify the relevant ratio
We need to compare gap width to wavelength. Case A: 8 cm / 2 cm = 4. Case B: 2 cm / 2 cm = 1.
Step 2 · Predict Case A (gap = 8 cm)
The gap is four times the wavelength. Diffraction is small. Most of the wave continues as plane waves on the other side, with only slight curving at the edges of the gap.
Step 3 · Predict Case B (gap = 2 cm)
The gap is equal to the wavelength. Diffraction is strong. The wave spreads out from the gap as circular wavefronts, almost as if a point source were placed at the gap.
Step 4 · State the rule
Diffraction is most pronounced when the gap width is close to the wavelength of the wave.
Practice questions.
State the condition for maximum diffraction at a gap.
Answer: When the width of the gap is roughly equal to the wavelength of the wave.
A radio wave of wavelength 100 m and a microwave of wavelength 3 cm both approach a window of width 1 m. Which one diffracts more strongly? Explain.
Answer: The radio wave. Its wavelength (100 m) is much greater than the gap (1 m), so it diffracts strongly. The microwave (3 cm) is much smaller than the gap, so it barely diffracts.
In a ripple tank, the wavelength is 1 cm. The gap is 5 cm. Sketch (in words) what the diffraction pattern looks like.
Answer: Mostly straight wavefronts continuing through the gap, with slight bending near the edges of the gap on the far side.
Explain in terms of wavelength why diffraction is much easier to observe with sound waves than with visible light.
Answer: Sound has wavelengths comparable to everyday objects (centimetres to metres), so it diffracts heavily around them. Light has wavelengths roughly a million times smaller, so visible diffraction requires gaps below a millimetre.