Understanding Sight Requires Understanding Light Understanding the Eye-Brain
The Eye & Brain (- are part of how we see.) http://www.michaelbach.de/ot/mot_adaptSpiral/index.html
Meet our “Eye-Brain”. We will use these often as observers on diagrams in this unit.
The tip of the candle flame emits light in all directions. But we only see the light that enters our Eye-Brain.
We don’t see light directed away from our eyes unless it is reflected into our Eye-Brain by something.
On diagrams in this unit, we will tend to ignore all light that does not enter our Eye-Brains.
The path light takes is called a Ray.Diagrams that show how light moves from the object to the eye are called Ray Diagrams.
Light is a type of energy our eyes are sensitive to.This is called an Operational definition because it only tells us how we detect light but it does not tell us what light is.
Light travels in straight lines. This is called Rectilinear Propagation. Since no ray of light can go straight from the Eye-Brain’s feet into your eyes, you can’t see them. Experience has trained our Eye-Brain to expect that things are located in the direction that the light comes from.
Newton was the first to note that white light breaks into the colours of a rainbow when it passes through a prism. These colours are called a Spectrum.
When light from the Sun (White Light) is broken down, it’s Spectrum looks like this. White Light is made up of shades of Red, Orange, Yellow, Green, Blue, and Violet. A mnemonic for remembering the order of rainbow colours is the name Roy G Biv.
When light from some sources is broken down, it’s Spectrum looks like a series of lines. All spectrum can be further separated until they appear as a series of lines. The above LineSpectrum is from light coming from an iron heated until it gives off light.Most light sources are made up of many different colours.
Newton noted that the individual coloured lines could not be separated further. These lines are pure colours and are identified by a frequency and wavelength number.
A Conceptual definition gives a better understanding of what light is. The atoms in sources that produce light are surrounded by electrons that can be excited to higher energy levels. As these electrons fall back to their original energy level, they give off the energy in the form of light.
There are many different ways for electrons to fall back. Each jump backdown produces a different coloured line of light. The Conceptual definition of light is; Light is produced when electrons change energy levels.
There are many ways for atoms to gain energy. For instance, Incandescence is the emission of light from a body due to its temperature. The atoms gain energy by being heated.
Our ears cannot hear some high sound frequencies that dogs can. Similarly, our eyes cannot “see” some of the colours of light.
For instance, you cannot see if an iron is hot or not. Our eyes are not sensitive to the “colour” the iron emits but our hands can feel it. The “colour” hot irons emit is called Infra Red Radiation. Some snakes can see Infra Red Radiation.
Similarly, you cannot see water heating in a microwave oven. Our eyes are not sensitive to the “colour” the water can absorb. The “colour” that water is heated by is called Microwave Radiation.
The Visible LightSpectrum is a very small part of a much larger spectrum called the Electromagnetic Spectrum.
Mnemonics for Remembering the Electromagnetic Spectrum from Long to Short Wavelength My Favourite from Short to Long Wavelength
Visible Lightcan be produced from many energy sources. Astronomical objects, Bioluminescences, Triboluminescence, Chemoluminescence, Fluorescence, Phosphorescence, Incandescent, Combustion, and Light-emitting diode are types of Luminous sources .
Atoms in Luminous Objects emit light rays in all directions produced from other energy sources.
Atoms in Non-Luminous Objects scatter the light rays from Luminous Objects in all directions.
Atoms in all objects produce or scatter light rays. This diagram only shows light rays from atoms at the top and bottom of the objects.
To make diagrams simpler we only show one ray of light from the top and bottom of objects.Rays illuminating Non-Luminous objects are not shown.
To make things even simpler, we sometimes only show the rays coming from the top of the object. We always only show the light rays that enter the observer’s eye.
Rays of light travel from the object to the observer’s eye through a Medium. A TransparentMedium allows nearly all the rays to pass straight through unaltered. Air is a transparent medium.
An OpaqueMedium absorbs or scatters all the rays. A text book is an opaque medium.
A TranslucentMedium transmits and scatters the rays. The medium indicates whether it is being illuminated but the object cannot be clearly seen. Wax paper is a translucent medium.
On a ray diagram, the Object’s Location or Distance from the Eye-Brain is where all light rays appear to originate or come from.
To make the ray diagram simpler, we often only use one ray entering the eye. In this case, the Object’s Location or Distance from the Eye-Brain is where all rays originate if the eye changes position.
Two eyes in front of a head can determine the object’s location better because the rays entering each eye can be used by the Eye-Brain to triangulate where the object is. This is called Depth Perception. Try the Pencil Touch Activity
The effect of two eyes on depth perception of the Eye-Brain is at the root of many visual curiosities. One of the oldest is called the Floating Finger. Floating Finger Procedure Hold your left and right forefingers about 30 cm in front of your eyes. Hold them horizontally about 2-3 cm apart. Focus your eyes at a far point. Do not focus on your fingers. Wiggle the fingers slightly up and down. Try closing one eye at a time.
Floating Finger Did you see a piece of finger floating in the air? Explanation: The eyes are focused on a point located far away. The Eye-Brain uses two slightly different sets of rays to triangulate the distance. But we still see objects that are closer to the eyes.
Left Eye Image The image from the left eye and the image in the right eye are combined in our brain. We see the overlap finger. The overlap is seen more clearly because it is seen by both eyes. You cannot see the floating piece of finger with one eye closed. We are not able to see depth well with only one eye. If we use only one eye, image depth can only be determined by their relative size.
Right Eye Image The image from the left eye and the image in the right eye are combined in our brain. We see the overlap finger. The overlap is seen more clearly because it is seen by both eyes. You cannot see the floating piece of finger with one eye closed. We are not able to see depth well with only one eye. If we use only one eye, image depth can only be determined by their relative size.
Images from Both Eyes combined in the brain The image from the left eye and the image in the right eye are combined in our brain. We see the overlap finger. The overlap is seen more clearly because it is seen by both eyes. You cannot see the floating piece of finger with one eye closed. We are not able to see depth well with only one eye. If we use only one eye image depth can only be determined by their relative size.
The effect of two eyes on depth perception of the Eye-Brain is also used in Magic Eye or Random Dot pictures.
The apparent Size of the object depends on the angle between rays coming from the top and bottom of the object.Large objects have a large angle between the rays.
Objects that are farther away have a small angle between rays coming from the top and bottom. This makes them appear small to the Eye-BrainObjects that are close have a large angle between the rays, and the object appears larger.
If the Eye-Brain is confused about the location of the objects, then the object’s apparent size (dotted shapes) can be confused.The Eye-Brain may assume both objects are at the same distance. Then the distant object looks small compared to the close object.
Animals with eyes on the side of their heads can only see the object with one eye at a time. They cannot use the triangulation method to determine the object’s location. These animals must use the object’s apparent size to determine its location. Their depth perception will be poor.
The Moon Illusion is an optical illusion in which the Moon appears larger when it is near the horizon than it does when it is higher up in the sky.