A wave can be described as a travelling disturbance that transports energy from one point to another - Leaving Cert Physics - Question 8 - 2021

Electromagnetic waves do not require a medium to travel through. They can propagate through a vacuum, which distinguishes them from mechanical waves such as sound.

Transverse waves are characterized by oscillations that are perpendicular to the direction of wave travel. An example of this is light waves.

In contrast, longitudinal waves have oscillations that are parallel to the direction of wave travel. Sound waves in air are a common example of longitudinal waves.

Diagram: A labelled diagram can help illustrate these differences.

Transverse waves (e.g., light) & Longitudinal waves (e.g., sound)

To find the speed of the radio waves, we use the formula:

$v = f imes ext{λ}$

where:

• $v$ = speed of the wave,
• $f$ = frequency = 107 MHz = $107 imes 10^6$ Hz,
• $ext{λ}$ = wavelength = 2.804 m.

Now substituting the values:

$v = (107 imes 10^6) imes 2.804 = 3.00 imes 10^8 ext{ m/s}$

Thus, the speed of the radio waves is approximately $3.00 imes 10^8$ m/s.

An example of the reflection of sound waves is an echo. When a sound is produced, such as clapping hands, it travels through the air and encounters a surface (e.g., a wall) that reflects the sound back. The person who made the sound hears it again after a brief delay, which is the echo of the original sound.

The refractive index ($n$) can be calculated using Snell's Law:

$n = \frac{\sin(i)}{\sin(r)}$

where:

• $i$ = angle of incidence = 23°,
• $r$ = angle of refraction = 15°.

Calculating:

$n = \frac{\sin(23°)}{\sin(15°)} = \frac{0.3907}{0.2588} \approx 1.51$

Therefore, the refractive index of the glass is approximately 1.51.

To illustrate diffraction, extend the wave fronts as they pass through the narrow gap to show the spreading out effect.

Diagram:

Waves spreading out from the slit.

An example of an interference experiment with sound waves can be set up using two speakers emitting the same frequency from different locations.

1. Setup: Position two speakers a fixed distance apart, facing a listener.
2. Method: Play a continuous tone simultaneously from both speakers.
3. Observation: Move the listener around the area between the speakers and note varying loudness.
4. Conclusion: The listener will experience areas of constructive interference (louder sounds) and destructive interference (softer sounds). This demonstrates sound wave interference.

Polarisation refers to the orientation of the oscillations in a wave. For light waves, polarisation restricts vibrations to a single plane, while sound waves, being longitudinal, cannot be polarised.

Diagram:

Shows vibrations in a single plane for light waves.