A: To turn invisible, at night...

The sky is dark, overcast, and starless. It’s between 1959 and 1999 when Anthony R.E. Sinclair studied buffalo and wildebeest in Serengeti Park, Tanzania. A hill looms as a darker mass against the dark sky; black treetops fringe the dark-blue horizon.

[Corel] A zebra’s stripes may make him invisible at night when lions prowl.

Sinclair dons "night vision" goggles to watch the animals with enhanced cat-like sight. Now the ground appears black and the sky a greenish glow on the phosphorous screen. A wildebeest silhouettes black against the screen’s sky. Suddenly one moving shape disappears, then reappears seconds later. Sinclair blinks and wonders. It happens again and again.

He takes off his goggles and sees nothing amiss. Puzzled, he plays a spotlight over the scene. Zebras! —among the wildebeest. The zebras were invisible in the dark —even through night goggles. The wildebeests—wandering behind the "night ghost" zebras— had disappeared.

Human (and cat) eyes have two light sensors— rods and cones. We use the motion-detecting rods at night and the color-detecting cones in daytime. The rod sensors may not see stripes at night that the cones do so easily in bright daylight.

"Aha!" Thought Sinclair. Zebras don’t care how conspicuous they appear in the day. Lions, their predators, hunt at night. That’s when zebras need to vanish and they do.

Other theories abound but I like the night-invisible theory best. Simple camouflage doesn’t cut it because a zebra’s reaction to danger is movement—not freezing. They are noisy, active beasts that live in the open (and run fast: 40 mph, 64 kph). However, maybe moving camouflage works. The mingling stripes of a herd in motion may confuse a lion, making it difficult to pick a single target.

One other idea makes sense: identification. Each zebra’s stripes form a unique pattern, like fingerprints. Mothers and foals identify each other in a flash. Plains and Mountain Zebra families form for life; their members find each other by stripes.

Tests have shown that black and white stripes painted on flat boards attract zebras. Stripes foster a strong herd instinct by pulling zebras together to face danger or flee together.

By the way, the zebra is a black animal with white stripes. An animal found in 1977 had white splotches and rows of white dots (poorly formed stripes) on a black background. So, striping results from suppressing the dark color (melanin).

Carl Anderson, Regensburg University: Zebra notes

Video for cats

Serengeti: The story of zebra stripes

Michael Kantner: Herds of zebra information

Q: White light is composed of all wavelengths, right? So what causes dispersion if all colors travel at the same speed? Why does violet bend more than red in a prism or a transparent medium? —Michael B.

A: White light is composed of all wavelengths or colors. Right. However, all colors don’t travel at the same speed through a medium. Light traveling through a vacuum goes at speed "c" ("c" = 299,790 km/s). Light waves oscillating through a medium goes slower than "c." How much slower depends on how much the light interacts with the medium’s vibrating electrons. If the color (i.e., light frequency) matches the electron’s natural or resonant frequency, the light interacts most and therefore slows most.

The electrons of air, water, and glass have a resonant frequency in the ultraviolet part of the spectrum. (That’s why glass absorbs ultraviolet rays from the Sun and protects you from sunburn.) That’s also why higher frequency colors near the ultraviolet (such as violet) slow more than lower frequency colors like red.

Since violet and red travel at different speeds through the glass prism— they bend by different amounts, which causes the dispersal. Moreover, they bend by different amounts because light picks a path through a medium that takes the least time. Violet is slower than red light so it must spend less time in the glass and go a shorter distance. That makes it bend more. (Related: figure illustrating how the colors bend.)

Further Surfing:

WonderQuest: Rainbows

Tom Henderson, The Physics Classroom: Refraction

Q: I'm reading a book about the Crusades. I wonder if there ever were female knights. I mean, not just women wearing the title of knight, but also wearing armour and fighting from horseback with lances? —Bert Meijers, Vaal, The Netherlands

A: Women did wear armor and fought from horses like knights during the Crusades. Probably none had been formally knighted.

[Corel] Heavy chain mail armor covered the body and the full-faced helmet covered the face. Impossible to see inside—was it a woman?

During the Second Crusade, Ida of Austria fought in the company of Duke Welf of Bavaria, and disappeared during the battle of Heraclea (1101) when Western forces were wiped out.

Two Muslim sources tell of dead women found on the battlefield after a battle outside Acre on 25 July 1190. Western sources say only ‘common people’ fought on that day so these women probably fought on foot. The common folk had lost faith with incompetent noble commanders and attacked the Muslims on their own.

An Arab historian, Imad al-Din, wrote of one Western noblewoman who, during the Third Crusade (1190 - 1192), "was a queen in her own land, and arrived accompanied by five hundred knights with their horses and money, pages and valets, she paying all their expenses and treating them generously out of her wealth. They rode out when she rode out, charged when she charged, flung themselves into the fray at her side, their ranks unwavering as long as she stood."

(Answered May 2, 2003)