A student shines a laser beam through a narrow slit in a piece of card. A screen is placed 2.0 m beyond the slit. Fig. 1.1 shows the experimental setup from...

(a) The wave behaviour is diffraction. [1]

Diffraction is the spreading of waves as they pass through a gap or around an obstacle. It is a fundamental property of all waves, not just light.

(b) The slit width (0.1 mm = 100 μm) is small enough to be comparable in scale to the wavelength of the light (650 nm). [1] Diffraction is most significant when the gap size is close to the wavelength of the wave. [1]

Although the slit is still much wider than a single wavelength, it is narrow enough for noticeable diffraction. If the gap were millions of times the wavelength, light would pass through essentially as a straight beam.

(c) A broad central bright region is seen on the screen. [1] Narrower bright and dark bands (fringes) appear on either side of the central maximum. [1]

This is a single-slit diffraction pattern. The central bright fringe is always the widest and brightest. The intensity of the side fringes decreases with distance from the centre. The dark regions occur where waves from different parts of the slit interfere destructively.

(d) When the wider slit (1.0 mm) is used, the central bright region becomes narrower. [1] There is less spreading because the gap is now much larger relative to the wavelength, so the light passes through more like a straight beam. [1]

This is a key relationship: the wider the slit (relative to wavelength), the less diffraction occurs. Conversely, a narrower slit produces more spreading.

(e) The frequency of the light does not change as it passes through the slit. [1]

Frequency is determined by the source and does not change when a wave passes through a gap, reflects, or refracts. Only wavelength and speed can change (e.g. when entering a different medium), but passing through a slit in air does not change the medium.