In search of light

Light from the Sun manifests itself in various forms. Tapping into one such form called polarisation, certain birds, reptiles and mammals find their way day and night, writes S Ananthanarayanan

Birds are known for their remarkable quality of vision. In proportion to their body, they have bigger eyes. The colour-sensitive nerve endings in the eyes of birds, fish and reptiles are of four kinds, against only one kind that humans have, and this helps some of them see in the ultra-violet light.

Insects have compound eyes, which help them cover a very wide angle of view by each eye and also helps in detecting any fast movements. And then, some snakes have ‘pits’, which are similar to eyes and can detect the warmth of prey in pitch darkness.

But a remarkable feature of birds and insects is that they can sense a property of light called polarisation, which helps them deduce the position of the Sun, even if it has just set or is covered by clouds. In fact, some birds can even tell the position of the moon with the help of polarisation.

For instance, kites and hawks have forward-facing eyes that give them the benefit of binocular vision. This particular phenomenon also helps in the migration of numerous birds across the world.

But the best that human eyes can do is to tell whether the light is bright or dim, although the property of polarisation of light is used in special sunglasses to cut the glare due to reflected light.

Stefan Greif, Ivailo Borissov, Yossi Yovel and Richard A Holland, scientists at Belfast, Seewiesen, Germany and Tel Aviv report in the journal Nature Communications, that they have found that bats  can make out the polarisation of light, and this helps calibrate their magnetic compass, so that they can navigate at night.

Bats use sound waves to detect and pinpoint prey, but they need other means to know their way about and get back to their roosting place after a night out in search of food.

What is polarisation?

Polarisation is a property of waves which arises because of movement or oscillation in different directions. That is, the movement happens in a direction perpendicular to the wave and not in sync with it.

The other kind of wave is like sound wave, or a wave that travels down a coiled spring, when one end is struck. In this kind of wave, the movement is to and fro, in the same direction as the wave.

The case of light is like that of the rope, except that a beam of light consists of billions of waves, with the rope being shaken in every possible direction. When there is a fixed direction, we say that the wave is polarised in that direction.

The waves on water are hence, always polarised in the vertical, that is, the up-down direction, and the rope in the direction that the person shakes the loose end. But in a beam of light, everything is mixed up and there is no polarisation.

The direction of movement of electric and magnetic effects in the case of light  (which is an electro-magnetic wave) becomes important when light passes through some materials or when it reflects off a surface.

When passing through some materials, the speed of light depends on the direction of polarisation. The phenomenon of polarisation is strongest at dusk, when the Sun’s rays are slanting.

Red, hot ball of fire

The hot parts of the Sun which give rise to sunlight have all kinds of orientations and the light is not polarised. But the brightness in the daytime is never direct sunlight but is light that is reflected or scattered light which is generally polarised to some extent. The direction of polarisation depends on which way the Sun is shining.

This is the feature that helps birds and insects to work out the location of the Sun, and hence get a sense of direction.

The way birds and insects can work this trick has to do with the structure of their eyes. Detection of light in the eyes is set in motion by a reddish-purple biological pigment called rhodopsin, which bleaches instantly when exposed to light, and then gradually regains its original colour. But the bleaching action is fastest if the axis of polarisation is along the axis of the rhodopsin molecule itself.

In humans and mammals, the rhodopsin molecules are randomly oriented and the detection is the same for all kinds of light. But in insects and some other animals, the photoreceptor cells are in tubes, called microvilli, which are arranged in parallel.

As the rhodopsin molecules are aligned to the long axes of the tubes, the visual cell itself is sensitive to light of a given polarisation.

During the day, birds find their way around using stationary objects, like houses or trees, which are clearly visible, and also the position of the Sun. But in the dark, they use the direction of the earth’s magnetic field.

To use the Earth’s magnetic field, during the night, the birds first need to correlate the direction of the field with visual signposts. This action is possible even when the Sun is not in sight, thanks to birds being aware of polarisation.

Mammal abilities

While birds and insects have been known to make use of the polarisation of light, this ability has not been seen among mammals. In this context, the discovery that the female greater mouse eared bat uses polarisation cues at sunset to find her way back home, with the help of the earth’s magnetic field, opens a new area of study, of the working of the visual biology of mammals.

The trial was with 70 adult female bats, who were exposed to light of different polarisations in experimental boxes that simulated the sky at sunset. The direction the bats took to go back home when it was dark changed according to the direction of polarisation of the light in the boxes.

This is a surprising discovery, as bats do not have any of the structural features of the eyes of birds, insects or fish, to be able to perceive polarisation of light. How the bats detect polarisation of light and how they use it needs to be studied and analysed to further understand their
biology.

(The writer can be contacted at [e-mail])