Send a wave at a wall, into shallower water, or past the edge of an obstacle. Three different behaviours follow, and all three can be seen on a single afternoon with a ripple tank.
The Key Idea
Reflection bounces a wave off a surface without changing its speed or frequency. Refraction bends a wave when it changes medium. Diffraction spreads a wave around obstacles or through gaps. All three are observed in the ripple tank.
The Ripple Tank
One piece of equipment, three demonstrations.
A shallow glass tank of water with a vibrating bar that produces straight wavefronts, or a small dipper for circular ones. A bright lamp above casts shadows of the crests onto a screen below. By placing a barrier, a glass block, or a narrow gap in the water, you can show all three behaviours in sequence.
SECTION 01
Reflection.
When a wave hits a flat barrier, it bounces back. The angle at which it leaves matches the angle at which it arrived, measured against a line drawn perpendicular to the barrier (the normal).
Reflection. The angle of incidence i equals the angle of reflection r, both measured from the normal.
During reflection, the wave keeps the same speed, frequency, and wavelength. Only the direction changes.
SECTION 02
Refraction.
When a wave crosses from one medium into another, it changes speed. If it enters the new medium at an angle, that change in speed makes it bend. This is refraction.
The Examiner's Trap
When water waves move into shallower water, they slow down. Because of the equation v = fλ, students often assume that both the frequency and the wavelength must decrease. This is wrong.
The frequency of a wave is entirely fixed by whatever is vibrating to create it. It never changes when crossing a boundary. Only the speed and wavelength change.
During refraction:
Speed changes (slower in shallower water)
Wavelength changes (shorter when slower, since v = fλ and f is fixed)
Frequency stays the same (the source is unchanged)
Direction changes if the wave enters at an angle
If the wave hits the boundary perpendicular, it still slows down, but its direction does not change.
SECTION 03
Diffraction.
When a wave passes through a gap or around the edge of an obstacle, it spreads out into the region behind. This spreading is diffraction.
Diffraction. Plane waves pass through a gap and spread into perfect circular ripples on the other side.
How much a wave spreads depends on the size of the gap compared to the wavelength. The smaller the gap (or the longer the wavelength), the more dramatic the spreading. This is treated in detail in the Extended-only section: Diffraction in detail.
During diffraction, just like reflection, the speed, frequency, and wavelength stay the same. Only the shape of the wavefronts changes.
Worked Example
In a ripple tank, plane water waves travel from deeper water into shallower water. The boundary between the two regions is at an angle to the wavefronts. Describe what happens to the speed, wavelength, frequency, and direction of the waves.
Step 1 · Identify the behaviour
This is refraction. The waves cross from one medium (depth) into another, at an angle.
Step 2 · Speed
Water waves travel slower in shallower water. So the speed decreases.
Step 3 · Wavelength and frequency
Frequency stays the same because it is set by the source. Since v = fλ and v has decreased, the wavelength must also decrease.
Step 4 · Direction
Because the boundary is at an angle, the waves bend toward the normal as they enter the slower medium.
Practice questions.
State which property of a wave changes during reflection: speed, frequency, wavelength, or direction.
Answer: Only direction changes.
A water wave passes from deep water into shallow water. Which one of the following remains the same: speed, frequency, wavelength?
Answer: Frequency. It is determined by the source.
Explain why a wave does not bend when it crosses a boundary perpendicular to the boundary, even though its speed changes.
Answer: Both ends of each wavefront slow down at the same instant, so there is no twisting and no direction change. The wavelength still changes.
Which behaviour, reflection, refraction, or diffraction, would you use to explain why you can hear a person talking around a corner even though you cannot see them?
Answer: Diffraction. Sound has a long wavelength and spreads around the corner; visible light has tiny wavelengths and barely diffracts at corner-sized gaps.