A-Level Physics (IGCSE Edexcel)

3.7 - Diffraction

1What diffraction is and the impact of gap size on diffraction
2Diffraction of waves around obstacles
3The role of monochromatic light in observing clear diffraction patterns
4How laser light creates distinct fringe patterns
5Comparison of diffraction patterns produced by monochromatic and white light
6Huygen's principle

5 min

What is diffraction?

$Illustration showing wave diffraction through different sized openings.$

Diffraction occurs when waves, like light or sound, spread out or bend as they pass through openings or move around obstacles. The extent of diffraction is influenced by the size of the opening compared to the wavelength of the wave.

Negligible diffraction - Occurs when the gap is significantly larger than the wavelength of the wave.
Noticeable diffraction - Happens when the gap size is about the same as a few wavelengths.
Maximum diffraction - Seen when the gap size is approximately equal to the wavelength.

For example, sound waves can diffract through doorways, making it possible to hear someone from around a corner. Light diffraction, however, is less noticeable in everyday life because typical openings like doorways are much larger than the wavelengths of visible light.

Diffraction around obstacles

Waves can also diffract when they encounter obstacles, bending around the edges of the obstacle. This creates a 'shadow' zone where the direct waves are blocked.

Wider obstacles: These cause more blocking of the waves, resulting in longer shadows.
Narrower obstacles: They allow more diffraction, leading to shorter shadows.

Using monochromatic light

For clear observation of light diffraction, a monochromatic light source is ideal. Monochromatic light consists of a single wavelength and hence a single colour. White light, which contains multiple wavelengths, does not produce distinct diffraction patterns.

Laser diffraction patterns

When a laser (which is monochromatic) shines through a narrow slit, it creates a pattern of bright and dark fringes on a screen. This phenomenon results from interference:

Bright fringes: These are areas of constructive interference where wave peaks align.
Dark fringes: These are areas of destructive interference where wave peaks and troughs align and cancel each other out.

$Diagram showing laser diffraction patterns with light waves passing through a narrow slit and creating bright and dark fringes on a screen.$

White Light vs Laser Light

$Diagram showing laser diffraction patterns with bright and dark fringes and white light spectrum.$

When white light, which comprises various wavelengths, passes through a slit, it creates a spectrum of colours instead of distinct fringes.

Red light has a longer wavelength and diffracts the most.
Violet light has a shorter wavelength and diffracts the least.

For each maxima, red light appears furthest from the central position and violet light closest to the central position.

Understanding Huygens' Principle

Huygens' Principle, developed by Dutch physicist Christiaan Huygens in the 17th century, provides a fundamental model for understanding wave propagation. This principle is crucial for explaining various wave phenomena, including diffraction.

Huygens' Principle states:

Every point on a wavefront acts as a source of secondary spherical wavelets. These wavelets spread out in the forward direction at the speed of the wave. The new wavefront is the surface tangent to all of these secondary wavelets.

Applying Huygens' principle to wave diffraction through a slit:

As the wavefronts approach the slit, only wavelets from points within the slit contribute to the new wavefront. The resulting wavefront curves outwards beyond the slit.