13.6 The Human Eye

Recall: 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” In a camera, the shutter is the small opening that allows light into the camera In the human eye, this is your pupil The pupil is the small black spot inside your eye that is surrounded by your iris, the part of your eye that is coloured mmons/a/a3/Eye_dilate.gif mmons/a/a3/Eye_dilate.gif

Pupil 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 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 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” 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 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 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… 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

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