Eclipsed Sun

Last updated on September 16, 2023

Exploring the Phenomenon of the Eclipsed Sun

Have you ever witnessed a sunset where the sun appeared to be a vivid shade of purple? This captivating sight, known as an "eclipsed sun," can leave you in awe of the wonders of atmospheric optics. While the naked eye may perceive the sun as completely invisible during such an event, digital cameras can reveal a stunning purple hue lingering in the sky for a few precious minutes. Let's delve into the science behind this phenomenon and uncover the secrets that lie within.

When Philip Laven embarked on a flight from Geneva to Warsaw, little did he know that he would bear witness to this captivating spectacle. As he looked out of the airplane window, he noticed the sun gradually disappearing behind a blanket of clouds. However, when he peered through his camera viewfinder, a completely different scene unfolded before his eyes – a vibrant purple sunset. The contrast between what he saw with his naked eye and what his camera captured sparked his curiosity.

What causes this striking difference between our visual perception and the camera's portrayal of an eclipsed sun? The answer lies in the sensitivity of digital cameras to near-infrared radiation. As the sun's rays pass through water droplets within the cloud bank, they scatter and obscure the direct visible light of shorter wavelengths. However, a small fraction of the sun's infrared radiation manages to transmit through the droplets, making it visible to the camera sensor.

The flattened appearance of the solar image captured by Laven's camera is a result of the photograph being taken at a relatively high altitude. When sunlight passes through the dense lower atmosphere, it undergoes differential refraction twice – once upon entering and again upon leaving. This double refraction causes the rays to bend and distort, resulting in a flattened image of the sun.

The peculiar violet color observed in Laven's photograph is actually an artifact produced by the camera sensor. CCD sensors, commonly found in digital cameras, have a sensitivity range that includes infrared light (750-1150 nm), in addition to visible light (400-700 nm). To prevent unwanted infrared interference, cameras are equipped with infrared-blocking filters. However, these filters often allow some residual infrared rays to pass through, leading to the capture of infrared-sensitive images.

It's fascinating to note that the infrared sensitivity of a camera can vary depending on its make and model. You can conduct a simple experiment to test your own camera's response to infrared radiation. Point your camera at your TV remote control from a distance of about six inches and observe whether the remote's infrared signal is visible through the camera's display. This experiment will provide insight into your camera's infrared sensitivity and its potential to capture an eclipsed sun.

In conclusion, the phenomenon of an eclipsed sun offers a captivating glimpse into the realm of atmospheric optics. While our eyes may perceive the sun as invisible during this event, digital cameras with their sensitivity to near-infrared radiation can reveal a mesmerizing purple sunset. By understanding the interplay between sunlight, water droplets, and camera sensors, we gain a deeper appreciation for the intricate mechanisms at work in our atmosphere. So, keep your eyes open and your camera ready – you never know when you might witness the enchanting beauty of an eclipsed sun.

On a flight from Geneva to Warsaw, Philip Laven (see his excellent Optics of a water drop ) saw the low sun disappear behind clouds. But his camera viewfinder showed a different scene - a purple sunset. Image ©Philip Laven

"The amazing thing was that, with my unaided eyes, the sun was completely invisible. However, the purple sun remained clearly visible on my camera for about 2 minutes "

Digital cameras are sensitive to near infrared radiation. Here, the water droplets in the cloud bank have scattered and obscured the direct visible light of shorter wavelength but still transmit a small fraction of the sun's infrared.

The solar image is quite flattened because the picture was taken at fairly high altitude and rays from the sun then undergo differential refraction twice - on entering and leaving the dense lower atmosphere.

The peculiar violet colour is an artifact of the camera sensor. CCD sensors respond to infrared in the range 750-1150 nm (visible light is about 400 - 700nm). Cameras therefore have to contain infrared blocking filters but these usually allow some residual rays through. The infrared sensitivity depends on the camera, test your own by pointing it at your TV remote control from a distance of 6 inches or so.

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Reference Atmospheric Optics

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