Sound and light travel as waves - Leaving Cert Physics - Question 7 - 2016

Apparatus

• Bell jar
• Electric bell / phone
• Battery
• Vacuum pump

Procedure

1. Set up the electric bell inside the bell jar and connect it to the battery.
2. Initially, the air is present in the jar, and the bell can be heard ringing.
3. Use the vacuum pump to remove the air from the jar gradually.
4. Observe that as the air is removed, the sound of the bell becomes fainter until it is no longer heard.

Observation/Conclusion

This experiment shows that sound requires a medium (air) to travel, as it becomes inaudible in the absence of air.

Explanation of Total Internal Reflection

• Total internal reflection occurs when light travels from a denser medium (like glass) to a less dense medium (like air) at an angle greater than the critical angle.

Diagram

Insert diagram here showing incident light, angle of incidence (i), angle of refraction (r), and normal line.

Key Points

• When the angle of incidence exceeds the critical angle, the light reflects entirely back into the denser medium instead of refracting out.

1. Telecommunications: Optical fibres are used to transmit data over long distances with minimal loss.
2. Medical instruments: They are employed in endoscopes for minimally invasive surgeries to view internal organs.

Calculation

To find the critical angle ( heta_c) for total internal reflection, we can use Snell's law:

$n_1 imes ext{sin}( heta_c) = n_2 \ ext{, with } n_1 = 1.44 (glass) \ ext{ and } n_2 = 1.00 (air).$

The critical angle is calculated as:

$\text{sin}( heta_c) = \frac{n_2}{n_1} = \frac{1.00}{1.44} \approx 0.694 \implies \theta_c \approx 43.98^{ ext{o}}$

The unit of sound intensity level is the decibel (dB).

A sound-level meter might be used in a workplace to monitor environmental noise levels, ensure compliance with safety regulations, protect workers' hearing from excessive sound levels, and assess the need for sound insulation or other measures.