CHAPTER 1THE WORLD

THROUGH OUR SENSES

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1.1 SENSORY ORGANS AND THEIR FUNCTIONS

1.   Sensory organs are used to detect every changes in the environment. (a)   Sensory organs are possessed by human and all animals. (b)   Sensory organs allow the body to respond to the stimuli surroundings. Stimuli from the surroundings. Stimuli are changes that happen in the environment.  (c)   Sensory organs have receptors that receive the stimuli and then, send them as impulses to the brain to be analysed. The brain will then, give a response through the related effectors. Examples of effectors are muscles and glands.

2.  The sensory organs found in humans are the skin, eyes, nose,   ears and tongue.

3. Table 1.1 shows the stimuli for the sensory organs found in our body. 2

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1.2 The Skin and the Sense of Touch1.   The skin is the outermost layer of the human body which covers and protects the human body.

2.   The skin is a sensory organs which is sensitive to touch.

3.   The human skin consists of two layers:      (a)   The epidermis layer which consists of dead cells and acts as a protector.      (b)   The dermis layer consists of living cells, blood 

vessels, nerves and sweat glands. The dermis also  has receptors  which are sensitive to head,  cold, contact (touch) and pressure.

4.   Receptors are the ends of the nerves which are very sensitive to stimuli.

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5.   Each receptor is connected to a nerve. When  stimulated, it sends a nerve signal known as an  impluse to the brain to be interpreted.

6.   Pain receptors are the closest to the skin surface.  This is followed by touch receptors, heat receptors  and cold receptors. Pressure receptors lie deep  down in the adipose tissue beneath the dermis  layer.

7.   Different parts of the skin have different levels and sensitivity. The skin sensitivity depends on:      (a)   The depth of receptors in the skin. The palms of  our hands, the lips and the neck more sensitve than  the soles of our feet.      (b)   How close together the receptors are. The parts  of the skin which have receptors close to one  another are more sensitive.  

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1.3 The Nose and the Sense of Smell1.   The nose is a sensory organ which is sensitive to smell.

2.   The cavity of the nose is lined by 2 types of cells:      (a)   Glandular cells which secrete slime ( muscus ).      (b)   Cells of the smell receptors.

3.   The cells of the smell receptors are found on the upper part of the nasal cavity, which are connected to the nerve endings that in turn convey smell impulses to the brain.

4.   Chemical substances, inhaled through the nose, dissolve in the mucus and stimulate the sensor cells of smell. Then, impulses are sent to the brain through the nerves to be interpreted.

5.   When we have flu, the thick layer of mucus in the nose hinders these sensory cells from being stimulated and we are then, unable to smell as usual. 7

6.   Hair and mucus in the nasal cavity function to filter dust from the air so that only clean air can enter the lungs.

7.   The sensivity of smell of animals such as cats, rats and dogs is greater than that of humans, which is relatively quite weak.

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1.4 The Tongue and Sense of Taste1. The tongue is the sensory organ for taste. It can detect salt, sour, sweet and bitter tastes.

2. There are small bumps on the surface of the tongue known as taste buds.

3. The taste buds are cells which are sensitive to taste.

4. Different parts of the tongue have different tastes.

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5. Most food have the combination of all types of tastes.

6. When we eat, the chemical substances from the food dissolve in the saliva and stimulate the taste buds.

7. Then, the receptors on the taste buds send impulses to the brain to be interpreted.

8. Our taste and smell sensory organs help us to feel the palatability of food. This is because the mouth cavity and nose cavity are connected. Therefore, the taste and smell of food can be experienced at the same time.

9. When the nose is pinched while eating, the palatability of the food that is being eaten cannot be tasted.

10. When someone is having a flu, the taste of food cannot be detected because too much of mucus block the sensory cells.

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1.6 The Ears and the Sense of Hearing and Balance

1. The ears are the sensory organs for hearing and are sensitive to sound.

2. The human ear is divided into three parts: (a) Outer ear - filled with air (b) Middle ear - filled with air (c) Inner ear - filled with fluid

3. Each part of the ear has its own function as shown in Table 1.3.

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4. The hearing mechanism: (a) The earlobe collects and directs sound waves into the eardrum through the

ear canal. (b) The eardrum vibrates and the sound vibration in tranferred to the ear bones (ossicle). (c) The ossicles strenghen these vibrations snd convey them to the oval window. (d) The vibrations of the oval window cause the fluid in the cochlea to produce

nerve impulses. (e) The nerve impulses are sent to the brain by the auditory nerve to be interpreted.

5. The ear as a balancing organ: (a) Apart from functioning as a hearing organ, the ear also controls the balance

of the body. (b) Any bodily movements will stimulate the receptors in the semicircular canals to produce impulses. (c) The brain will interpret these impulses and direct the muscles to respond

and to balance up the body.

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1.7 SENSE OF SIGHT

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•  The eye is the sensory organ of sight and responds to light.

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• Changes in the size of the pupil under different situations.

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The sight mechanism

    1. Light is reflected off an object into our eyes.

2. The light travels through the pupil and the eye lens.

3. Finally the light is focused onto the retina.

4. The image formed on the retina is real, inverted and smaller than the object.

5. The optic nerve then sends the nerve impulses from the retina to the brain. The brain interprets the image as upright.

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6. The formation of an image on the retina of the eye to the stage where we can see is summarised in Figure.

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7. The condition of the eye lens looking at near and distant objects is shown below :

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8. The flow chart below summarises the route of light rays from the object entering the eye.

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LIGHT AND SIGHT

Characteristics of light

Reflection of light

1.   Light rays are reflected by an opaque surface.

2.   A smooth and shiny opaque surface, like a plane mirror, reflects nearly all the light rays that fall on it.

3.   According to the Law of Reflection,       (a)   the incident ray, reflected ray and the normal are all on the same plane.      (b)   the angle of incidence is equal to the angle of reflection.

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4. The Law of Reflection is obeyed only if the parallel light rays fall onto a uniform surface. The reflected light rays are also parallel and in order.

5. If the parallel light rays fall on a non-uniform rough surface, the reflected light rays will not be parallel or in order but dispersed.

6. The knowledge of the reflection of light is used in the following instruments : (a) the periscope - used in submarines to see the situation on the surface of

the sea. (b) kaleidoscope - produces attractive patterns of small objects in it.7. The light ray is reflected when it is directed towards the plane mirror. The characteristics of the image formed are as follows : (a) virtual ( cannot be formed on a screen ). (b) vertical. (c) of the same size as the object. (d) the distance of the image behind the mirror is the same as the distance of

the object in front of the mirror. (e) laterally inverted.

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Refraction of light

1.   Refraction of light takes place when light travels through two different media with different       densities at an angle.

2.   The speed of light changes when it moves from one medium to another with a different density which causes the light to be refracted (bent).

3.   The following shows the three situations of the movement of light rays through two different media.

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4. The following examples shows how light travels from one medium to another with different densities.

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5. Daily phenomena of refraction of light are shown below :

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Eye defects

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Short-sightedness

3.   The defect may be caused by      (a)   abnormally long eyeballs.      (b)   eye lens that are abnormally thick. this happens because the ciliary muscles are weak 

and are unable to make the eye lens thinner.

4.   The defect can be corrected by wearing concave lenses ( diverging lenses ).

5.   A concave lens divergas the light rays before they enter the eye.

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Long-sightedness

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1.   A long-sighted person can see distant objects clearly but near objects appear blur.

2.   Long sightedness occurs because the image of a near objects falls behind the retina.

3.   The defect may be caused by      (a)   abnormally short eyeballs.      (b)   eye lens that are abnormally thin. This happens because the ciliary muscles are weak and are unable to make the eye lens thicker.

4.   The defect can be corrected by using convex lenses ( converging lenses ).

5.   A convex lens converges the light rays before they enter the eye.

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Summary of short-sightedness and long-sightednessand correction of defects

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Astigmatism

1.   Astigmatism is caused by the irregular curvature of the cornea.

2.   All the light rays from an object do not meet at a point on the retina. On the other hand, some light rays are focused on the retina while others are focused either in front or behind the retina.

3.   In many cases, astigmatism causes blurred vision for either near or distant objects.

4.   To correct astigmatism, the optician recommends cylindrical lenses ( asymmetrical lens ).

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Limitation of the sense of sight

Optical illusion

1.   Sometimes what we see may not appear to be the real thing. This is because the brain cannot interpret accurately what is actually seen by the eye.

2.   This limitation of the sense of sight is known as optical illusion ( confusion of the brain ).

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Blind spot

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Stereoscopic vision and monocular vision

Stereoscopic vision

1.   Stereoscopic vision is vision involving both eyes.

2.   The brain will combine the vision from both eyes to from both eyes to from a three-dimensional image.

3.   This enables us to estimate distances accurately.

4.   The stereoscopic field of vision is narrow.

5.   Predators usually have stereoscopic vision.

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Monocular vision

1.   Monocular vision is vision involving one eye only. This makes estimating distances accurately difficult.

2.   Monocular vision produces a flat image.

3.   The monocular field of vision is wide.

4.   Preys usually have monocular vision.

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Comparison between stereoscopic vision and monocular vision

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Use of optical instruments

1.   Optical instruments can be used to help overcome the limitations of the sense of sight.

2.   The microscope is an optical instrument which helps us see fine and small objects.

3.   The telescope and binoculars are optical instruments that help us see distant objects.

4.   The periscope is used in submarines to see above the sea level.

5.   X-ray machines enable us to observe our bodies' internal organs.

6.   A special machine called the ultrasonic scanner can produce an image of a foetus in a pregnant woman's womb on a screen. 

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SOUND AND HEARING

Production of Sound

1.   Vibrations produce sound :      (a)   Sound is a form of energy produced by vibration.      (b)   When an object vibrates, the kinetic energy from the object is converted into sound

energy.                   (c)   Vibrating objects that produce sound are :             i.   Musical instuments such as guitar, voilin and drum when played.            ii.   A tuning fork when knocked.           iii.   Air at the mount of a tube containing water when blown.           iv.   The tissues in our vocal cords vibrate when we talk.            v.   Sound produced by animals when their limbs are moved :                  - Vibrations of the wings of bees and mosquitoes produce sound.                  - Grasshoppers produce sound when their hind legh are brushing against  their

wings, causing their wings to vibrate.

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Transfer of Sound

1.   Sound can be transferred from one place to another through a medium.

2.   Sound can move through a solid, a liquid or a gas. Sound is transferred through the air when we listen to someone talking.

3.   Sound moves fastest through solid, followed by liquid and slowest through a vacuum as there are no particles in a vacuum.

4.   The arrangement of particles in matter influence the transfer of sound. Compact arrangement of particles in a solid enables the vibration to be transferred quickly.

5.   Particles in a gas are very far apart from each other. Therefore, the transfer of vibration is not efficient.    

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The reflection and absorption of sound 

1.   When sound waves are blocked by an object, they may be reflected or absorbed by the object.

2.   An object which has hard and smooth surfaces is a good sound reflector.       Examples : Plank, glass, metal.

3.   Reflected sound is known as an echo. Echo does not occur in a small room because sound is reflected very quickly.

4.   An object which has soft and rough surfaces is a good sound absorber. Therefore, soft materias are normally used reduce echo especially in a hall. Actions to reduce echo are :      (a)   The floor of a hall is covered by carpets.      (b)   Soft cushions and curtains are put in a hall or big room.      (c)   The walls are lined by sponge or cardboard punched with holes.

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5.   Some equipments are invented using the principle of echo to benefit mankind.

6.   Echo is used to prevent ships from colliding with rocks under the sea. Echo is also used to trace  fishes in the ocean and to determine the depth of the ocean.

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Hearing Defect

1.   The most common hearing defects are the inability to detect sound and the difficulty of hearing withease.

2.   Deafness may be caused by several factors :      (a)   Damage of the ossicles.      (b)   Damage of the eardrums.      (c)   Damage of the cochlea.      (d)   Damage of the auditory nerve.

3.   Bacterial or viral infections and high fever may lead to damage of the inner ear.

4.   Long exposure to loud sound may increase the chance of becoming deaf.

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5.   Some of the hearing limitations can be corrected by using modern devices.      (a)   Hearing aids can be used to help people with hearing problems.      (b)   Surgery can be carried out to replace damaged ossicles and to repair

damaged eardrums.      (c)   Implantation of electronic gadgets into the ears can help deaf people to

hear again.

6.   Nonetheless, a severe damage of the auditory nerves cannot be corrected.

7.   Looking after the ears :      (a)   Prevent the ears from being exposed to loud sound, especially while

listening to music.      (b)   Avoid digging the ears with sharp objects.      (c)   Avoid from inflicting tight slaps onto the ears.      (d) Clean up the ears with cotton buds regularly so that the ear canal is not

blocked.

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Limitations of the Sense of Hearing

1.   The range of hearing frequency.      (a)   Human beings :             i.   The human ear can only detect sound between a frequency range of

20Hz to 20 000 Hz.            ii.   The rearing range differs from one individual to another.                  - It is harder for old people to hear because their eardrums are less

elastic.                  - For individuals who are exposed to continues sound pollution like

loud sound of vehicles or machines, their ability to hear will decrease.

      (b)   Animals :             i.   Some animals can detect the ultrasonic frequencies that humans are

not able to. The ultrasonic frequencies are sound with frequencies exceeding 20 000 Hz.             ii.   The range of hearing frequency for several animals is shown in the

following table.     

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2.   Devices to overcome the hearing limitations

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3.   Stereophonic hearing      (a)   Hearing by using both sides of the ears is known as stereophonic hearing.      (b)   Stereophonic hearing allows us to determine the direction of sound accurately.             i.   A sound coming from the right side will stimulate the right ear first.            ii.   The sound waves will then, reach the left ear. the impulses are sent to the brain to 

be interpreted earlier than the left ear.           iii.   The right ear will hear the sound louder than the left ear.           iv.   The differences in the loudness or speed of the sound that reaches the ears allows 

us to determine the direction or the source of sound.      (c)   Stereophonic hearing is important to humans and animals because it can help to 

determine the direction or source of a sound. This is important because :            i.   It can help avoid danger such as enemies, predators or moving vehicles.           ii.   It can help animals to obtain their food.      (d)   The direction of sound is difficult to determine using only one ear.

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STIMULI AND RESPONSE IN PLANTS

1.   Plants can detect and respond to stimuli around them.

2.   The response by plants to stimuli is called tropism.

3.   There are two types of tropism :      (a)   Positive tropism - response by plants toward s the stimulus.      (b)   negative tropism - response by plants away from the stimulus.

4.   Plants respond to light, gravity and water. there are also plants that respond to the stimulus of touch.

5.   The tropic movements are importants to plants  because these movements help the plants get necessities like light and water and minerals. This enables plants to grow healthily. 

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6.   Examples of tropic responses and nastic movement are given as follows :

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7.   Tropic differences between the responses of the plant shoots and plant roots are given below.

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