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Science 8 Chapter 6 – Human Vision and Extended Human Vision Notes

6.1 Human Vision

• Human eyes can focus on objects both near and far and adapt to the brightest of

lights and dimmest of moonlight.

• Humans have one vision system for colour and another for shades of grey.

• How you ask? Let’s look closely at the human eye.

• Light enters your eye through the PUPIL.

• The pupil is an opening that appears dark because light passes through it without

reflecting back.

• The IRIS is the coloured circle of muscle surrounding the pupil. The iris is the

structure we refer to when we speak about the colour of someone’s eye.

• Covering the iris and pupils is a transparent tissue called the CORNEA. The cornea

is made up of cells that are transparent enough to let light through.

• Surrounding the cornea is an opaque tissue called the SCLERA. We see the sclera as

the white region surrounding the iris.


• Behind the pupil is a flexible convex lens. The light rays pass through the lens and

are focused on a screen at the back of the eye, which is called the RETINA.

• The retina is where images are formed. Special light sensitive cells (rods and cones)

detect the image on the retina.

• Other cells convert these images into electrical signals which are sent to the brain

through a thick nerve called the OPTIC NERVE.

• Light passes through a focussing system that involves the cornea, lens, aqueous and

vitreous humour.

• AQUEOUS HUMOUR both supports and provides nutrients to the cornea.

• VITREOUS HUMOUR provides supports for the lens and gives the eye shape.

• The light rays are focused as soon as they pass through the cornea.

• The cornea refracts incoming light rays so they converge on the retina.

• The cornea does most of the focusing and the lens does the fine focusing.

• The lens has the ability to fine tune our focus by automatically changing its shape.

• When the eye muscles contract there is more tension on the lens and this allows the

lens to become thicker.

• This thicker lens allows us to see near objects.


• When looking at objects in the distance, these same muscles relax, increasing the

tension on the lens making it thinner.

• Try putting a finger really close to your eye and try to focus, what do you notice or

feel?

• How does the eye form an image?

1. Light enters the eye from the object, this is the light which is reflected back and

gives the object it’s colour

2. Light rays which are at the top of the object come together at the bottom or the

retina

3. Light rays which come from the bottom of the object come together at the top of

the retina.

4. The image that is formed is inverted or upside down

5. The image which is formed is then transmitted to your brain which interprets the

image as being upright.


• The area where the optic nerve enters the retina does not have any light sensing

cells. This area is known as the BLIND SPOT.

The Blind Spot

Close your left eye and hold this page about 8-9 inches from your right eye. Look at the cross and slowly move the page forward and backward until the black circle disappears. This will happen when light rays from the black circle fall on the place where the optic nerve enters the retina - commonly referred to as your "blind spot".

• Now that we can form the image, where does colour come from?

• Once the image is focussed on the retina, the cells that absorb the light can do their

work.

• Some cells detect bright light and others detect low light.

• These cells come in two basic shapes, RODS and CONES.

• Rod cells can absorb almost any colour of light, but they absorb green light

particularly well. Even so, the rod cell does not tell us about colour, just shades of

light and dark.


• Thus, rods are responsible for our black and white vision. In low light conditions,

rods help us to see shapes and movement.

• Cones cells allow us to see colours. We have three kinds of cone cells, which each

possess some slightly different pigments.

• Recall that by using red, green and blue it is possible to see all the colours.

• If the brain gets an equal amount of all three pigments we see white. The

combinations and balances of the pigments is what makes us see colour.


• Great, so now that I know how to see, why do I need glasses?

• When people have troubles focusing they often will get glasses to correct their

vision.

• The shape of your eye can change. This change of shape can affect your ability to

focus and thus you may require glasses.

• As people get older, the flexibility of their eyes decreases, making it harder to focus

on objects

Normal Vision

• Light from a distant object enters the cornea and converge at the retina producing a

clear image.

Near-sighted Vision

• People who can see nearby objects but cannot bring distant objects into focus.

• Concave lenses are given to these people to bring spread out the light and make a

clearer image.


Far-sighted Vision • People who can see distant objects but cannot bring near objects into focus.

• Far sighted people have convex lenses that help focus the light into the eye and

allow them to see objects that are close up


• Some people need vision correction because their cornea has a distorted shape. This

condition is known as ASTIGMATISM.

• A normal cornea looks like a soccer ball (spherical); an astigmatic eye has an

irregular shape.

• This condition causes an image to be focused on more than one point on the retina,

resulting in blurred vision.

• Astigmatism can be corrected using glasses, contact lenses or laser eye surgery.

• BLINDNESS can be any vision impairment that keeps people from doing important

life activities.

• Most can detect some light, but some people may not be able to detect any light at

all.

• Most people who are legally blind can perceive some light or even have limited

amount of vision.

• Snow Blindness: painful condition that is temporary or partially blinding caused by

overexposure to the glare of the sunlight, especially in the snow planes. Treatment

includes resting in a dark room.

• Night Blindness: condition in which it is difficult or impossible to see in dim light.

Most common cause is the rod cells losing their ability to respond to light.


• Colour Blindness: ability to only see in shades of grey. Occurs in 1 to every 40 000

people.

• Although colour blindness is rare, colour deficiency is rather common. Occurring in

about 8% of males and 1% of females.

• Colour vision deficiency is an inability to distinguish certain colours. Most common

is the inability to tell red and green apart. Both may appear yellow.

Colour Blindness from Birth

***Colour Blindness Activity***


6.2 Extending Human Vision

• Microscopes use two convex lenses with

relatively short focal lengths to magnify small

objects.

• Some Telescopes work the same way that

microscopes do. However, the objective lens in a

telescope has a longer focal length because the

objects viewed are further away.

• The simplest telescopes use two lenses. These lenses bend the light to focus it,

which is way a telescope designed this way is called a REFRACTING

TELESCOPE.

• The objective lens limits this telescope because you need to increase the lens size to

increase the magnification. Weight problems, tubes design problems etc


• Cameras work by gathering and bending light with a convex lens. The lens projects

an image onto a light detector and records a digital image of the scene.

• Cameras have similarities to Human Eyes:

Human Eye Camera Lens Lens Iris Diaphragm Retina CCD Detector Eye Lid Lens cap

• Laser light: have all the same wavelengths, all rays are traveling in the same

direction and all of the crests and troughs are lined up.

• Because lasers only contain one wavelength, the light does not refract into a

rainbow.

• Lasers are routinely used to remove cataracts, correct vision, re-attach retinas, stop

bleeding, and reshape corneas, all through laser surgery.


• Simple way to transport light is to use an optical fibre.

• Optical fibres are transparent glass fibres that can transmit light from one place to

another.

• Total Internal reflection occurs in optic fibres, which is a type of reflection in which

light strikes a boundary between two materials and is completely reflected.

• This type of technology allows us to transmit images insides a wire, which makes

television, internet, and video signals possible!

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