Form 2 Science Chapter 1 p6a20

Form 2 Chapter 1 The world through our senses yschow@smkbpj(a)

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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

Tongue

Taste

Chemical in foods

Skin

Touch

Pressure, heat, cold, touch

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Pathway from stimulus to response. Stimulus

Response

Receptor in sensory organ

Sensory nerves

Effectors (muscle)

Brain yschow@smkbpj(a)

Motor nerves 4

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

Heat receptors

Heat

Pain receptors

Pain

Pressure receptors

Large pressure

Touch receptors

Small pressure (touch ) yschow@smkbpj(a)

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• 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 sensitive to touch.

Braille letters are specifically designed symbols for stimulating the fingertips, which allows blind people to read yschow@smkbpj(a)

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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. yschow@smkbpj(a)

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• When we catch a cold, too much mucus is produced and this makes the receptors less sensitive.

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Cold = flu (selsema 感冒) 感冒

1.4 Sense of Taste yschow@smkbpj(a)

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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 receptors. • These receptors are sensitive to sweet, salty sweet salty, sour and bitter tastes. yschow@smkbpj(a)

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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 produced. • The impulses are sent to the brain for interpretation. yschow@smkbpj(a)

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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 three sections: i) outer ear, ii) middle ear and iii) inner ear. yschow@smkbpj(a)

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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 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..

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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 22 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 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) 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

Have an astigmatism

Close your right eye and hold this page about one arm’s length from your left eye. Look at the figure .

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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.

It's amazing how our brain works. This should be proof enough, we don't always see what we think we see. yschow@smkbpj(a)

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Text book pg 24

Blind spot

• • •

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.

When images fall on the blind spot, they cannot be seen. yschow@smkbpj(a)

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Stereoscopic (binocular) vision • Stereoscopic vision is a vision involving both eyes. eyes • Humans and most predators have stereoscopic vision.

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Advantages of stereoscopic vision (a) Able to see objects in three dimensions. (b) Able to estimate the distance accurately.

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Monocular vision • Monocular vision is a vision involving only one eye. eye. • Animals of prey normally have monocular vision. • Monocular vision has a wider scope of vision. vision • This enables the prey to detect the presence of predators easily yschow@smkbpj(a)

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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. • A medium is needed for the sound to travel. • Hence, sound cannot travel through vacuum vacuum.

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Sound can be reflected • Sound can be reflected by smooth and hard surfaces and it is absorbed by soft and rough surfaces. • Echo 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 !! Use earplug or earmuffs Loud music cause hearing loss yschow@smkbpj(a)

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How to overcome hearing loss ?

Hearing aid

Surgery

Artificial cochlea

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Human hearing limit • Our ears can only detect sound of frequencies between 20 Hz and 20000 Hz. Hz • Sounds with frequencies 20 000 Hz and • 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. • Stereophonic hearing helps us to determine the 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 in response to an external stimulus. (b) Nastic movements The direction of the response is not dependent on the direction of the stimulus

Venus fly trap

Pitcher yschow@smkbpj(a)

Plants

Mimosa pudica 44

Types of tropisms

(a) Phototropism : Response to light

(b) Geotropism : Response to gravity

(c) Hydrotropism : Response to water

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(d) Thigmotropism: Response to touch

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• When the part of the plant grows towards the stimulus, it is called positive tropism. • When the part of the plant grows away from the stimulus, it is called negative tropism. yschow@smkbpj(a)

Shoot is negative geotropism

Roots are positive geotropism .

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