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The human eye

Investigation : model of the human eye

This demonstration shows that:

  1. The eyeball has a spherical shape.
  2. The pupil is a small hole in the front and middle of the eye that lets light into the eye.
  3. The retina is at the back of the eyeball.
  4. The images that we see are formed on the retina.
  5. The images on the retina are upside down. The brain inverts the images so that what we see is the right way up.

You will need:

  1. a round, clear glass bowl
  2. water
  3. a sheet of cardboard covered with black paper
  4. a sheet of cardboard covered with white paper
  5. a small desk lamp with an incandescent light-bulb or a candle and a match

You will have to:

  1. Fill the glass bowl with water.
  2. Make a small hole in the middle of the black cardboard.
  3. Place the black cardboard against one side of the bowl and the white cardboard on the other side of the bowl so that it is opposite the black cardboard.
  4. Turn on the lamp (or light the candle).
  5. Place the lamp so it is shining through the hole in the black cardboard.
  6. Make the room as dark as possible.
  7. Move the white cardboard until an image of the light bulb or candle appears on it.

You now have a working model of the human eye.

  1. The hole in the black cardboard represents the pupil. The pupil is a small hole in the front of the eyeball that lets light into the eye.
  2. The round bowl of water represents the eyeball.
  3. The white cardboard represents the retina. Images are projected onto the retina and are then sent to the brain via the optic nerve.

Tasks

  1. Is the image on the retina right-side up or upside down? Explain why.
  2. Draw a simple labelled diagram of the model of the eye showing which part of the eye each part of the model represents.

Structure of the eye

Eyesight begins with lenses. As light rays enter your eye, they pass first through the cornea and then through the crystalline lens . These form a double lens system and focus light rays onto the back wall of the eye, called the retina . Rods and cones are nerve cells on the retina that transform light into electrical signals. These signals are sent to the brain via the optic nerve . A cross-section of the eye is shown in [link] .

A cross-section of the human eye.

For clear vision, the image must be formed right on the retina, not in front of or behind it. To accomplish this, you may need a long or short focal length, depending on the object distance. How do we get the exact right focal length we need? Remember that the lens system has two parts. The cornea is fixed in place but the crystalline lens is flexible – it can change shape. When the shape of the lens changes, its focal length also changes. You have muscles in your eye called ciliary muscles that control the shape of the crystalline lens. When you focus your gaze on something, you are squeezing (or relaxing) these muscles. This process of accommodation changes the focal length of the lens and allows you to see an image clearly.

The lens in the eye creates a real image that is smaller than the object and is inverted

( [link] ).

Normal eye

Defects of vision

In a normal eye the image is focused on the retina.

Normal eye

If the muscles in the eye are unable to accommodate adequately, the image will not be in focus. This leads to problems with vision. There are three basic conditions that arise:

  1. short-sightedness
  2. long-sightedness
  3. astigmatism

Short-sightedness

Short-sightedness or myopia is a defect of vision which means that the image is focused in front of the retina. Close objects are seen clearly but distant objects appear blurry. This condition can be corrected by placing a diverging lens in front of the eye. The diverging lens spreads out light rays before they enter the eye. The situation for short-sightedness and how to correct it is shown in [link] .

Short-sightedness

Long-sightedness

Long-sightedness or hyperopia is a defect of vision which means that the image is focused in behind the retina. People with this condition can see distant objects clearly, but not close ones. A converging lens in front of the eye corrects long-sightedness by converging the light rays slightly before they enter the eye. Reading glasses are an example of a converging lens used to correct long-sightedness.

Long-sightedness

Astigmatism

Astigmatism is characterised by a cornea or lens that is not spherical, but is more curved in one plane compared to another. This means that horizontal lines may be focused at a different point to vertical lines. Astigmatism causes blurred vision and is corrected by a special lens, which has different focal lengths in the vertical and horizontal planes.

Questions & Answers

which gas is ideal gas
ANAMIKA Reply
how to calculate emf when you are given two circuits with different external resistance, current but no internal resistance?
Andiswa Reply
you have to find the current of whole both circuits nd add em...
Amanzialwandle
calculate the distance between two charge +4nc and -3nc if electrostatic is 0.005N
Malaza Reply
use the coulombs law equation where there is F u substite by 0.005 then u cross multiple
Neldo
what is atomic
motheo Reply
what is charge
motheo
why do we need to add oil to an engine
Kamohelo
we need to add all to an engine 2 lubricate the bearings and the shops to avoid friction that will lead to overheating and knocking
Easyas
charge is a unit of matter that expresses the extent to which it has more or less electrons than protons
Easyas
how to calculate the distance between Q1 and point P if you are given distance between point P and Q2
Lwando Reply
how do we calculate the magnitude of the force between two objects
Ntombi Reply
we use Newtons law ...fnet=ma
Fikile
how do you calculate the enery change in enthalpy reaction, can someone explain it to me guys
Buyie Reply
∆H(enthalpy change)= Hproducts- Hreactants.... use this formula, H stands for enthalpy which is regarded as energy
Andiswa
atomic model john dalton
Alliieyza Reply
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crippah
what is a normal force
tumelo Reply
normal force is the line that is perpendicular to the surface
Thabo
normal force is the force exerted to an object by the surface , perpendicular to the surface
Fhatani
I want to join
Londiwe
better explanation of FARADAY'S LAW OF INDUCTION
Kgaugelo
what is the formular of energy
Wilka Reply
E=f\q
Wonder
E=hf
Mondli
doesnt it like depend on the type of energy
Benjamin
what does (1s2 2s2 and 2p6) mean?
Ryan Reply
That is Albau Diagram I guess!! it represent number of electrons in each energy orbit!! the numbers 1s, 2s&2p, 3s and so on represent energy orbit and then the other numbers after s & p represent number of electrons
Andiswa
That is SP notation
Kgaugelo
What are the differences between pd and emf?
Santosh Reply
How to calculate magnitude of Friction force
Tanaka Reply
in Newton
Obakeng
yeh
Obakeng
in newton
Fikile
Onother Question?
crippah
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Fikile
First of all in wat Grade ae u..?
crippah
grde 11
Fikile
Oka nd hw is the properties of a substance afected by the intermoleculaer forces...
crippah
hi guys I m in grade 11 I need help with electrostatics
Serati
what is newton
Ndamulelo Reply
look for the x components n y components then after y add the x components separate n y separate then u use the Pythagoras theorem 2 find the resultand
Abigail Reply
Thank you very much
Luvuyo
how do I find 🔍 the critical angle
Ntandokazi
angle of incident and angle of reflection must always approach normal line thats where u will see 90° which is critical point
Tumelo

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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