Blue Grotto – Ocean's Glowing Floor
Siwei (Serena) Lin
Award: Top 100
Award: Honorable Mention for Writing
School: st mark's school
Teacher: autumn glenn
Category: Natural
Photo #21121
Blue Grotto – Ocean's Glowing Floor
My family and I visited Capri's famous Blue Grotto in the Mediterranean last summer. To enter the 1-meter-high opening, I had to press myself flat in a rowboat while pulling on overhead metal chains. At first glance, the cave seems to be illuminated from below. The seawater glows electric blue, and the floor appears brighter than the surface above. This striking effect is created by the way sunlight enters and interacts with the cave.
Although the opening above the water is narrow, a much larger entrance lies below the surface. Sunlight mainly enters through this underwater opening, passes through the water, and reflects off the seafloor layered with white sand. Since the reflected light rises from below, the cave appears to shine from its bottom.
The blue color results from water's selective absorption of light. Longer wavelengths such as red, orange, and yellow are absorbed more strongly, while shorter blue wavelengths penetrate farther and scatter back toward the observer. The white seafloor also contributes to the strong glow of seawater, as white absorbs little light and reflects the most. The glow is brightest around midday, when more sunlight enters the cave at a more normal angle to the water, resulting in less reflection and more transmission due to the properties of light. The white sand then diffuses this blue-rich light, making the underwater floor appear unusually luminous.
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