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What Is Light Diffraction?

Diffraction is the physical phenomenon that occurs when a light wave encounters an obstacle or a narrow opening. While we often think of light traveling in perfectly straight lines (rays), it actually behaves like a fluid wave. When light hits a sharp edge or passes through a small hole, it bends and spreads out into the region that should be a shadow. A classic analogy is sound waves. If someone speaks around a corner, you can still hear them because the sound waves diffract or bend around the wall. Light does the exact same thing, but because the wavelength of light is microscopic, the bending is only noticeable when the obstacle or opening is equally microscopic.

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What Is Light Diffraction?

Diffraction is the physical phenomenon that occurs when a light wave encounters an obstacle or a narrow opening. While we often think of light traveling in perfectly straight lines (rays), it actually behaves like a fluid wave. When light hits a sharp edge or passes through a small hole, it bends and spreads out into the region that should be a shadow. A classic analogy is sound waves. If someone speaks around a corner, you can still hear them because the sound waves diffract or bend around the wall. Light does the exact same thing, but because the wavelength of light is microscopic, the bending is only noticeable when the obstacle or opening is equally microscopic.

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The Huygens-Fresnel Principle

To understand why this happens, physicists use the Huygens-Fresnel principle. This theory states that every point on a wavefront acts as a source of tiny new circular waves called wavelets. When light passes through a slit, the wavelets at the edges are no longer contained by the rest of the wave. They spread out sideways. These spreading wavelets interfere with each other. Where the peaks of the waves meet, they create bright spots (constructive interference). Where a peak meets a valley, they cancel each other out to create dark spots (destructive interference). This interaction creates the characteristic fuzzy or ringed patterns seen in diffraction.

Visual Halos and Glare

In the human eye, diffraction is often the cause of visual halos. Under normal conditions, the cornea and lens are transparent and regular, allowing light to pass through cleanly. However, if the cornea becomes swollen (edema) or if the lens develops cloudiness (cataract), the cellular structure becomes irregular. The water droplets or protein clumps act like a diffraction grating. They scatter the incoming light, splitting it into its component colors. This is why a patient with cataracts or high eye pressure often sees rainbow-colored rings around streetlights or car headlights at night.

The Pinhole Trade-off

Diffraction imposes a strict limit on visual sharpness known as the diffraction limit. When the pupil of the eye constricts to a very small size, such as in bright sunlight, it reduces optical aberrations and sharpens the image depth. However, if the pupil gets too small (below 2 millimeters), the light waves are forced to squeeze through such a tiny aperture that they diffract aggressively. The spreading waves blur the image on the retina. This creates a physiological trade-off where the sharpest vision usually occurs with a pupil size of roughly 3 to 4 millimeters, balancing the reduction of aberrations with the avoidance of diffraction.

Everyday Examples (CDs and Eyelashes)

You can see diffraction in daily life without a laboratory. The rainbow colors reflected off the back of a CD or DVD are caused by diffraction. The tiny data tracks on the disc are spaced so closely that they act as a grating, separating white light into a spectrum. Similarly, if you squint your eyes tightly while looking at a light source, you might see long rays of light shooting out. This is caused by the light diffracting around your eyelashes and the edges of your eyelids.

FAQs on Light Diffraction

Is it the same as refraction?

No. Refraction is the bending of light as it passes from one medium to another, like air to water. Diffraction is the bending of light as it passes around an edge or through an opening, without necessarily changing the medium.

Does it affect photography?

Yes. Landscape photographers often want everything in focus, so they use a small aperture like f/22. However, the small hole causes diffraction, which makes the fine details of the photo look soft or blurry compared to f/8.

Why are the halos colored?

Diffraction depends on wavelength. Red light waves are longer and bend more than blue light waves. This separation spreads the white light into a spectrum, creating a rainbow effect with red on the outside and blue on the inside of the halo.

When to See Your Eye Doctor

If you suddenly start seeing rainbow halos around lights, especially if accompanied by eye pain or blurred vision, this is a medical emergency. It is a classic symptom of acute angle-closure glaucoma, where high pressure forces water into the cornea, creating a diffraction grating that warns of potential vision loss.

References

https://www.physicsclassroom.com/class/light/Lesson-3/Diffraction-and-Interference

https://micro.magnet.fsu.edu/primer/lightandcolor/diffraction.html

https://eyewiki.aao.org/Diffractive_Optics

https://www.nature.com/articles/s41598-021-88906-8