2021-01-23NewScientist

54 | New Scientist | 23 January 2021

Seeing small

What is the smallest animal with

eyes and could it see a molecule?

Hillary Shaw

Newport, Shropshire, UK

It depends what one means

by “see”, “eye” and “molecule”.

A single cell could be light-

sensitive, but it would only “see”

shades of light and dark. Does

seeing imply some degree of

comprehension, rather than

just instinctual action such as

light-avoidance? That would

necessitate an eye that could

focus and form an image on a

retina and a brain capable of

understanding that image.

The smallest image that could

then be “seen” would be the size

of one light-detecting cell, which

is considerably larger than a

molecule, unless you count

things such as long-chain

polymer plastics as a molecule.

We are almost certainly the

only animal whose brain “knows”

what a molecule is, anyway.

Lewis O’Shaughnessy

Nottingham, UK

Camera-type eyes like those

possessed by humans, with a lens

to focus light captured through

an aperture onto photoreceptors,

are fascinatingly complex with

many different tissues playing

unique and essential roles.

This complexity limits the

minimum size of such eyes, which

is why smaller animals often have

proportionally larger eyes. Still,

this kind of eye can be remarkably

small – minute vertebrates

possess these complex eyes,

including Brookesia micra, a

tiny species of chameleon.

For even smaller eyes, insects

are a good place to look. Their eyes

are much simpler than ours

Want to send us a question or answer?

Email us at [email protected]

Questions should be about everyday science phenomena

Full terms and conditions at newscientist.com/lw-terms

The back pages Almost the last word

GI

RO

SC

IEN

CE

/SC

IEN

CE

PH

OT

O^ L

IBR

AR

Y

and so the images are much

less clearly defined – a bit like

reducing the number of pixels

on your computer screen.

We could look even smaller

(and this is where we stretch

the definition of an eye). Some

bacteria can produce light-

sensitive proteins, which might

guide their navigation. Not quite

an eye as we would normally

consider it, but not bad for

single-celled organisms.

Richard Bradford

Sheffield, South Yorkshire, UK

The problem is that, no matter

how small the animal, it wouldn’t

be able to discern an individual

molecule using visible light.

To make out an object, the

wavelength of light hitting it

has to be shorter than the size

of the object being observed.

The wavelength of visible light

ranges from about

400 nanometres (violet) to

700 nm (red) whereas a water

molecule is about 0.27 nm across

its largest dimension. Even a

large molecule, such as DNA, has

a diameter of only about 2 nm.

In a spin

Earth spins round the sun, the sun

round the galaxy and galaxies spin

on their axes. Where does all this

spin come from? The big bang?

Mike Follows

Sutton Coldfield,

West Midlands, UK

The formation of star clusters

and galaxies is due to fluctuations

in the density of cosmic matter,

and hence gravity’s pull. The fact

that all these objects are spinning

is due to the conservation

of angular momentum.

Clumps of dark matter formed

first, and acted as “seeds” for the

formation of structures, providing

gravitational potential wells that

pulled in atoms of normal matter.

If these potential wells and all

the matter falling into them had

all been perfectly spherically

symmetric (that is, symmetric

along any axis) then there would

be no spin. However, when one

atom doesn’t fall towards the very

centre of the well, it produces a

tiny torque, which becomes

accentuated as the clump of

matter collapses. This is due

to the conservation of angular

momentum, observed when

a figure skater spins faster as

they pull in their arms.

The change in spin radius as

a vast molecular cloud collapses

into a star is so enormous that

an imperceptible spin in the

cloud manifests itself as a

spinning solar system with

planets orbiting a star.

Guy Cox

St Albans, New South

Wales, Australia

It is a case of spin or fall in. After

the big bang, matter moved away

from its origin at high speed. We

know from the cosmic microwave

background that the distribution

of matter wasn’t uniform – if it

had been, the universe as we

know it wouldn’t exist.

Wherever there was a

concentration of matter, gravity

tended to pull it closer together,

and eventually the first stars were

formed. For matter to avoid falling

into a star, it had to have sufficient

angular momentum to orbit it,

forming the accretion disc from

which planets coalesced. These

planets in turn attracted material,

some of which circled around

them and became moons. The

same applies at a larger scale, with

stars attracting other stars and so

forming galaxies. The big bang

gave linear motion. Gravity

turned it into orbital motion.

This week’s new questions

Pull the other one If there are other universes, would our

universe feel their gravity? Don Kearley, Sydney, Australia

Fear and trembling Why do our hands shake when we are

nervous? Michael Crouch, Norwich, Norfolk, UK

In a multiverse, can the

different universes feel

the gravity of the others?

“ No matter how small

an eye is, it wouldn’t

be able to discern

a single molecule

using visible light”