Chapter 1 Science Form 2

Science Form 2
CHAPTER 1
THE WORLD THROUGH OUR SENSES

 Chapter 1.1
1.Humans have five sensory organs: Skin,tongue,nose,ears and eyes.
----a)A stimulus is a change in the environment that produces a reaction in a living organism.
2.Relation between sensory organs, sensory and the stimuli detected:
3.Sense is the ability of the sensory organ to detect a stimulus.
4.The sensory organs enable an organism to detect changes that occur in its environment.
5.Table 1.1 shows the relation between the sensory organs,senses and stimuli.

6.Nerve impulses pathway from stimulus to response:

Receptor in the touch sensory organ:
7.Sense of Touch
   

8.Sense of taste:

9.Sense of smell:

NOTE: Olfactory Bulb is smell receptor

1.The nose is a sensory organ of smell

2.The sensory receptors are sensitive to chemicals in the air.They are located in the upper nasal and covered with mucus.

3.When chemical vapor dissolves in the mucus, it stimulates the smell receptors. Information is produced and sent to the brain to be interpreted.The brain will identify the smell.

10.Sense of hearing:

NOTE:The semicircular canals and the Eustachian tube are NOT involved in the hearing mechanism.

1.The ear is a sensory organ which defect sound.

2.The path taken by the sound waves from the ear to the brain (mechanism of hearing) is as follows:

 Sound→Ear pinna→Ear canal→Eardrum→Ossicles→Oval window→Cochlea

→Auditory nerve→Brain

Table 1.2 

Table 1.3 Non-hearing mechanism

11.Sense of Sight:

a)The eye is a sense organ that detects light.

b)It is made up of three layers

   i)Sclera (outer layer)

   ii)Choroid (middle layer)

   iii)Retina (innermost layer)

c)The parts of the eye that help to focus the light onto the retina are: 

   i)  the cornea

   ii)  the aqueous humor

   iii)  the lens

   iv)  the vitreous humor 

d)The eye is able to focus the light from an object on the retina by changing the thickness of the lens.

e)The light path in the human eye is shown below:

  Light →Cornea→Aqueous humor→Pupil→Eye lens→Vitreous humor→Retina→Optic nerve→Brain

Structure of the human eye

Functions of the different parts of the eye

Chapter 1.7 Light and Sight

 Reflection and refraction of light

1.Light can be reflected and refracted.

2.Light travels in straight line

3.Reflection of light is the deflection of light when light rays fall on the surface of an object.

4.Refraction of light is the bending of light that occurs due to the changes its speed when moving from a medium to another medium of different densities.

Reflection of light

Reflection of light

5.Refraction of light can be observed in our daily life.

a)An object in the water appears to be nearer to the surface

b)A drinking straw placed in a glass of water appears bent

6.The dentist uses the principle of light reflection when examining teeth using a mirror.
7.A convex mirror is placed around a bend of a road for safety precaution.
8.Others examples of daily appliances which make use of the principles of light reflection are
 a)periscope           b)vehicle rear-view mirror
9.We can see an object because the object reflects light into our eyes.
Defects of vision and ways to correct them

1.The various defects of vision include:

a)Long-sightedness

b)Short-sightedness

c)Astigmatism

d)Color blindness

e)Presbyopia

Comparison between short-sightedness and long-sightedness

Short sightedness (myopia) and ways to correct the eye sight

Long-sightedness (hyperopia) and ways to correct the eye sight

2.Astigmatism is a vision defect caused by the irregular curve of the cornea.It can be corrected by wearing glasses with cylindrical lenses or by surgery.

Limitations of sight

1.We are unable to see objects that are too small, near or far away.

2.Some common examples of the limitations of sight are

(a) blind spot

(b) optical illusion

(c) monocular and stereoscopic visions

3.Optical illusions

Optical illusions

Optical illusions occur because

i) our sense of sight has its limitations,

ii) the brain cannot interpret accurately what the eyes see.

iii)This is because our brain is unable to interpret the image correctly even through our eyes are able to see the diagram clearly.This phenomenon is known as optical illusions.

4.Blind spot

(a) The blind spot is present in every eye.

(b) The blind spot does not have any receptors sensitive to light and cannot detect any image that falls on it.

Step 1:Close your right eye

Step 2:Focus the sight of your left eye on the +  symbol

Step 3:Move your eye closer to the screen slowly until the black dot symbol disappears from your view.

The black dot disappears when its image falls on the blind spot of the left eye.
5.Stereoscopic and monocular visions

Comparisons between stereoscopic and monocular vision

Chapter 1.8

1)Sound is produced by vibration

2)Kinetic energy changes into sound energy when an object vibrates.

3)Sound travels in the form of sound waves through a medium

4)Definition of medium?

5)Sound cannot travel through a vacuum.
6)When sound hits an obstacle,the sound waves will be absorbed or reflected by the obstacle.
7)Sound travels fastest through solids, followed by liquid and gases.
8)

absorption and reflection of sound

9)

Sound cannot travel through a vacuum

a)Before the air is sucked out:

  (i)The hammer vibrates.

  (ii)Sound is heard.

b)After the air is sucked out:

   (i)The hammer still vibrates.

   (ii)Sound is NOT heard.

c)Conclusion:

 (i) Sound needs a medium to travel.

 (ii) Sound cannot travel through a vacuum.
10)Defects of hearing 

a) Deafness is a defect in the hearing mechanism.
b) Deafness may be due to the damage of the following parts of the ear.
   i)Eardrum
   ii)Ossicles
   iii)Cochlea
   iv)Auditory nerve
c)Surgery can be carried out to repair damaged eardrums and replace fused ossicles.
d)A hearing aid is a small amplifier that can amplify sound. It can be used by people with impaired hearing.
11.Limits of hearing
a) Human beings cannot hear all frequencies of sound.
b) Some old people hear a smaller range of sound compared to younger people because their eardrum have lost their elasticity.
c) A stethoscope is used by doctors to listen to the heartbeats of patients.
d) A loudhailer is used to give high quality sound as well as high volume.
e) Sonar is used in marine research to detect high frequency sounds produced by aquatic mammals.
Stereophonic  hearing  
1.Stereophonic hearing means hearing with both ears.
2.Stereophonic hearing enables us to determine the direction of the sound.

1.9 Stimuli and Responses in Plants

  1.Plants respond to stimuli in two ways, namely tropisms and nastic movements.
  2.Tropisms or tropic movements are responses to stimuli which come from one direction.These movements are usually slower.

Table 1.7 Tropisms

3.

Tropisms in plants

After 1 week,

(a) the shoots respond to light, that is growing in the direction of light (positive phototropism).

(b) the roots respond to water, that is growing towards the water source(positive hydrotropism).

(c) the response of roots to water is stronger compared to the response to gravity.

4.

 

After 1 week,
(a) all the shoots grow upwards, showing  negative geotropism.
(b) all the roots grow downwards, showing positive geotropism.

5.Nastic movements are responses to stimuli which come from any direction. These responses do not depend on the direction of stimuli.

Mimosa pudica before stimulus Mimosa pudica response to stimulus

After mimosa pudica closed

Venus fly trap

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