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

1. Apparatus: We will use a bell jar, electric bell or phone, battery, and a vacuum pump.

2. Setup: Connect the bell to the battery and place it inside the bell jar. Ensure that the bell jar is sealed properly.

3. Procedure: Turn on the electric bell, and observe the sound it produces. Next, use the vacuum pump to remove the air inside the bell jar.

4. Observation: As air is removed from the bell jar, the sound produced by the bell becomes progressively quieter and eventually inaudible when the air is completely evacuated.

5. Conclusion: This demonstrates that sound requires a medium (air) to travel, as it cannot propagate in a vacuum.

Total internal reflection occurs when light traveling from a denser medium to a less dense medium hits the boundary at an angle greater than the critical angle.

Diagram:

         air                   glass
           |                     |
           |  Normal           |
           |                    |
           |  i > c          |    

• In the diagram, 'i' represents the angle of incidence which is greater than the critical angle 'c'.
• When this occurs, all of the light is reflected back into the denser medium (glass) instead of refracting into the air.

• Telecommunications: Optical fibres are widely used in telecommunications to transmit data over long distances at high speeds.
• Medical Instruments: They are also used in endoscopy to supply light to help visualize internal areas of the body.

To find the minimum angle of incidence, we use the formula for critical angle:

$n_1 \sin(c) = n_2 \sin(90^\circ)$

Given that the refractive index of the optical fibre (n_1) is 1.44 and refractive index of air (n_2) is 1:

$1.44 \sin(c) = 1 \sin(90^\circ)$

This simplifies to:

$\sin(c) = \frac{1}{1.44} \approx 0.694$

Now, using the inverse sine function:

$c = \arcsin(0.694) \approx 43.98^\circ$

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

A sound-level meter may be used in a workplace to measure sound levels in order to ensure they are within safe limits. Excessive noise can lead to hearing loss and other health issues, so monitoring sound intensity helps in maintaining a safe working environment.