The eye is meant to image a point of light, for example a star, as precisely as possible on your retina so that you perceive it as, well, a point. And objects, like the text you try to read, will appear sharp if each of their individual points is imaged as a point.

Looking at a bright star without your spectacles you get an idea of how your eyes fail at that task: what should be a point is smeared out in various ways (depending on what opticians call aberrations of the optical system, or how well your eyes can focus, or both). This means that different rays of light (from the same star) entering different parts of an eye are sent not to the expected focus point but slightly nearby, and together adding to a blurry image of what should be a point.

Now by looking through a tiny gap you cut out most of these rays, which makes the blurry patch smaller, more like a point, and improves the sharpness of your vision.

Of course there are limits to that "stopping down": the image gets dimmer and, with a really small gap, the wavy nature of light actually starts increasing the blur again.

Cool, thanks for posting!

As others have said, light's behavior depends on the circumstances.

Demonstrating its wavy nature is very easy. All you need is some point light source (e. g. a dia projector with a piece of carboard with a small hole over the objective, a street lamp far away, &c. – do NOT use a laser, you don't want to shine that in an eye) and a piece of aluminium foil with a pinhole in it. Looking at the first through the second you'll see an Airy disk with some rings around it.

It gets mind-blowing if you make a second pinhole in the foil, as near as possible to the first one. Can you guess the result? Spoiler: >!You'll still see the disk and rings, but now with dark interference bands over them.!<

But exhibiting light's particle behavior is difficult and would need a physics lab. You can look up Einstein's 1905 Nobel prize for the photoelectric effect, it's about that.

The next best thing would perhaps be a cloud chamber video?

So the somewhat unsatisfying answer, as we crave clear-cut ones, is that a quantum is neither a particle nor a wave but… a quantum, even if it can behave like the one or the other, depending on the question asked (the experimental setup).

Somebody gave a metaphor for that. Take a cylinder. Seen from one direction it looks like a disk. From another, like a rectangle. Yet it is neither.

> As a result of events that occurred during the early history of mammals, eutherian mammals retain only two of the four cone opsin gene families found in many other vertebrates. Very likely during this same time frame, the elaborate system of coloured oil droplets characteristic of photoreceptors in many vertebrates were also abandoned, as was a portion of the specific retinal circuitry dedicated to processing colour information (Jacobs & Rowe 2004). These changes left most eutherian mammals with a single dimension of colour vision. Primates subsequently escaped this restriction by evolving a series of visual system alterations that provided opportunities for expanded colour vision.

From Evolution of colour vision in mammals

This means that mammals generally have poor color vision, primates like us being an exception. No need for flashy displays, they couldn't see them.

As for birds (aka avian dinosaurs), a lot of them have four color receptors, one more than our three, and their plumage is in fact more complex and dazzling than what we humans can even see. (Similar to flowers who sport designs only visible in the ultraviolet, meant for insect eyes.)

Fishes generally have good color vision, even into the ultraviolet, but as light decreases with depth those living deep down can be left only with rods in their retinas, i. e. be color blind.

In both cases, fishes and birds, the display of colors is used to attract mates, sometimes to scare rivals away.

Fantastic shapes (which can be found in mammals too, in the form of weird, cumbersome horns, for example) are thought to be the result of sexual selection, the females (generally) "preferring" these as a sign of health and thus good genes and lack of parasites. (A bird with a ridiculously oversized tail has to be good at survival to make it to the mating season.)

A last point about fishes: water drag increases with temperature. So while the big ones like sharks, tuna, &c., are always streamlined, it makes no sense for small tropical ones, and they can indulge in attractive body forms instead. But you don't see that in cold waters.