form 2 science chapter 1

Form 2

Chapter 1

The world through

our senses

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1.1 Sensory Organs and Their Functions

• A sensory organ is an organ that enables the body to respond to

stimuli.

• A stimulus is a change in the surroundings that can be detected by

the sensory organs.

• The five sensory organs are eye, ear, nose, tongue and skin.

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• The ability of the sensory organs to detect stimuli is

called senses.

Sensory organ Sense Stimuli

Eye Sight Light

Ear Hearing Sound

Nose Smell Chemical in air

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Nose Smell Chemical in air

Tongue Taste Chemical in foods

Skin Touch Pressure, heat, cold,

touch

Pathway from stimulus to response.Stimulus

Receptor in

sensory organ Effectors

(muscle)

Response

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Sensory

nerves Brain

Motor

nerves

(muscle)

1.2 Sense of Touch

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1.2 Sense of Touch

• The skin is a sensory organ which responds to the

sense of touch.

• There are five types of receptors in the skin.

Receptor Stimuli (sensitive to)

Cold receptors Cold substances

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Cold receptors Cold substances

Heat receptors Heat

Pain receptors Pain

Pressure receptors Large pressure

Touch receptors Small pressure (touch )

• The sensitivity of the skin depends on

(a) the thickness of epidermis

(b) the number of receptors present

• Fingertip and neck are more sensitive .

• Elbow , knee and back side are not so • Elbow , knee and back side are not so

sensitive to touch.

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Braille letters are specifically designed symbols for

stimulating the fingertips, which allows blind people to

read

1.3 Sense of Smell

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1.3 Sense of Smell

• Mucus is produced to help keep the

receptors moist.

• When you inhale, the chemicals

from the food enter the nasal cavity

and dissolve in the mucus.

• The smell receptors are stimulated.

• The impulses from the smell

receptors are sent to the brain by

the olfactory nerves for

interpretation.

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• When we catch a cold, too much mucus is

produced and this makes the receptors less

sensitive.

yschow@smkbpj(a) 10Cold = flu (selsema 感冒感冒感冒感冒)

1.4 1.4

Sense of

Taste

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1.4 Sense of Taste

• The tongue contains many taste

buds. The taste buds are the taste

receptors.

• There are four types of taste • There are four types of taste

receptors.

• These receptors are sensitive to

sweetsweet, saltysalty, soursour and bitterbitter tastes.

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How do you taste?

• The taste buds are able to detect

the taste of the food when the

food is dissolved in saliva.

• The taste receptors will be

stimulated and impulses are stimulated and impulses are

produced.

• The impulses are sent to the brain

for interpretation.

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1.5 Sense of Hearing

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1.5 Sense of Hearing

• The ear is a sensory organ for hearing and

balancing.

• The car can be

divided into divided into

three sections:

i) outer ear,

ii) middle ear and

iii) inner ear.

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How do we hear ?

1. The pinna collects sound

waves and directs them

into the auditory canal

and to the eardrum.

2. The eardrum begins to

vibrate and the

vibrations are

transferred to the

ossicles

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3. The ossicles magnify the vibrations and pass them

to the oval window.

4. The oval window transmits the vibrations to the

cochlea.

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5. The cochlea converts the vibrations into impulses.

6. The impulses are sent by the auditory nerves to the

brain for interpretation.

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1.6 Sense of Sight

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How do we see ?

• When you look at an object, the light rays from the object enter the

eye.

• The light rays are refracted by cornea, aqueous humour, lens and

vitreous humour.

• An image is formed on the retina. Impulses are produced and are

sent to the brain by the optic nerves..sent to the brain by the optic nerves..

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1.7 Light and Sight

• Light is a form of energy and light

travels in straight lines. This causes

the formation of eclipses.

• Light cannot travel through opaque

objects. Thus, shadows are formed.

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Light can be reflected

• When light hits a surface, some of it bounces off or is

reflected.

• Mirrors are very shiny surfaces designed to reflect nearly

all the light that hits them.

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When you look in a flat

mirror, you see a reflection

of yourself which is the

same size as you but back

to front.

Light can be refracted

• When light travels from one medium to another of

different density, its speed changes.

• This causes the light ray to bend.

• This is known as refraction.

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Effect of light refraction

a) The swimming pool appears to be shallower

than its actual depth.

b) A straw in a glass of water appears to be b) A straw in a glass of water appears to be

bent.

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Vision defects

There are three defects of vision:

(a) Short-sightedness ( Rabun jauh 近视)

can see near objects clearly but not distant objects.

(b) Long-sightedness ( Rabun dekat 远视)(b) Long-sightedness ( Rabun dekat 远视)

can see far objects clearly but not near objects.

(c) Astigmatism (Rabun silau 散光)

both far and near objects are blur.

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Short-sightedness Long-sightedness Astigmatism

can see near objects

clearly

can see far objects clearly both far and near objects

are blur

The images of distant

objects are formed

in front of the retina.

The images of nearby

objects are formed

at the back of retina.

Caused by irregular

surface of the cornea.

Using diverging

(concave) lens.

using converging (convex)

lens.

Using cylindrical lenses.

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Testing astigmatism ( text pg 23)

Normal eye

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Close your right eye and hold this page about one arm’s length

from your left eye.

Look at the figure .

Have an astigmatism

Optical illusion

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Optical illusion

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• Sometimes our brains do not accurately

interpret what we see.

• This phenomenon is known as optical illusion.• This phenomenon is known as optical illusion.

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It's amazing how our brain works.

This should be proof enough, we don't always see what we think we see.

Blind spot

Text book pg 24

• This boy is chasing a butterfly - time to end this madness.

• Close your left eye and look at the boy with your right eye. Then move your

head closer to or further from the screen until ... the butterfly disappears !

• You can't see the butterfly because it's exactly in front of your blind spot, the

place where the optical nerves enter the eye.

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When images fall on the blind spot, they cannot be seen.

Stereoscopic (binocular) vision

• Stereoscopic vision is a vision

involving both eyesboth eyes.

•• HumansHumans and most predatorspredators have

stereoscopic vision.stereoscopic vision.

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Advantages of stereoscopic vision

(a) Able to see objects in three

dimensions.

(b) Able to estimate the (b) Able to estimate the

distance accurately.

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Monocular vision

• Monocular vision is a vision involving only one eyeone eye..

•• Animals of prey Animals of prey normally have monocular vision.

• Monocular vision has a wider wider scope of visionscope of vision.

• This enables the prey to detect to detect the presence of predators easilypredators easily

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The various devices used to overcome the

limitations of sight include

• microscope,

• magnifying glass,

• telescope,

• binoculars,

• ultrasound scanning device,

• X-ray and

• periscope.

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1.8 Sound and Hearing• Sound is produced when objects vibrate.vibrate.

• A mediummedium is needed for the sound to travel.

• Hence, sound cannot travel through vacuumvacuum.• Hence, sound cannot travel through vacuumvacuum.

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Sound can be reflected

• Sound can be reflected by smooth and hard surfaces reflected by smooth and hard surfaces

and it is absorbed by soft and rough surfaces.

•• EchoEcho is the reflected sound. Echo can be used to:•• EchoEcho is the reflected sound. Echo can be used to:

(a) estimate the depth of sea

(b) identify a school of fish

(c) detect the presence of submarines

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Hearing defects

There are two major types of

(a) The first type involves the outer and middle ear.

For example, the earwax can block sound waves and cause

temporary loss of hearing.

(b) The second type involves damage to the inner ear.

For example, toxins are produced as a result of diphtheria or

scarlet fever. These toxins damage the cochlea and cause

permanent loss of hearing.

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Protect our ears

Don’t do this !!Don’t do this !!

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Use earplug or earmuffs

Loud music

cause hearing loss

How to overcome hearing loss ?

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Hearing aid Artificial cochlea Surgery

Human hearing limit

• Our ears can only detect sound of frequencies

between 20 Hz and 20000 Hz20 Hz and 20000 Hz.

• Sounds with frequencies 20 000 Hz and

• above are ultrasonic sounds. These sounds can • above are ultrasonic sounds. These sounds can

be detected by animals such as bats, cats and

dolphins.

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Stereophonic hearing

• Stereophonic hearing is hearing with both ears. hearing with both ears.

• Stereophonic hearing helps us to determine the determine the

direction of sound.direction of sound.

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1.9 Stimuli and Responses in Plants

• Plants respond to

stimuli like light,

water , touch and

gravity.

• Different parts of the

plant respond to

different stimuli.

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• There are two types of responses:

(a) Tropism

Tropism is the directional growth of the part of a plant directional growth of the part of a plant in

response to an external stimulus.

(b) Nastic movements

The direction of the response is not dependent on the not dependent on the The direction of the response is not dependent on the not dependent on the

direction of the stimulusdirection of the stimulus

yschow@smkbpj(a) 44Venus fly trap Pitcher Plants Mimosa pudica

Types of tropisms

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(a) Phototropism :

Response

to light

(c) Hydrotropism :

Response

to water

(b) Geotropism :

Response

to gravity

(d) Thigmotropism:

Response

to touch

• When the part of the

plant grows towards

the stimulus, it is

called positive

tropism.Shoot is Shoot is

negative negative

geotropism geotropism

• When the part of the

plant grows away

from the stimulus, it is

called negative negative

tropism.tropism.

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Roots are Roots are

positive positive

geotropism .geotropism .