Explain the terms diffraction and interference - Leaving Cert Physics - Question 10 - 2019

Interference occurs when two waves meet, resulting in a combination of their amplitudes. This can lead to constructive interference (where the waves reinforce each other) or destructive interference (where they cancel each other out).

In Young’s experiment, monochromatic light passes through two narrow slits, creating a pattern of bright and dark fringes on a screen due to the interference of light waves. A labelled diagram would illustrate the slits, the light source, and the resulting fringe pattern on the screen. The bright fringes are located where constructive interference occurs, while the dark fringes are where destructive interference occurs.

The experiment demonstrates that light behaves as a wave through the phenomenon of interference. The formation of alternating bright and dark fringes can only be explained by the wave nature of light, as particles would not generate such a pattern.

Given the distance between bright fringes and the geometry of the setup, we can use the formula:

$an θ = \frac{λ}{d}$

Substituting values gives:

• For n=6, $an θ = \frac{0.825}{1.25} = 0.378$
• Using $nλ = d \sin θ$, the wavelength can be calculated:

1. Move the screen further away from the slits.
2. Decrease the distance between the slits.
3. Utilize light with a larger wavelength.

When the ciliary muscles contract, the lens in the eye becomes thinner, reducing its focal length and thereby decreasing its power. This allows the eye to focus on distant objects.

The first harmonic will show a quarter wavelength in the pipe, while the second will show a wavelength that covers the length of the pipe with one closed end. Diagrams should be included to illustrate these patterns.

To find the wavelength, we can use the formula:

$λ = 2 \times (L_{open end} - L_{closed end}) = 2 \times (49.8 - 16.7) cm = 0.66 m$

To find the speed of sound:

$c = f \cdot λ = (512 Hz) \cdot (0.66 m) = 337.92 m/s\approx 339 m/s$