Notes Science Form 2 Chapter 1 The World Through Our Sense English Version

1. The Wourld Through Our Senses
Sensory organ, senses and stimuli
Stimuli – changes that occur in the surroundings.
Example : sound, chemical sunstances and light intensity
Sensory organs – Organs that detect stimuli
Each sensory organ is sensitive to only certain types of stimuli. For example, the eyes
are only sensitive to light but not to sound.
Sense – The ability of an organism to detect a stimulus
Human beings have five sensory organs
Senses of smell
Senses of touch Senses in humans senses of taste
Senses of hearing senses of sight
Relation between sensory organ, stimuli, and senses in human beings
Sensory organ Stimulus /stimuli detected Sense
Eyes Light Sense of light
Ears Sound Sense of hearing
Nose Smell Sense of smell
Skin Touch, pressure, pain, heat and
cold
Sense of touch
Tongue Taste (chemicals in Food) Sense of taste
Response to stimuli
Nerve impulses
NervesStimulus
Receptor in
sensory organ
Nerves
Respons Nerve impulses
Brain
Effector
Common pathway in detecting a stimulus and producing a response in human beings

2. Sense of Touch
1. The skin is a sensory organ that gives us our sense of touch. It can detect stimuli such
as touch, pain, pressure, heat and cold.
2. Refer to the diagram, skin consists of two main layer, the epidermis and dermis.
Below the skin is a layer of fatty tissue.
3. The skin has five different receptor;
3.1 The touch receptor – sensitive to slight pressure
3.2 The pain receptor – sensitive to pain
3.3 The heat receptor – sensitive to heat
3.4 The cold receptor – sensitive to cold
3.5 The pressure receptor – sensitive to heavy pressure
4. The sensitivity of skin depend on;
a) How close together the receptor are
Example : the lips and fingertips are very sensitive to touch due to the large
concentration of touch receptor.
b) How deep the receptor are
Example : The receptor with thick epidermis are less sensitive. The epidermis
on the neck is thinner than that on the knee. So neck more sensitive than the
skin at the knee.
c) The sense of touch is very important to the blind because it helps them to identify
object and feel their way around and to reads book in Braille .

3. Sense of Smell
1. Smells are actually chemicals present in the air.
2. The nose can detect these chemicals.
3. The smell receptors or sensory cells are located at the roof of the nasal cavity.
Refer to diagram. This position allows the smell receptor to have the maximum
exposure to the air current.
4. How smells are detected;
a) When air is breathed in, the air current passes through the nasal cavity. The
chemicals present in the air dissolve in the mucous lining and stimulate the
smell receptor.
b) These receptors send out nerve impulses to the brain. The brain interprets
the message and identify the smells.

4. Sense of Taste
1. The tongue is the sensory organ related to the sense of taste.
2. It is sensitive to four basic tastes – sweet, salty, sour and bitter.
3. The surface of the tongue is covered with little bumps. These bumps contains
tiny structures called taste buds. Each taste bud contains many taste receptors
which are sensitive to chemicals in food.
4. There are four types of taste buds , one for each basic taste. Different areas of
the tongue are sensitive to different tastes. Refer to the diagram;
5. How taste detected ;
a) When we chew our food, the chemicals of the food dissolve in the salive.
b) The dissolved chemicals stimulate the taste receptors in the taste buds to
produce nerve impulses.
c) Messages are sent to the brain to be interpreted. This enables us to identify
the taste of the food.

5. 6. The nasal cavity and the mouth cavity are connected. This allows the taste and
smell of food in the mouth to be detected at the same time.

6. Sense of Hearing
The human ear has three main parts – the outer ear, the middle ear and the inner ear.
Part Structure Function
Outer ear
a) Pinna (ear flap)
b) Ear cannal
Shape like funnel. Made of
cartilage and skin.
A 2.5 cm long, narrow tube lined
with hair.
Collects and directs sound waves
into the ear canal
Directs sound waves to the
eardrum
Middle ear
a) Eardrum
b) Ossicles
c) Oval window
d) Eustachian tube
A thin , stretched membrane at the
end of the ear canal
Three small bones
A thin , small membrane at the end
of the ossicles
A narrow tube that connects the
middle ear to the back of the throat.
Vibrates when sound waves hit it
Amplify vibrations and transfer
them from the eardrum to the oval
window.
Transfers vibrations from the
ossicles to the cochlea
Equalises the air pressure on both
sides of the eardrum, thus
preventing itu from damage.
Inner ear
a) Cochlea
b) Auditory nerve
c) Semicircular
canals
A called tube filled with fluid and
lined with tiny hair-like sensory
cells.
Nerve fibres that connect the ear to
the brain
Three semicircular cannals
situated at right angles to each
other. Contain fluid and sensory
cells.
Detects vibrations and converts
them into nerve impulses
Carries vibrations and converts
them into nerve impulses
Detect the position and movement
of the head to help us keep our
balance (Note: They are not
involved in the hearing
mechanism.

7. 8. How sounds are detected by the ear :
a) The pinna collects sound waves and directs them along the ear canal to the eardrum.
b) When the sound waves hit the eardrum, it vibrates at the same frequency as the sound
waves. The vibrations are then transferred to the ossicles.
c) The ossicles amplify for the vibrations about 20 times before transferring them
to the oval window.
d) The vibration of the oval windows causes the fluid in the cochlea to move in the
form of waves . The movement of the fluid stimulates the sensory cells or
receptors in the cochlea. Nerve impulses are produced.
e) The auditory nerve carries the impulses to the brain.
f) The brain interprets the impulses as sounds.

8. Sense of Sight
1. The eyes are the sensory organs of sight. They are sensitive to light.
2. The eyelids and eyelashes keep dust and dirt out of the eyes.
3. The eye brows help to prevent sweat from running into the eyes.
4. The wall of eyeball is made up of three layers.
(a) Sclere – tough, outer layer
(b) Choroid – black, middle layer
(c) Retina – sensitive to light , innermost layer
The structure of the human eye and the function of the different parts

9. Part Stucture Function
Sclera White, fibrous, outer layer that
is opaque and tough
Protects and maintains the shape
of the eyeball
Choroid Middle layer that contains
black pigment and many blood
vessels
- The blood vessels supply
nutrients and oxygen to the eye.
- The black pigment absorbs light
and therefore prevents reflection
of light inside the eye which
would make an image less sharp.
Retina The innermost layer of the
eyeball. It contains many
photoreceptors known as cones
and rods
- Detect light and produces nerve
impulses
- The cones detect colours in
bright light, enabling us to have
colour vision
-The rods enables us to see in dim
light . They detect shades of grey
in dim light.
Cornea The curved and transparent
layer of the sclera at the front
of the eye.
Its curved surface helps to refract
(bend) light onto the retina.
Conjunctiva The thin , transparent layer of
membrane in front of the
cornea
Protects the cornea
Iris The coloured part of the eye
that is continous with the
choroids. It is made up of
muscles
Controls the size of the pupil and
thus the amount of light entering
the eye.
Pupil The hole in the centre of the
iris
Controls the amount of light
entering the eye.
Lens A transparent bioconvex and
leastic disc.
-It refract and focuses an image
onto the retina.
-The thickness of the lens is
changed to focus near and distant
objects.
Ciliary body Make up of strong muscle Contract and relaxes to change
the thickness of the lens
Suspensory
ligaments
Strong fibres that connect the
lens to the ciliary body
Hold the lens in its position
Aqueous humour A watery transparent liquid
that fills the space between the
cornea and the lens
- Helps to focus the image onto
the retina
-Helps to maintain the shape of
the eyeball
-The cornea , the conjunctiva and
the lens obtain food and oxygen
from the blood vessels in the
choroids layer by diffusion
through this liquid.

10. Vitreous humour A thick , jelly-like and
transparent substance that fills
the space behind the lens
-Helps to focus the image onto
the retina
-Helps to maintain the shape of
the eyeball
Yellow spot(fovea) The most sensitive spot on
the retina. It is located directly
opposite the pupil
Detect light or image that fall
on it
Blind spot A point on the retina where
the optic nerve leaves the eye.
There are no photoreceptors
here.
Images that fall on this spot
cannot be detected.
Optic nerve Made up of nerve fibres that
connect the retina to the brain.
Carries nerve impulses from the
retina to the brain.
5. How wee see
1. When we look at an object, light rays from the object enter each eye through the
pupil. The intensity of the light influences the size of the pupil.
a) When it is very bright , the size of the pupil decreases to let in less light.
b) When it is dim, the size of the pupil increases to let in more light.
2. As the light rays pass through the cornea, the aqueous humour , the lens and vitreous
humour, the rays are refracted to form an image on the retina.
3. The image is upside down and smaller than the actual object.
4. The brain interprets the impulses and we can see the object the right way up. The
interpretation also allows us to know the shape, the colour , the size and the distance of
the object.

11. 5. Eyes have the ability to focus on objects a different distance. Refer to diagram;
Light and Sight
Properties of light;
a). Light travels in straight lines within the same medium
b). Light extremely fast and has speed of 3.0 x 10 8 ms-1 in air.
c). Light can be reflacted when it hits a surface
d). Light can be refracted when it travels from one transparent medium to another.

12. 1. A beam of light is made up of a bundle of rays. It can be parellal , divergent or
convergent. Refer to the diagram.
2. Reflaction of light
a) Reflaction of light occurs when light bounces off the surface of an object. Refer
to diagram
b) When a parallel beam of light hits a very smooth surface, it is reflected as a
parallel beam. Its called ‘regular reflection’. It also produced sharpe image.
c) When a parallel beam of light hits a rough surface cause the reflected light to head
off in all directions. This type of reflection is called irregular of diffuse reflection.
Refer to the diagram;

13. Defect of vision and ways to correct them
Short-sightness
(myopia)
Long-sightness
(hypermetropia)
Similarity
Both are caused by the conditions of lens and eyeball
Differences
- Sees near objects clearly
- Distance object are blurred
Condition
of vision
- Sees distant objects clearly
- Near objects are blurred
- The lens is too thick
- The eyeball is too long
- The lens is too thin
- The eyeball is too shortCauses

14. Agtigmatism Colur-
blindness
Long-
sightedness
Defect of vision
Presbyopia
Short-
sightdness

15. Blind spot
Optical illusion
Monocular
vision
Limitation of
sight
Cannot see
tiny objects
Stereoscopic
vision
Cannot see
very distance
object
Diagram 4 : Monocular and stereoscopic vision

16. Magnifying glass Binoculars
PeriscopeUltrasound
scanning
Devises to overcome
limitation of sight
Magnetic
Resonance Imaging
(MRI) machine
x- ray machine
Telescope
Stimuli and Responses in Plants
a) Tropisms
Tropism are growth responses of plants to external stimuli
b) Geotropism
Geotropism is the growth movement shown by plant in response to gravity
c) Phototropism
Phototropism is the growth movement shown by plants in response to light
d) Hydrotropism
Hydrotropism is the growth movement shown by plants in response to water.
e) Thigmotropism
Thigmotropism is a response by plants to touch or contact with the solid structure

17. Structure questions
1. Figure 1 shows the cross section of the human ear
(a) Name the parts labeled M to Q
(b) Which labeled parts of the ear responsible in carrying out the following
functions ?
(i) Changing sound vibrations to electrical messages
(ii) Amplifying sound vibrations
(iii) Helps in balancing by detecting the position of the head
(c) Complete the sequence below to show how sound is detected by the human
ear.
Pinna
Auditory Nerve

18. 2. Figure 2 shows the cross section of the human eye.
(a) label the cornea and retina in Figure 2
(b) The following sequence shows the path of light rays from the object entering
the human eye.
Cornea Aqueous humou Pupil R Vitreous humour Retina
(i) Mark R in Figure 2
(ii) State the function of R
(c) A student suffers from a detect of vision due to his slightly long eyeball.
(i) Name the defect of vision the student is suffering from
(ii) Mark X along the line EF to show the position of the image that is
formed in his eye
(iii) What kind of lense can be used to correct this defect?