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

The pattern on the screen is formed due to the interference of light waves emanating from the two slits.

1. The slits act as coherent sources of light waves, meaning they emit waves with a constant phase difference.
2. As the waves from the slits overlap, they create points of constructive interference (bright fringes) and destructive interference (dark fringes).
3. Constructive interference occurs when the path difference between the waves is an integer multiple of the wavelength, resulting in increased brightness.
4. Destructive interference occurs when the path difference is a half-integer multiple of the wavelength, leading to reduced brightness or darkness in those regions.

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

When the distance between the slits is increased, the distance between the fringes (or spots) decreases. Consequently, the pattern on the screen becomes more compact.

To demonstrate that sound is a wave motion, consider the following experiment:

1. Setup: Use two loudspeakers connected to a signal generator. Position them a few meters apart.
2. Process: Walk in front of and parallel to the speakers while observing the sound.
3. Observation: At certain points, you will hear variations in the sound level (loud and soft) as you move.
4. Conclusion: This variation in sound intensity is due to constructive and destructive interference of sound waves, illustrating that sound travels as waves.

Longitudinal Waves:

• These waves consist of compressions and rarefactions where the direction of the vibrations is parallel to the direction of wave propagation.

Transverse Waves:

• These waves consist of peaks and troughs, where the direction of the vibrations is perpendicular to the direction of wave motion.

An experiment to demonstrate that light waves are transverse can be conducted as follows:

1. Setup: Use a laser pointer and a piece of polarized film.
2. Process: Shine the laser through the polarized film and then rotate the film.
3. Observation: As the film is rotated, the intensity of the light passing through the film will change, indicating dependence on the orientation of the polarization.
4. Conclusion: This behavior confirms that light waves oscillate in a direction perpendicular to their direction of travel, consistent with transverse wave properties.