This unit is about lenses.It talks the different types and their uses,the Human eye and the camera.


A lens is a transparent material with curved side capable of refracting light.

The curved surfaces of a lens help to bend or refract light passing through the lens.

Types of lenses.

There are two types of lenses.

  • Convex lens.
  • Concave lens.

Convex lens (converging lens)

Is a lens which is thicker in the middle and thinner at the edges.

illustration of Convex lens


  • When a parallel beam strikes a convex lens comes together at a point in front of the lens.

Concave lens (diverging lens)

  • This is a lens which is thinner in the middle and thicker at the edges.

illustration of Concave lens

illustration of  Convex meniscus and Concave meniscus

NB: The converging meniscus and diverging meniscus are used in spectacles.

When a parallel beam of light reaches the concave lens it spreads outwards after passing through the lens.

Uses of lenses.

  • Lenses are used in photographic cameras.
  • Lenses are used in microscopes used by doctors to see germs.
  • Used in spectacles worn by people with eye defects.
  • Used as magnifying glasses.
  • Used in projectors which focus information on film slides into big pictures on the screen.
  • Used in binoculars to see distant things in magnification.

In general lenses are used in optical instrument.

Optical instruments.

Optical instruments are instruments which use either lenses, prisms, plane mirrors or curved mirrors.

Examples of optical instruments.

  • Cameras
  • Spectacles
  • Microscopes
  • Magnifying glasses.
  • Telescopes
  • Binoculars
  • Projectors

Dispersion of light.

Dispersion of light is the splitting of white light into the seven colours of the spectrum.

  • Dispersion of light is due to refraction of light.
  • A spectrum is a band of seven distinct colours.
  • A spectrum is formed when white light is split by the act of a prism.
  • A prism is a device that splits white light into seven colours.
  • An example of a natural light spectrum is a rain bow.

Colours of the spectrum.

Primary colours and secondary colours.

A primary colour is one that cannot be obtained by mixing other colours e.g. red, blue and green.

A secondary colour is colour made by mixing two primary colours e.g. yellow, magenta, peacock blue pr cyan.

How to make secondary colours.

  • Red   + green = Yellow.
  • Red + Magenta = white.
  • Blue + yellow = Cyan or peacock blue.

Illustration of the spectrum

White is a universal colour.

A pin hole camera

Characteristics of images formed by a camera.

  • The images are real.
  • The images are inverted.
  • The images are diminished.


  • This layer contains light sensitive cells called rods and cones.
  • Human being see clearly during day because they have more cones than rods.
  • Cones help in day light and colour light.
  • Rods help in dim and night vision.
  • It is on the retina when the images are formed.

Fovea (yellow spot)

  • It has the highest concentration of cones.
  • It gives accurate interpretation of the image and is where the sharpest image is formed.

Blind spot.

  • This spot doesn’t have any light sensitive cell.
  • It is where the optic nerve leaves the eye and also where blood vessels and nerves join the optic nerves.


  • Refracts light rays and focus the image on the retina.

Suspensory ligament

  • Holds the lens in position by attaching it to the ciliary body.

Aqueous and vitrous humour.

  • These are salt solutions, sugar solution and proteins in water.
  • They refract light to produce an image on the retina.
  • They help to maintain the shape of the eye.

The optic nerve

Transports nerve signals to the brain for interpretation.

Eye Lashes

  • They help to trap large air borne particles.

Tear glands.

  • They lie under the eyelids.
  • They secrete a solution of sodium hydrogen carbonate and sodium chloride (Tear).
  • They have an enzyme which kills bacteria.

NB:The eye has the ability to focus near and far objects on the retina by changing the shape of the lens.

This focusing of near and far objects by the lens is called accommodation.

Characteristics of images formed by the eye.

  • The images are real.
  • The images are inverted.
  • The images are diminished.

The Human Eye

The eye is an organ of sight.

It is spherical in shape and enclosed in a socket of the skull called the orbit.

The structure of the eye.

Functions of parts of the eye.

The eyelids.

  • They cover and protect the eye.
  • Blinking can be voluntary or by reflex action.
  • Blinking distributes a fluid (tears) over the surface of the eye to prevent it from drying.
  • Tears clean up the eye and kills some germs which enter the eye.


  • Is a thin layer which lies inside the eyelid.
  • It is kept moist and clean by a slow continuous stream of liquid from the tear glands.


  • It is a tough non-elastic coat around the eyeball.
  • It supports and maintains the shape of the eyeball.


  • It is a transparent part of the sclerotic. It helps to refract and converge light.


  • It has a dense net work of blood capillaries supplying food and oxygen to the eye.
  • It is pigmented black to reduce internal reflection of light within the eye.


  • It regulates the size of the pupil and controls the amount of light entering the eye.
  • It also determines the colour of the eye.


  • Admits light into the eye.

Eye defects:

  • It is the inability for an eye to focus certain distance normally.


  • The eye ball being too long or eye lens being too thick.
  • This causes the image from distant objects be brought to focus infront of the retina.
  • Short sightedness can be corrected by wearing spectacles with diverging lens (concave lenes).
  • Eye strain.
  • Abnormal shape of the eye ball.
  • Abnormal shape of the lens.
  • Colour blindness.

Examples of eye defects.

  • There are four eye defects in common in humans namely.

a)Short sightedness (myopia)

b)Long sightedness (hypermetropia)

c)Old age sight (presbyopia)


  • Short sightedness

Short sightedness is a condition when a person can only see near objects clearly but cannot see distant objects.

Normal eye sight.

Both near and distant objects can be focused on the yellow spot on the retina.

Short sightedness (myopia)

BB4HXF Myopia, or nearsightedness, can be corrected with concave lenses to allow near objects to be brought into focus by the eye.

This occurs when the eye ball is longer than the normal or when the lens is too thick and the objects close to the eye can be focused properly but the point of focus for distant objects is infront of the retina.

Correction of short sightedness.

Short sightedness is corrected by wearing spectacles with diverging lenses (concave

Long sightedness:

Long sight is a condition when certain people can see distant objects clearly but can not see near by objects.

Small or short eye ball or too thin eye lens.

The above causes the image from close objects be brought to focus behind the retina.

Illustration of long sightedness

BB4HY9 Hyperopia, or farsightedness, can be corrected with convex lenses to reduce the eye’s effort to bring an object into focus.

This occurs when the eyeball is shorter than the normal or when the eyeball is small or the lens is too thin.

Distance objects can be focused properly but the point of focus for close objects is behind the retina.

Correction of long sightedness

Long sightedness is corrected by wearing spectacles with convex lenses.

Old age sight (presbyopia)

When the lens loses its elasticity it can no longer change in shape.

It becomes suitable for only distant vision (long sight). Old age sight people usually require reading glasses which have converging lenses. This happens in old age above sixty years.


It is the most common of all eye defect.

It is caused by the surface of the cornea not being perfectly smooth or spherical.

This results is blurred vision.

Astigmatism is corrected by wearing spectacles with cylindrical lenses.



  • It causes the swelling of conjunctiva.
  • It is caused by gonorrhea.

There are three types of conjunctivitis.

  • Acute conjunctivitis.
  • Chronic conjunctivitis.
  • Gonorrhoea conjunctivitis.


  • The white part of eye becomes pink.
  • Waterly discharge from the eyelid with mucus and pus.
  • Scratching and burning sensation in the eyelid.
  • Looking at light cause pain.


Caused by bacteria.

  • It is highly contagious and infectious disease.
  • It is common in places with poor hygiene and overcrowding where water is scarce and people don’t wash hands and their eyes

How trachoma spread

  • By houseflies.
  • Sharing hands wit an infected person.
  • Sharing of the same basin with an infected person.
  • Shaking hands with another infected person.

Signs and symptoms

  • Redness and itching on the eye.
  • Waterly discharge from the eyelids.
  • Swelling of the eyelids.
  • Pain while looking at light.

River blindness.

  • It is caused by a tiny filaria worm (onchocerca vulvulus).
  • It is carried by a small hump called black fly known as a black fly or simulium fly.
  • This fly breeds in fast flowing rivers.

Signs and symptoms

  • Itching skin rashes.
  • Severe skin itching.

Prevention and control.

  • Spraying using insecticides against the adult fly.
  • Treatment of the infected person.

Other diseases.


  • An inflammation of the margin of the eyelid.
  • The eyes itch and burn and swell.


  • This is when the lens of the eye becomes grey and opaque.
  • They are caused by an injury or continued exposure of the eye to high temperature.


  • Caused by increased internal pressure of fluids.
  • It can come about by itself or progress from another diseases.


  • This is a small inflammation on the eyelid. It looks like a small boil.
  • It is usually a sign of poor general health, anaemia or diabetes.

Corneal ulcer.

It is caused by an injury to the cornea.

Care of the eye.

  • Don’t strain your eyes by reading;
  • Don’t rub your eyes with dirty fingers.

a)Very small prints with too little or direct sunlight.

b)In moving vehicles.

c)In wrong postures like in bed.

  • Don’t expose your eyes to very bright or glaring light.
  • Always wash your eye with clean water and soap, every morning and evening.
  • Never look directly at the sun, it may spoil your retina.
  • If there is anything wrong with your eyes visit an eye specialists.
  • When reading use a correct distance of about 30cm.
  • Don’t share towels or clothes with people who have sick eyes.


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ASSIGNMENT : LENSES-P.7 assignment MARKS : 10  DURATION : 1 week, 3 days

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