Tag Archives: Optics

Banks et al.: Why Do Animal Eyes Have Pupils Of Different Shapes?

Human eyes have round pupils, but there is considerable variation in the animal kingdom, from the vertical slit pupil of a cat, to the horizontal slot of a goat, as pictured above. So Martin S. Banks and his colleagues asked the question “Why?” in an article published in Science Advances in August 2015. They started … Continue reading Banks et al.: Why Do Animal Eyes Have Pupils Of Different Shapes?

Fata Morgana

As the weary traveller seesIn desert or prairie vast,Blue lakes, overhung with trees,That a pleasant shadow cast; Fair towns with turrets high,And shining roofs of gold,That vanish as he draws nigh,Like mists together rolled,— Henry Longfellow “Fata Morgana” (1873) I took the photograph above in Kolyuchin Inlet, in the Russian Far East, one evening in … Continue reading Fata Morgana

More About Converging Rainbows

A couple of months ago I received this lovely picture from Mick Shaw, which I use with his permission. The sun is reflecting off a thin layer of sea-water covering the sand-flats of Morecambe Bay, and producing a pair of reflected-light rainbows in tandem with the usual primary and secondary arcs. Reflected-light rainbows were the … Continue reading More About Converging Rainbows

Long-Exposure Bicycle Spokes

The Boon Companion has been experimenting with long exposure times and intentional camera movement, of late. She was just about to discard the motion-blurred cyclist above as a failed experiment when something about the image caught my eye. In the thirtieth-of-a-second exposure, the bicycle wheel has rolled a short distance. But why do the spokes … Continue reading Long-Exposure Bicycle Spokes

Why Do Mirrors Reverse Left And Right But Not Up And Down?

Reflection: A transformation under which each point in a shape appears at an equal distance on the opposite side of a given line—the line of reflection. It’s not often I have occasion to shout at the television, but a recent episode of the BBC’s long-running television series QI precipitated just such an outburst. The cause … Continue reading Why Do Mirrors Reverse Left And Right But Not Up And Down?

Tertiary Rainbows, etc

In my last two posts about rainbows, I discussed the formation of the primary and secondary rainbows, respectively, tracing their origins to specific light paths through falling raindrops. The primary rainbow ray follows a path like this:For a raindrop at the apex of the rainbow arc, sunlight enters near the top of the drop, bounces … Continue reading Tertiary Rainbows, etc

Rainbow Rays

The COVID-19 lockdown, in my part of the world, has produced an outpouring of children’s rainbow art—often stuck up in people’s windows, but sometimes sketched on the pavements, too. I’ve been struck by the generally good command of spectral colours on display, with red on the outside and an appropriate progression towards violet on the … Continue reading Rainbow Rays

The Strange Shadows Of Apollo

In a previous post, I explained how all the manned moon landings were made with the sun low in the sky behind the Lunar Module, so that long shadows accentuated terrain features, making it easier to locate a safe place to land. But this meant that the LM landed facing into its own shadow, so … Continue reading The Strange Shadows Of Apollo