Describe the use of a ripple tank to show: (a) reflection at a plane surface (b) refraction due to a change in speed caused by a change in depth (c) diffraction due to a gap (d) diffraction due to an edge

3.1 General Properties of Waves – Ripple Tank Demonstrations

a) Reflection at a Plane Surface

Imagine a ball rolling on a flat floor and hitting a wall. It bounces back at the same angle it came in. In a ripple tank, water waves travel towards a vertical wall and bounce back. The key rule is angle of incidence equals angle of reflection:

\$\thetai = \thetar\$

This is why we see a clear, symmetrical pattern of waves on the other side of the wall.

• Place a vertical barrier in the tank.
• Generate waves with a vibrating plate.
• Observe the reflected waves forming a mirror image.
• Measure angles with a protractor to confirm equality.

b) Refraction due to a Change in Speed Caused by a Change in Depth

Think of a skateboarder going from a smooth track to a rough one: they slow down. Similarly, water waves travel faster in deeper water and slower in shallow water. When a wave crosses a depth boundary, its speed changes, causing the wavefront to bend – this is refraction.

The relationship is given by Snell’s law for waves:

\$\displaystyle \frac{v1}{v2} = \frac{\sin \theta1}{\sin \theta2}\$

In a ripple tank, you can create a shallow strip by placing a shallow tray inside the tank. Waves entering the shallow region bend towards the normal (the line perpendicular to the boundary).

1. Mark a shallow strip in the tank.
2. Generate waves on one side of the strip.
3. Watch the waves bend as they enter the shallow area.
4. Use a ruler to measure the angles before and after the boundary.

c) Diffraction Due to a Gap

Picture a stone dropped into a pond near a doorway. The water spreads out on the other side, creating a fan‑shaped pattern. In a ripple tank, a narrow slit (gap) allows waves to pass and spread out, producing a diffraction pattern.

The main features:

• Central bright fringe (maximum).
• Alternating dark and bright fringes due to constructive and destructive interference.
• Fringe width depends on the slit width and wavelength.

The positions of the minima can be estimated by:

\$\displaystyle y_m = \frac{m \lambda L}{a}\$

where \$m\$ is the order of the minimum, \$\lambda\$ is the wavelength, \$L\$ is the distance to the screen, and \$a\$ is the slit width.

• Insert a narrow slit in the tank.
• Generate waves and observe the fan‑shaped pattern.
• Mark the positions of bright and dark fringes.

d) Diffraction Due to an Edge

Imagine a stone thrown into a pond next to a wall. The waves bend around the edge, creating a series of concentric circles on the other side. In a ripple tank, a single vertical edge (like a half‑wall) produces a diffraction pattern that looks like a series of bright and dark bands parallel to the edge.

Key points:

• Wavefronts bend around the edge.
• Interference between waves from the two sides of the edge creates fringes.
• Fringe spacing is related to the wavelength and distance from the edge.

1. Place a half‑wall in the tank.
2. Generate waves on one side.
3. Observe the diffraction pattern forming on the other side.
4. Measure fringe spacing and compare with theoretical predictions.

Summary Table of Wave Properties