Heiligenschein

Last updated on September 16, 2023

Heiligenschein: Exploring the Phenomenon of Bright Dewdrops

Have you ever noticed a mesmerizing halo of light surrounding dewdrops on leaves? This enchanting optical phenomenon is known as the Heiligenschein. While the existing content provides a brief overview, let's delve deeper into the fascinating intricacies of this captivating display of light.

The Science Behind Heiligenschein

When sunlight interacts with dewdrops on leaves, a remarkable transformation occurs. The spherical dewdrops act as lenses, bringing sunlight to a crude focus just beyond their rear-facing surface. This focal point, approximately 20% beyond the diameter of the droplet, creates a bright spot on the leaf's surface.

As light from this spot scatters in all directions, some of it returns through the droplet along nearly the same paths as it came. This phenomenon, known as backscattering, causes each drop to 'backscatter' sunlight most prominently in directions toward the sun. Consequently, dewdrops shine brightly at the antisolar point, forming the Heiligenschein. Interestingly, if you explore your surroundings diligently, you might even spot a speckled dewbow about forty degrees away from the antisolar point.

The Role of Droplet Size and Placement

While Heiligenschein does not rely on diffraction, the size and placement of the droplets are crucial for its manifestation. The drops must be small enough to cling to leaves in a distinctive manner. However, they do not need to be extremely tiny. Even larger droplets can contribute to this phenomenon.

When drops and small hairs on leaves possess the right dimensions, sunlight focuses on the leaf's surface, generating the bright spot that gives rise to Heiligenschein. Hence, it is the intricate interplay between droplet size, leaf hair placement, and sunlight that produces this ethereal spectacle.

Unveiling the Color Palette

Although Heiligenschein appears white to the human eye, its true nature lies in the saturation of individual points of light on the eye's color receptors. Additionally, cameras also capture this phenomenon as a white hue due to the same optical effect. So, despite the abundance of vibrant colors in nature, Heiligenschein presents itself as a shimmering white halo.

Beyond Dewdrops: Other Instances of Antisolar Lighting

The antisolar point seems to be nature's favorite location for illuminating optical phenomena. While Heiligenschein arises from geometric optics and dewdrops, other occurrences captivate us through different mechanisms. Let's explore a few fascinating examples:

• Glory: This stunning optical phenomenon emerges from diffraction effects in small particles suspended in the atmosphere. It often graces our presence during flights or while standing on mountaintops.
• Retroreflection: Certain minerals and crystals possess the ability to retroreflect light, creating enchanting displays where light is reflected back toward its source.
• Coherent Backscattering: Forests, structured soils, the Moon, and even Mars exhibit a captivating glow due to shadow hiding and coherent backscattering. This opposition effect creates an intriguing contrast in brightness.
• Gegenschein: Even the dark night sky holds its own subtle glow opposite to where the sun resides. This faint counter glow is caused by interplanetary dust backscattering light.

Spherical Aberration: Imperfections of Spherical Lenses

While dewdrops act as lenses for Heiligenschein, it is important to note that spheres are not perfect lenses. Spherical aberration, a phenomenon observed in such lenses, causes rays passing close to the center to be less focused compared to those passing further away. This imperfection contributes to the unique characteristics of Heiligenschein and adds to its allure.

In conclusion, the Heiligenschein presents a captivating interplay of light, droplets, and leaves. This ethereal halo of brightness enchants us with its shimmering beauty. From the delicate balance of droplet size and placement to the diverse range of phenomena that illuminate the antisolar point, nature continues to amaze us with its intricate optical displays. So, the next time you encounter dewdrops on leaves, take a moment to appreciate the enchanting Heiligenschein and let it transport you into the magical world of atmospheric optics.

Dew drop near a leaf.

The droplet rests on small hairs which separate it from the leaf's surface.

The drop brings sunlight to a crude focus. When this is at the leaf surface, some light from the bright spot is scattered backwards through the drop to form the heiligenschein.

The heiligenschein does not need very small droplets because it does not depend on diffraction. However, the drops must be small enough to hang on leaves in a special way.

Spherical dew drops act as lenses. and bring sunlight to a crude focus ~20% of their diameter beyond their rear facing surface. Some drops rest on the tips of small hairs and do not touch the leaf surface. When the drops and hairs have the right dimensions, sunlight is focussed in a bright spot on the leaf. Light from the spot scatters in all directions but some returns through the droplet along almost the same paths as it came. The net effect is that each drop 'backscatters' sunlight, returning it most brightly in directions towards the sun. Dew thus shines brightly at the antisolar point to form a heiligenschein. But if the lawn or field is large enough check forty degrees away from the antisolar point for the same droplets can also form a speckled dewbow.

Why is the heiligenschein not green? It is said to look white because the individual points of light saturate the eye's colour receptors and to some extent cameras.

Nature is fond of lighting the antisolar point. The Glory arises from diffraction effects in small particles. Larger droplets resting on leaves give us the heiligenschein via geometric optics. Some minerals and crystals retroreflect. Forests, structured soils, the Moon and Mars appear to glow because of shadow hiding and coherent backscattering, the opposition effect. Even the dark night sky has its faint glow opposite where the sun rests, the gegenschein or counter glow made from light backscattered by interplanetary dust.

. Spheres make rather poor lenses. Rays passing close to the centre are more weakly focussed than those passing further away. This is spherical aberration.

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Reference Atmospheric Optics

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