Why some Animals see much better than We do after Dark?

Why cant we see in the dark? Is it because our eyes aren't strong enough? Have we always been unable to see in the dark like before cave men?

Let's start with explaining darkness here. There is no darkness. It's just  absence of light. Now if there is no light, you can not see anything. Now for the science part. You are able to see stuff because there is light(photons,  to be specific)  emitted by that particular object which travel through space and are absorbed by photo receptive cells in your eyes (rods and cons ). When light strikes our retina, the brain creates images. When no light strikes our retina, it doesn't create images. This is what we observe as brightness during the day. There is no actual brightness in the objective reality, only in our subjective perception of it. I understand this is beyond the grasp of most people but here's a question.

What do you call the ability to visualize the shape, size and location of an object without touching it?

I call it sight. No animal can see in the complete dark, where there is a complete absence of photons. (light particles), except the ones using sound; echo location,. Our eyes convert this photons into information, which our brains convert into images. It's these images we see. When we dream we also see images and there is no light involved. Some synesthesia sufferers can hear colors. This is when sound waves trigger visual sensations. This proves that our visual sensations occur in the brain and not in the eyes. The purpose of the eyes is to detect light and convert it into electrochemical impulses. Now as we know that the visual system works on sensing and perceiving light waves.

Light waves vary in their length and amplitude:

a) Wave length (also referred to as frequency, since the longer a wave, the less often/quickly it occurs) - affects color perception (ex., red=approx 700, yellow approx 600)

b) Wave amplitude (this is the size/height of the wave) - affects brightness perception.

What does that mean? How does wave length affect out visual perception ?

The visible spectrum is the portion of the larger electromagnetic spectrum that we can see.  The electromagnetic spectrum encompasses all of the electromagnetic radiation that occurs in our environment and includes gamma rays, x-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. Human vision is confined to a small portion of the electromagnetic spectrum called visible spectrum. It extends between 450 and 750 nanometers wavelengths - a very small distance, since a nanometer (nm) is one billionth of a meter. Below 450 nm wavelength is the Ultraviolet spectrum and above 750 nm wavelength is the Infrared spectrum.

This still doesn't explain, why our night vision isn't as good as some animals ? 

Since, humans see only in the visual light spectrum, while some animals see not only in the visual spectrum but into both the Ultra-violet and Infrared spectrum as well. They can sense body heat, people cannot. Other species can detect other portions of the electromagnetic spectrum. For instance, honeybees can see light in the ultraviolet range, and some snakes can detect infrared radiation in addition to more traditional visual light cues. If you put a cat in a completely darkened room with no ambient light whatsoever, it will be blind. It will be able to still navigate by detecting objects with it's whiskers and detect air currents around said objects. Alright, this explains how wavelength is an important factor for our visual perception.

But, what about Wave amplitude ?

In humans, light wavelength is associated with perception of color . Within the visible spectrum, our experience of red is associated with longer wavelengths, greens are intermediate, and blues and violets are shorter in wavelength. The amplitude of light waves is associated with our experience of brightness or intensity of color, with larger amplitudes appearing brighter.

The process of vision cannot be understood without some knowledge about the structure of the eye:

Structure of the Eye:

• Cornea - the round, transparent area that allows light to pass into the eye. 
• Iris - the colored part of the eye, is a ring of muscle.
• Lens - which lies behind the pupil and iris, can adjust its shape to focus light from objects that are near or far away. 
• Retina - inner membrane of the eye that receives information about light using rods and cones. The functioning of the retina is similar to the spinal cord - both act as a highway for information to travel on. Light passing through the cornea, pupil, and lens falls onto the retina at the back of the eye. 
• Pupil - The iris surrounds an opening called the pupil, which can get bigger or smaller to allow different amounts of light through the lens to the back of the eye. In bright light, the pupil contracts to restrict light intake; in dim light, the pupil expands to increase light intake.

Rods & Cones - many more rods (approximately 120 million) than cones (approx 6.4 million).

• Cones - visual receptor cells that are important in daylight vision and color vision, allowing people to see in color. The cones work well in daylight, but not in dim lighting. This is why it is more difficult to see colors in low light. Most are located in the center of the retina called the FOVEA, which is a tiny spot in the center of the retina that contains only cones, visual acuity is best here. So, when you need to focus on something you attempt to bring the image into the fovea. 

Seeing In Color - we can see many colors, but only have 3 types of cones that receive information about color. We have cones that pick up light waves for red, green, and blue. But have you ever wondered that the way we see colors is the same for other species too.

    

Does color exist? People just assume that because we see colors, that they actually exist in the world. In other words, that when they see the color red, that red is a real, physical, tangible, "thing". But is it, or is color just a matter of our perception? If we had different types of nervous systems, we would see things differently (literally) and so wouldn't we think those other things we saw were the real "things"?      

• Rods - visual receptor cells that are important for night vision and peripheral vision. The rods are better for night vision because they are much more sensitive than cones.   In addition, the rods are better for peripheral vision because there are many more on the periphery of the retina. The cones are mostly in and around the fovea but decrease as you go out. To see best at night, look just above or below the object this keeps the image on the rods. 

• Role of Rhodopsin - The molecules of Rhodopsin in the rods undergo a change in shape as light is absorbed by them. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light. When exposed to light, it immediately bleaches, and it takes about 30 minutes to regenerate fully. Most of the adaptation occurs within the first five or ten minutes in the dark. Rhodopsin is less sensitive to the longer red wavelengths of light. So many people use red light to preserve night vision.

• Role of Tapetum - Many animals have a tissue layer called the Tapetum lucidum in the back of the eye that reflects light back through the retina. This increases the amount of light entering into the retina. This is found in many nocturnal animals and some deep sea animals. This causes the phenomenon of eye shine in these animals. Tapetum lucidum is absent in human eye.The shining of eyes in Dogs and Cats in vehicle head light is due to this reflective retina.

Nocturnal Animals

Nocturnal animals are more active at night than during the day. These animals sleep during the day, often in a burrow or den. Many animals, like desert animals, are nocturnal in order to escape extreme daytime heat. Special Adaptations: Nocturnal animal have special adaptations that help them survive in the dark.

The retina of nocturnal animals is almost entirely composed of rods. The other type of vision cells, cones, is absent or almost absent, leaving nocturnal animals with virtually no color vision. The photosensitive pigment inside the rods, rhodopsin, is particularly sensitive to low levels of light.

During the day, in a daylight adapted eye, the rhodopsin breaks down so rapidly, it is ineffective for visual perception. At night-time, in the rod-rich eyes of dark-adapted animals, rhodopsin is created faster than it breaks down.

Therefore, the threshold of light needed to stimulate the eye is reduced. It is just a minute fraction of the light needed to activate a cone cell for vision during the day.

However, despite being more sensitive to light, the low number of cones means nocturnal animals have sacrificed visual acuity. They must get by with somewhat fuzzy, unfocused images. Only by greatly exaggerating the size of their eyes (and therefore the retinal image), can dark-adapted animals develop reasonable resolution to their images.

Large eyes in Nocturnal Animals

Nocturnal animals have large eyes, wider pupil, large lens and increased retinal surface to collect more light. Some animal species have evolved tubular eyes as part of their evolution to collect more light. Many nocturnal animals cannot move their eyes but they have extraordinary rotational ability of the neck. For example. Owls can rotate their neck through 270°. This helps to increase the night vision. Some animals have a spherical lens and widened cornea to compensate for reduced eye movement. This along with a large cornea increases the animal’s field of vision. So they can see better in night even without moving the head. The night vision of animals is due to one or more differences in the morphology and anatomy of their eyes. These adaptations include large eyeball, large lens, large optical aperture, more rods in the retina, presence of a tapetum, etc.

Physiology of Night vision

The vertebrate eyes have Photosensitive cells called Rods and Cones. Rods are elongated cells mainly confined in the periphery of the retina. These are meant for Dim vision in low light and for peripheral vision. Rods, are extremely light sensitive and their sensitivity is about 500 times greater than the sensitivity of cones. Only one photon is required to stimulate a rod to send a signal to the brain. Nocturnal mammals have rods with unique properties that make enhanced night vision possible. The nuclear pattern of their rods changes shortly after birth to become inverted. Inverted rods have heterochromatin in the center of their nuclei and euchromatin and other transcription factors along the border. The outer nuclear layer in nocturnal mammals is thick due to the presence of millions of rods present to process the lower light intensities of a few photons. Light is passed to each nucleus individually. Cones on the other had are pointed cells confined in the central part of the retina. These are meant for Central vision, Bright vision and Colour vision. Rods have photosensitive pigment called Rhodopsin and cones have Iodopsin.

Human beings have the number of cones in their eyes greater than the owl, while owl has a large number of rods in its eyes than human beings. Cones helps us to see during the day time while rods helps us to see during the night. Both human beings and owl has rods and cones, but the percentage of them present makes the difference. Another factor that influences perception is the context of the perceiver. People’s immediate surroundings create expectations that make them see in particular ways.