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Retinal image

It is the cornea-lens system that determines where light falls on the surface of the retina which results in discernible images.

The images are inverted and obviously very small compared with the world outside that they resolve. The inversion poses no problem. Our brains are very flexible and even when tricked by prisms will always turn the world right-side-up given time. The reduction in size is part of the process by which the fit of the image on the retina determines our field of view.

The images are real in the sense that they are formed by the actual convergence of light rays onto the curved plane of the retina. Only real images of this kind provide the necessary stimulation of rod and cone cells necessary for human perception.

Rods and cones

Both the photosensitive rods and cones form a regularly spaced mosaic of cells across the entirety of the retina – bar the absence of rods in the fovea centralis. Because there are 100 million rod receptors and 20 million cone receptors in each eye, rods are packed more densely per unit area. The synaptic connections of both rods and cones vary in function in different locations across the retina, reflecting the specialisations of different regions. This, for example, allows the eyes to deal with daylight and darkness and with what we see at the centre and periphery of our field of view.

Rods and cones are easily distinguished by their shape, from which they derive their names, the type of photo pigment they contain and by the distinct patterns of synaptic connections with the other neurons around them.

Neurons (nerve cells) are present throughout the human central and peripheral nervous systems and fall into three main categories: sensory, motor and interneurons. Rods and cones are both sensory neurons. Rods don’t produce as sharp an image as cone cells because they share more connections with other types of neurons. But a rod cell is believed to be sensitive enough to respond to a single photon of light whilst cone cells require tens to hundreds of photons to be activated.

The principal task of rod and cone cells alike is photo-transduction. This refers to the type of sensory transduction that takes place in the visual system. It is the process of photo-transduction that enables pigmented chemicals in the rods and cones to sense light and convert it into electrical signals. Many other types of sensory transduction occur elsewhere within the body enabling touch and hearing for example.

References: Functional Specialization of the Rod and Cone Systems: https://www.ncbi.nlm.nih.gov/books/NBK10850/

Retinal input

Visual input is initially encoded in the retina as a two-dimensional distribution of light intensity, expressed as a function of position, wavelength and time. This retinal image is transferred to the visual cortex where primary sensory cues and, later, inferred attributes, are eventually computed in the process of actualising our perceptions. Parallel processing strategies are employed from the outset to overcome the constraints of the individual ganglion cell’s limited bandwidth and anatomical bottlenecks as data approaches the optic nerves that connect each eye to the visual cortex.

References: DeYoe and Van Essen (1988): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2771435/

Retina

THE RETINA

Human beings see the world in colour because of the way their visual system processes light. The retina contains light-sensitive receptors, rod and cone cells, that respond to light stimuli. It is the variety of wavelengths and intensities of light entering the eyes that produces the impression of colour.

The retina is the innermost, light-sensitive layer of tissue inside our eyes. It forms a sheet of tissue barely 200 micrometres (μm) thick, but its neural networks carry out almost unimaginably complicated feats of image processing.

The physiology of the eye results in a tiny, focused, two-dimensional image of the visual world being projected onto the retina’s surface. Because of the optics of lenses, it appears upside down and the wrong way around. But no worry, sorting that out is child’s play for the human brain! The real challenge is that the photosensitive receptors in the retina must produce precise chemical responses to light and translate every minute detail of the image into electrical impulse ready to be sent to the brain where they produce visual impressions of the world. In a very limited sense, the retina serves a similar function to a photosensitive chip in a camera.

As research continues to reveal ever-increasing amounts of detail about these signalling processes across and beyond the retina, it required new thinking, not only of the retina’s function but also of the mechanisms within the brain that shape these signals into behaviourally useful perceptions.

The retina consists of 60-plus distinct neuron types, each of which plays a specialized role in turning variations in the patterns of wavelengths and intensities of light into visual information. Neurons are electrically excitable nerve cells that collect, process and transmit vast amounts of this information through both chemical and electrical signals. Retinal neurons work together to convert the signals produced by a hundred and twenty million rods and cones and send them along around one million fibres within the optic nerve of each eye to connections with higher brain functions. In this process rods and cones are first responders whilst ganglion cells are the final port of call before information leaves the retina.

There are three principal forms of processing that take place within the retina itself. The first organises the outputs of the rod and cone photoreceptors and begins to compose them into around 12 parallel information streams as they travel through bipolar cells. The second connects these streams to specific types of retinal ganglion cells. The third modulates the information using feedback from horizontal and amacrine cells to create the diverse encodings of the visual world that the retina transmits towards the brain.

As mentioned above, the image of the outside world focused on the retina is upside down and the wrong way around. But the human retina is also inverted in the sense that the light-sensitive rod and cone cells are not located on the surface where the image forms, but instead are embedded inside, where the retina attaches to the fabric of the eyeball. As a result, light striking the retina, passes through layers of other neurons (ganglion, bipolar cells etc.) and blood-carrying capillaries, before reaching the photoreceptors.

The overlying neural fibres do not significantly degrade vision in the inverted retina. The neurons are relatively transparent and accompanying Müller cells act as fibre-optic channels to transport photons directly to photoreceptors. However, some estimates suggest that overall, around 15% of all the light entering the eye is lost en-route to the retina. To counter this, the fovea centralis, at the centre of our field of vision, is free of rods and there are no blood vessels running through it, so optimising the level of detail where we need it most.

https://en.wikipedia.org/wiki/Retina

Refractive index

The refractive index of a medium is the amount by which the speed (and wavelength) of electromagnetic radiation (light) is reduced compared with the speed of light in a vacuum.

Refractive index (or, index of refraction) is a measure of how much slower light travels through any given medium than through a vacuum.
The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
The refractive index of a medium is a numerical value and is represented by the symbol n.
Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
If the speed of light in a vacuum = 1. Then the ratio is 1:1.
The refractive index of water is 1.333, meaning that light travels 1.333 times slower in water than in a vacuum. The ratio is therefore 1:1.333.
As light undergoes refraction its wavelength changes as its speed changes.
As light undergoes refraction its frequency remains the same.
The energy transported by light is not affected by refraction or the refractive index of a medium.
The colour of refracted light perceived by a human observer does not change during refraction because the frequency of light and the amount of energy transported remain the same.

Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
A fast (optically rare) medium is one that obstructs light less than a slow medium.
A slow (optically dense) medium is one that obstructs light more than a fast medium.
The speed at which light travels through a given medium is expressed by its index of refraction.
If we want to know in which direction light will bend at the boundary between transparent media we need to know:

Which is the faster, less optically dense (rare) medium with a smaller refractive index?
Which is the slower, more optically dense medium with the higher refractive index?

The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.

ROYGBV

ROYGBV is an acronym for the sequence of hues (colours) commonly described as making up a rainbow: red, orange, yellow, green, blue, and violet.

A rainbow spans the continuous range of spectral colours that make up the visible spectrum.
The human eye is tuned to the visible spectrum and so to spectral colours between red and violet.
ROYGBV are colours associated with a range of wavelengths rather than with unique values.
The visible spectrum is the small band of wavelengths within the electromagnetic spectrum that corresponds with all the different colours we see in the world.
The fact that we see the distinct bands of colour in a rainbow is an artefact of human colour vision.

RGB colour model

RGB colour is an additive colour model in which red, green and blue light is combined to reproduce a wide range of other colours.

The primary colours in the RGB colour model are red, green and blue.
In the RGB model, different combinations and intensities of red, green, and blue light are mixed to create various colours. When these three colours are combined at full intensity, they produce white light.
Additive colour models are concerned with mixing light, not dyes, inks or pigments (these rely on subtractive colour models such as the RYB colour model and the CMY colour model).
The RGB colour model works in practice by asking three questions of any colour: how red is it (R), how green is it (G), and how blue is it (B).
The RGB model is popular because it can easily produce a comprehensive palette of 1530 vivid hues simply by adjusting the combination and amount of each of the three primaries it contains.

Rainbow colours

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.

Rainbow

A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

Rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
Waterfalls, lawn sprinklers and other things that produce water droplets can produce a rainbow.
A rainbow is formed from millions of individual droplets each of which reflects and refracts a tiny coloured image of the sun towards the observer.
It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
When the sun is behind an observer then the rainbow will appear in front of them.

Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.

If the reflecting surface is very smooth, the reflected light is called specular or regular reflection.

Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of objects that the light has already encountered become visible to an observer.

Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in all directions and so no image is produced.