CURVED MIRRORS

Back to: Primary Seven science notes

CURVED MIRRORS:

They are two types: Convex and Concave mirrors.

Convex mirrors

Convex Mirrors are curved outwards.

They form small images.                            

A convex mirror

Convex mirrors are used as follows:

1. As driving mirrors to view traffic behind.
2. In supper markets they are used to monitor the activities in the building (room).
3. Security people use them to view dangerous objects hidden under cars.

Convex Mirrors are used for the above uses because they show a large area compared to plane mirrors.

Concave Mirrors:

These are curved inwards.

They form magnified images of objects.

Uses of concave mirrors

• They are used as reflectors of car head lamps, touches, bicycle and motor cycle lights.
• They are also used by dentists to view bad teeth in people’s mouth.
• They are used as shaving mirrors to view more details when shaving.
• They are used to make solar cookers.

LENSES:

1.     Lenses are transparent, curved pieces of materials.

2.     Lenses can be made from glass, clear plastic or any other curved transparent materials.

3.     There are two types of lenses:-

• Concave lens
• Convex lens

4.     When rays are passing through lenses they bend towards the thicker surfaces.

5.     Rays passing through the centre of the lens are not refracted.

CONCAVE LENSES:

1. This is a transparent material curved inwards. It is thinner in the centre and thicker at the outside edges.
2. When curved on only one side it is called a plano concave when curved on both sides it’s a biconcave.
3. When light rays fall on the concave lens, they pass through when they are spread outwards.
4. A concave lens forms a diverging beam. It is also called a diverging lens because it makes the rays to diverge as they pass through it.

5.     When viewed through  concave lenses  make things appear smaller.

CONVEX LENSES:

1. Convex lenses are transparent materials curved outwards.
2. Convex lenses are thicker in the centre and thinner at the outside edges.
3. When curved one side it is known as a Plano Convex.  When curved on both sided it is known as a biconvex.

 When light rays fall on a concave lens, they  converge as

 they pass through.

• They form a converging beam. That why it is also called a

coverging lens.

•  The point at which rays meet is known as focal point.

USES OF LENSES:

Lenses are used in the following optical instruments:-

• They are used in the eyes. (convex lenses) to focus light onto the retina.
• They are used in cameras to focus light into the film.
• Spectacles use lenses to correct eye defects.
• Microscopes and magnifying glasses use lenses to make objects appear bigger so that details are viewed properly.
• Projects use lenses to magnify images from the film.
• Telescopes and binoculars use lenses to view distant objects by making them appear nearer.

IMAGES FORMED BY LENSES:

1. Convex lenses form real images which are upside down and smaller than the objects.
2. Real images are those that can be formed onto the screen.

A PIN HOLE CAMERA:

This is a dark box which allows light through a tiny hole made on one side of the box. At the other side opposite the hole there is a translucent paper which acts as a screen. An extension of the dark box is made after the screen for better viewing of the image on the screen.

How it works:

1. Light from an object is reflected towards the pinhole cameracamera.
2.  Rays pass through the pin hole moving in a straight line.
3. The rays then fall on the screen and form an image.
4. The image formed by a pin hole camera has the following characteristics:-
5. It is real
6. It is smaller than the object
7. It is upside down

NOTE:

(i)     The smaller the hole the better the image formed.

        A wide hole gives unclear images.

(ii)    The pin hole works on the principle that light travels in a straight line.

A LENS CAMERA (PHOTOGRAPHIC CAMERA):

It is  a light proof box with the following functioning parts; The Diaphragm, aperture, lens, shutter, film and focusing ring.

 

Diagram of a lens camera

Functions of each part

1.     The Diaphragm:

1. It is made of opaque materials with a hole in the centre called Aperture.
   1. The diaphragm regulates the amount of light entering the camera.
   1. This is done by changing the size of the aperture.

2.     The Aperture:

The aparture is a small hole in the centre of the diaphragm where rays pass through to enter the camera.

3.     The Lens:

This is a convex lens made of glass. Its function is to focus light rays entering the camera.

4.     The Shutter:

i.   This is an opaque material that covers the diaphragm to prevent light from entering the  camera.

ii.  It opens for a fraction of a second to allow light into the camera during the photographing exercise.

5.     The Film:

It is a light sensitive plastic or piece of paper on which images are formed.

The images formed have the following characteristics:

• They are real
• Diminished
• Upside down
• Same shape as the object

6.     Focusing Ring:

This is the material that adjusts the distance of the lens from the film.

It does this by moving the lens either forward or backwards nearer the film.

How a lens Camera Works:

1. Rays from the  are reflected towards the camera.
2. The shutter opens for a fraction of a second to allow in the light.
3. The Diaphragm regulates the amount of light entering the camera.
4. When the light passes through the lens it is focused onto the film.
5. The film reacts to the lights to form an image.

Photographs are got after the film is developed (chemicals removed). And then the images are printed into photographs.

A GLASS PRISMSAND LIGHT SPECTRUM

1. When  light passes through a triangular glass prism, it is refracted and it splits into seven colours.
2. The seven colours into which the while light splits are Red, Orange, Yellow, Green, Blue, Indigo and Violet.
3. The group or band of the seven colours in which the white light splits is known as a light spectrum.
4. The splitting of the white light into a spectrum is known as the dispersion of light.

Diagram showing a light specrum

Spectrum can also be formed in the following ways.

• Putting a glass with water in the morning at the window sill so that the rays go through it.
• Reflecting sun light with a mirror partly dipped in a basin of water.

A RAIN BOW:

1. A rainbow is a natural spectrum formed when the sunlight is refracted by raindrops during a cloudy day.
2. The rain drops act as a prism and split the sunlight into seven colours i.e

Red, Orange, Yellow, Green, Blue, Indigo and Violet.

RIMARY AND SECONDARY COLOURED LIGHTS:

1.     Coloured lights which can not be got by mixing other coloured light are called Primary Colours. 2.     Red, Green and Blue are the primary coloured lights.

3.     All the other colours got after mixing other colours are known as Secondary Colours.

4.     Below are examples

(i)     Blue and Green when mixed you get cyan

(ii)    When Red and Blue lights are mixed you get Magenta

• Mixing Red and Green lights you get Yellow.
• When all colours are mixed you get White.

HOW WE ARE ABLE TO SEE COLOURS:

Objects appear to be of certain colours because they absorb other colours and reflect only that colour. i.e 

• Objects appear blue because they absorb all the other colours and reflect Blue
• Objects appear red because they absorb other colours and reflect Red.
• Objects appear white because they reflect all the colours and absorb none.
• Black objects absorb all the other colours and reflect none that is why they appear Black.
• Dull and Black colours absorb more light and heat than they reflect.

NOTE

1. Bright and White colours reflect more light and heat than they absorb.
2. For that reason, people in hot regions should put on white or bright colours to reflect off the heat. Cars and buildings are painted bright colours to reflect off the heat and light.

THE HUMAN EYE:

1).    An eye is the organ that uses light in order to function.

2).    It is shaped like a ball and it is enclosed in a part of the skull called Socket.

An ouside view of the eye

                                                          Part and Its Function

Eye Lids	These are tough coats that protect the eye by covering it. The closing can be voluntary or reflex action. (done without the will and consultation of the brain)
Conjuctiva	A thin layer covering the front part of the eye.
Eye Lashes	These trap foreign bodies from entering the eye easily.
Eye Brows	These prevent sweat from flowing into the eye.

THE INNER PARTS OF THE EYE

The function of the parts of the human eye:

.

PART	FUNCTION.
The Iris	It controls the amount of light entering the eye by regulating the size of the pupil. It is responsible for the colour of the eyes.
The convex lens	Refracts the light entering the eye so that it is focused on the retina.
Aqueous and vitreous humours	i. They keep the shape of the eye. ii. They also refract light so that it is refracted onto the retina.
The pupil	It allows light into the eye.
The choroid	is made up of capillaries which supply the eye with blood. This blood supply food and oxygen to the eye. It is opaque to prevent light from entering the eye and also to prevent internal reflection
The retina	It is a light sensitive part of the eye. It is where images are formed in the eye
The yellow spot (Fovea)	is a small bend in the retina where there is the highest concetration of light sensitive cells. If images are formed here they will clearly be seen.
The blind sport	It is a point where the nerves leave the eye . If images are formed here no image will be seen.

1. The retina is a light sensitive part of the eye.
2. It is where images are formed in the eye.
3. It is made up of two types of cells, the cones and rods.
4. The cones help in colour vision while the rods help in dim light and night vision.
5. The images formed at the retina are diminished,(smaller than the object), real and upside down.
6. The retina is connected to the brain by the optic nerves.
7. In the retina the images are changed to nerve impulses (messages). These messages are taken to the brain by the optic nerves.

NOTE

The tear glands are found under the top layer of the eye lids. They produce tears. Tears have the following functions:

• help to lubricate the eye ball.
• help to wash off foreign bodies from the eyes.
• help to kill some bacteria that go to the eyes.

Comparison of the human eye and pin hole camera:

Human Eye	Pin hole Camera
Pupil changes size to control light entering	Hole remains the same size
Has a convex lens	Has no lens
Focuses the light by changing the shape of the lens	Focuses the light by changing the distance of the camera   from the object.
Has eyelids to close the eye Images is upside down	Opening always open Image upside down
Images formed on the retina (Real Images)	Image formed on the screen (Real Images)
Images are smaller than the object	Images are smaller than the object

CARE FOR THE EYE:

Eyes are delicate sensory organs that need a lot of care. We can care for our eyes in the following ways:

1. Clean the eyes with plenty of clean water and soap. When eyes are not properly washed with plenty of water, one gets water cleaned diseases. Like:  trachoma
2. Avoid sharing face towels and handkerchiefs with eye infected people. This spreads eye diseases.
3. Use proper lighting when reading. Flickering lights and dim lights can destroy eyes when used for reading. Very bright light damages the retina.
4. –       Removal of foreign bodies from eyes should be done using the tip of a clean sterilised cloth or handkerchief.
5. They can also be removed by washing the eye with plenty of water.
6. Avoid rubbing the eye when a foreign body goes onto it. Avoid using a sharp object to remove the foreign body as it may scratch the eye.

5…    Treat eye infections immediately.

Eye Defects

Eye defects are abnormalities in the eyes which prevent some one from seeing properly.

Causes of eye defects

i.      Abnormal shape of the eyeballs.                  

ii.      Weak lenses

iii      Irregular shape of the cornea.

Examples of eye defects

i.      shortsightedness (myopia)                  

ii.      long sightedness (hypermetropia)

iii.     astigmatism                                       iv.    

SHORT SIGHTEDNESS:

1. This is an eye defect nearby objects are seen clearly but distant ones are nor clearly seen.
2. Short sightedness can also be called myopia or near sightedness.
3. Short sightedness is caused by elongated eye balls.
4. It can also be caused by the lens failing to change to become long and thin (short fat lens).
5. Short sightedness person the images from far will be formed infront of the retina.
6. Short sightedness can also be caused by continual use of eyes to close work i.e T.V or computers, ready writing.

Correction for short sight.

1. The correction for short sight is wearing spectacles with concave lenses.

• Concave lenses are used because they first diverge the rays before they enter the eyes.

LONG SIGHTEDNESS.

1. A long sighted person cannot see nearby objects clearly but can see distant objects clearly.

• Long sightedness can also be called far sighetedness or hyper metropia.
• Long sightedness is caused by eye balls which are shorter than normal.
• It can also be caused by the lens failing to become short and fat.
• Images from near are formed behind the retina in a long sighted people.

Correction of Long sightedness.

A convex lens is used to correct long sightedness.

A convex lens is used because it converges the rays before they enter the eyes.

Astigmatism                                                               Presbyopia