Seeing Things

Understanding Sight Requires

• Understanding Light• Understanding The Eye-Brain

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The Eye & Brain(are part of how we see)

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 that does not enter our eyes

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.

Can You see the Eye-Brain’s Feet?

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 that things are located in the direction 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

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 Line Spectrum is from light coming from 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 back down 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 can not hear some high sound frequencies that dogs can.

Similarly our eyes can not “see” some of the colours of light.

For instance you can not 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 can not see water heating in a Microwave oven. Our eyes are not sensitive to the “colour” the water can absorb.

The “colour” water is heated by is called Microwave Radiation

The Visible Light Spectrum is a very small part of a much larger spectrum called the Electromagnetic Spectrum

Radio Waves

Microwaves Infra Red Visible Spectrum

Ultra Violet

X-Rays Gamma Rays

Raul's Mother Is Visiting Uncle Xavier's Garden

Rabbits Mate In Very Unusual eXpensive

Gardens

Raging Martians Invade Roy G. Biv.

Using X-rays & Gamma

Rays

Mnemonics for remembering

the Electromagnetic Spectrum

from Long to Short Wavelength

Gamma Rays X-Rays Ultra Violet

Visible Spectrum

Infra Red

Microwaves Radio Waves

Girls/Guys

eXperience

Unusual

Vibrations

In My Room

My Favorite from Short to Long Wavelength

Visible Light can 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 observers eye.

Rays of light travel from the object to the observers eye through a Medium. A Transparent Medium allows nearly all the rays to pass straight through unaltered. Air is a transparent medium.

An Opaque Medium absorbs or scatters all the rays. A text book is an opaque medium.

A Translucent Medium 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: 1) Hold your left and right

forefinger about 30 cm in front of your eyes. Hold them horizontally about 2-3 cm apart.

2) Focus your eyes at a far point. Do not focus on your fingers.

3) Wiggle the fingers slightly up and down.

4) 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 brain/eye uses to 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 / Brain

Objects 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 Objects apparent size (dotted shapes) can be confused.

The Eye-Brain may assume both objects are at the same distance. Then distant object look small compared to close object.

Animals with eyes on the side of their heads can only see the object with one eye at a time. They can not use the triangulation method to determine the Object’s Location. These animals must use the Objects 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 near the horizon than it does while higher up in the sky.

One of the possible explanations of the Moon illusion is the eye is confused about the location of the moon and surrounding Objects. Therefore the apparent size is confused.