13.6 The Human Eye13.6 The Human Eye

Recall: the eye is a cameraRecall: the eye is a camera

You should remember that from earlier lessons, the human eye is much like a camera

It has a small pinhole to allow light to enter

It has a lens to change the direction of the light path

It has “film” located at the back of the eye to pick up light and “capture” the image

The “shutter”The “shutter”

In a camera, the shutter is the small opening that allows light into the camera

In the human eye, this is your pupilThe pupil is the small black spot inside

your eye that is surrounded by your iris, the part of your eye that is coloured

http://upload.wikimedia.org/wikipedia/commons/a/a3/Eye_dilate.gif

Pupil reactionPupil reaction

The pupil DILATES (gets bigger) when exposed to dark conditions

The pupil CONSTRICTS (gets smaller) when exposed to light conditions

This helps to protect your eye from over exposure to light

Pupil controlPupil control

DID YOU KNOW…In emergency situations, for example,

when you are panicked or scared, your pupil will tend to DILATE

Why might our bodies evolve this reflex?

The lensThe lens

The lens in the human eye is much like a water balloon

It is connected to the top and bottom of your eye by connective tissue that is elastic

This allows muscles in your eye to stretch the lens

The ability of the human eye to change the path of the light ray is known as ACCOMODATION

When the lens is stretched, it becomes thinner. Think of what happens if you pull on either side of a water balloon. This decreases the lens’ ability to change the light ray’s path.

When the lens is not stretched, it becomes thicker. This increases the lens’ ability to change the light ray’s path.

The retina: the “film”The retina: the “film”

When the light rays reach the back of the eye, it hits the RETINA which is composed of many ROD and CONE cells

ROD cells are more sensitive to light and are used for night vision

CONE cells are less sensitive to light (but can distinguish colour) and are used for vision during the day

The retina contains blood vessels that keep the cells healthy

FOVEA: contains the highest concentration of cone cells - this area is responsible for processing most of the incoming light

BLIND SPOT: contains no cone cells because all the blood vessels feeding the eye exit and enter here

A change of shapeA change of shape

The energy of the light rays causes the cells to change shape

This initiates an electric signal that travels towards the brain

The brain then interprets this signal

Mixed up visionMixed up vision

Because both our eyes receive information, our brain has to sort information coming in from our right and left sides

Remember that light entering into our eyes always lands in a position opposite to where it came from

If it came from the top, it will end up a the bottom

If it came from the left, it will end up on the right

It is a combination of this information that our brain uses to figure out where the object is in front of us

Our ability to figure out depth and position is known as STEREOSCOPIC VISION and is maximized when both our eyes are being used

Did you know…Did you know…

Optical illusions are a good way to help us understand that our eyes are complex information systems that can be “fooled”

It isn’t just about how light enters our eyes – our brain takes this information and makes many computations to help us understand motion, depth, colour and shape

Optical illusionsOptical illusions

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