A student used a laser, as shown, to demonstrate that light is a wave motion - Leaving Cert Physics - Question 7 - 2005

The pattern is formed due to the behavior of light as coherent waves emerging from the slits. The process involves:

1. Wave Overlap: The light waves emanating from the two slits overlap as they travel toward the screen.
2. Path Difference: As the waves overlap, a path difference between them leads to constructive and destructive interference.
   • Constructive Interference: When the crest of one wave aligns with the crest of another, they combine to produce brighter spots (fringes) on the screen.
   • Destructive Interference: Conversely, when the crest of one wave aligns with the trough of another, they cancel each other out, resulting in dark spots (fringes) on the screen.

When the wavelength of the light is increased, the distance between the fringes (or spots) also increases. This results in a more spread-out pattern on the screen.

Increasing the distance between the slits causes the distance between the fringes (or spots) to decrease, leading to a more closely spaced pattern.

To demonstrate that sound is a wave motion, one can conduct the following experiment:

1. Setup: Connect two loudspeakers to a signal generator, ensuring one speaker is positioned in front of the other.
2. Experiment: Walk in front of the speakers while altering the volume or frequency using the tuning fork.
3. Observation: Listen for variations in sound (loud and low) while noting any interference patterns, such as areas of louder sound (constructive interference) and quieter areas (destructive interference).
4. Conclusion: This observation illustrates that sound travels in waves, confirming that sound is a wave motion.

The key differences between longitudinal and transverse waves are:

1. Longitudinal Waves:
   • The direction of the vibrations is parallel to the direction of wave propagation.
   • Example: Sound waves, where compressions and rarefactions move along the direction of the wave.
2. Transverse Waves:
   • The direction of the vibrations is perpendicular to the direction of wave propagation.
   • Example: Light waves, where the electric and magnetic fields oscillate perpendicular to the direction of travel.

To demonstrate that light waves are transverse waves, one could perform the following experiment:

1. Setup: Use a laser pointer and pass the light through a polarizer.
2. Experiment: Rotate one polarizer while observing the intensity of the light passing through.
3. Observation: As the polarizer is rotated, the intensity of the light will decrease to zero when it is at 90 degrees to the first polarizer.
4. Conclusion: This indicates that the light wave is polarized and hence demonstrates the transverse nature of light waves, as they oscillate in a direction perpendicular to their propagation.