For most people, sight is a hugely important part of life. Sight first evolved 550 million years ago, as light-sensitive proteins in a single-celled photosynthesiser, which allowed it to identify the direction light was coming from, increasing its ability to photosynthesise! Over time, these built up and became incorporated into specialised cells, and slowly adapted to allow for the identification of direction and shape, by forming a dip, then a pinhole (eg. the pupil) and a lens to focus the light more effectively. Finally, colour vision evolved! In humans, we have 3 different types of cone cells that are sensitive to colour- S cones peak in the blue end of the spectrum, M cones in the green area, and L cones in the yellow area. This is called trichromacy and is shared by some other primates, and even marsupials. Most other mammals are dichromats (only 2 types of cones), but Fish, Birds and Insects often have a 4th cone to detect Ultra Violet light, so are tetrachromats.
What’s the smallest thing the eye can see?
Some animals can not only see a greater variety of colours but also have better eyesight in general, able to resolve greater detail! Deciding on which animal as the best eyesight is pretty difficult, as all are adapted to their environment and lifestyle, for example, prey/grazing animals like sheep and horses have a huge range of view, as their eyes are on the sides of their heads so can actually look behind them without turning their heads, useful for mealtimes when you don’t want to end up the meal! But perhaps the crown belongs to birds of prey. They are predators, so their eyes face forwards, but some, like owls, combat this narrow line of sight by having incredibly flexible necks. As well as this, they rely on having sharp vision to spot prey from far above- an eagle can see 8 times further than human, so is able to see its prey from 2 miles away!
Even though we might not have the best eyesight in the animal kingdom, it’s still pretty good- technically, you can see anything that reflects or emits enough light to be picked up by the rods and cones in your eyes (which is only 50-100 photons of more), which allows us to see stars from thousands of light-years away, covering just a couple of milli-arcseconds! To be able to see something as an object rather than a point of light, it has to cover about 1 arcminute (or 60 arcseconds), which will activate more than one cone cell. But on the day to day scale, the eye can usually resolve about 0.04mm- roughly the width of a hair- up close, or two headlights 3km away. Obviously, this varies for different people as everyone has different eyesight, but thanks to modern technology, poor eyesight can often be corrected with surgery or glasses, and binoculars and telescopes allow you to see even further!
Seeing Cells and beyond
This 0.04mm limit means that we can see single grains of salt, and even the largest known bacterium/some of the largest cells, unaided! To go beyond this, we have to use microscopes. Typical microscopes that you might find at school are optical microscopes, which use lenses to focus light, and can resolve down to about 200nm. This means it can be used to see cells and the largest viruses. Beyond this, to observe sub-cellular structures (though some techniques such as fluorescent confocal microscopes have a higher resolution than most optical telescopes so can be used to find subcellular structures of alive cells) and the majority of viruses, we have to use electron microscopes, which see down to 0.2nm! The most powerful of these, Scanning Tunnel microscopes, can even observe atoms and the structure of molecules! Being able to visualise these nanostructures can help us to develop new medicines or tiny computer chips.
How do eyes work?
Light enters through the pupil, which is essentially just a hole in your eye! Depending on how bright it is, your pupil changes size to change the amount of light entering. Between this and the pupil is the lens and vitreous humour, which together focus light onto the retina. Most of it is focussed onto the fovea and macula, the area that produces the sharpest image. This area has many cones but no rods, meaning that it isn’t very useful for night vision, which is why, when stargazing, you might see a faint star out of the corner of your eye, but when you turn to look at it, you can’t see it! Our retinas are similar to the photometer in a telescope and are covered by two types of photoreceptor cells. The rod cells are active in low light conditions, and cones are active in bright light and are capable of colour detection!
Telescopes work in a similar way to our eyes but have a wider aperture and greater focusing power, so can collect light from further away. They also can stare at the same spot indefinitely without having to blink, so can collect more light over time! This is why to take photos in the dark/of stars, you often need a longer shutter speed/exposure time. When Hubble took its famous deep field image, it was pointed at a seemingly empty patch of sky for more than 100 hours! Hubble Extreme Deep Field had more than 20 days of exposure time by collating images spread across a decade of observation. Both images only covered 1/24,000,000 of the night sky (2.5 arcminutes, or the same angular size as tennis ball 100m away) but contained thousands of galaxies, the oldest 13.2 billion light-years away! These were probably some of the first galaxies, as the universe itself only began half a million years before that!
Last year saw the first-ever image of a black hole taken and shown to the public. This used telescopes across the world to create a telescope with an aperture the diameter of the earth! Combining this with some clever coding and simulations created a pretty awesome image!
Back to earth
Despite all of this innovation, human eyesight is still fallible, and 40,000 people develop macular degeneration, the most common cause of age-related blindness, every year, and is set to increase as life expectancy rises. Macular Degeneration is when cells around the macula (usually responsible for the clearest, central vision) start to die off. Nowadays, there are more and more ways to accommodate for blindness, such as screen readers and alt text, and braille/guide dogs have helped improve quality of life for blind people. But it’s possible that in the future, blindness will be a difficulty of the past…
The ISS has resulted in many new discoveries, and one current research project is to ‘grow’ artificial, protein-based retinas on the space station! Its thought that manufacturing them in microgravity will help make them more stable and homogeneous, meaning that they will be more likely to work safely! This could potentially return sight to millions of people across the globe (for those who can afford it, as unfortunately, I doubt this will be cheap).
Another project has used electrical stimulation with electrodes in the brain to directly create images, without the need for light entering through the eyes! The study used complex sequences of electrical pulses to trace/outline letters, which the participants ‘saw’ as bright outlines!
So… from the animal kingdom to distant galaxies and black holes, to atoms, and our own cells and eyes, it’s safe to say that the science of sight and optics has helped us to uncover some amazing things!
What’s your favourite thing to look at using a telescope? Or a microscope? Do you have a favourite type of telescope/ microscope? Let me know in the comments!
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