# Explain why, in a double-slit interference experiment using monochromatic light, the bright fringes are equally spaced but their intensities are not all equal.

> IB DP Physics Higher Level (2023 syllabus) — C.3 Wave phenomena · Explain · 4 marks

> In a double-slit interference experiment, a screen is placed at a large distance from two narrow slits separated by a distance d. The slits are illuminated by a monochromatic plane wave.

## Mark scheme (4 marks)

1. Equal fringe spacing arises because constructive interference occurs wherever the path difference between the two sources equals a whole number of wavelengths (nλ), and these positions are equally spaced on the screen.
2. Each slit acts as a source of diffracted waves, so the intensity at any point on the screen is also modulated by the single-slit diffraction envelope.
3. Bright fringes near the centre of the diffraction envelope (near the central diffraction maximum) have higher intensity, while those further from the centre have lower intensity.
4. Some bright fringes from the interference pattern coincide with minima of the single-slit diffraction envelope and are therefore missing (have zero or near-zero intensity).

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