Light travels as a wave of electromagnetic radiation - Leaving Cert Physics - Question 9 - 2020

To find the wavelength ($\lambda$), we can use the formula:

$\lambda = \frac{v}{f}$

Where:

• $v$ is the speed of light ($3 \times 10^8$ m/s)
• $f$ is the frequency ($5 \times 10^{14}$ Hz)

Substituting the values gives:

$\lambda = \frac{3 \times 10^8 \text{ m/s}}{5 \times 10^{14} \text{ Hz}} = 6 \times 10^{-7} \text{ m}$

Thus, the wavelength of the orange light is $6 \times 10^{-7}$ m.

An experiment to demonstrate that sound requires a medium can be conducted using a bell jar and an electric bell. Place the electric bell in the jar. When the bell is turned on, sound can be heard as long as the jar is filled with air. However, if the air is evacuated from the jar using a vacuum pump, the sound will disappear, indicating that sound needs a medium (like air) to travel.

Reflection is the bouncing back of a wave when it encounters a barrier or a surface. It occurs when the wave fronts strike a smooth surface, causing the wave to return into the original medium at the same angle as it struck the surface (Law of Reflection).

To observe interference of sound waves, one can set up two loudspeakers emitting sound waves of the same frequency and amplitude. Place them at a certain distance apart in front of a wall. Use a microphone or a sensitive sound meter at various positions in front of the speakers to measure sound intensity. At certain positions, constructive interference (loud sound) will occur, while at others, destructive interference (soft or no sound) will be observed, demonstrating the principle of interference.

Sound waves are longitudinal waves, meaning the particle displacement is parallel to the direction of wave travel. Polarisation is a phenomenon that occurs with transverse waves, where the oscillations can be restricted to a particular direction. Since sound waves do not have this characteristic, they cannot be polarized.

A simple experiment to demonstrate the Doppler effect involves using a moving sound source, such as a buzzer, and a stationary observer. As the buzzer approaches the observer, the sound frequency perceived will increase, leading to a higher pitch. As it moves away, the frequency will decrease, resulting in a lower pitch. The frequency shifts can be observed in real time to illustrate the Doppler effect.

One common use of the Doppler effect is in medical imaging, specifically in Doppler ultrasound, where it’s used to measure the flow of blood in the body, allowing doctors to assess the health of the heart and blood vessels.