Science Form 2

Topic 1 – The World Through

Our Senses

Sensory organs and their functions…

tongue – taste (Taste)

Nose – smell (Smell)

Eyes – see (Sight)

Skin – touch (Touch)

Ears – listen

(Hearing)

Sensory organs and their functions…

Eyes

Skin

Nose Tongue

Ears

• Sensory organs are organs that detect stimuli.

• Stimuli (stimulus) are changes in the environment

that cause the body to react.

• Human beings have 5 sensory organs:

- eyes sight

- nose smell

- ears hearing

- tongue taste

- skin touch

• Sense is the ability of a sensory organ to detect

stimuli.

Sensory organs and their functions…

• The nervous system detects and responds to stimuli

• An organism receives a stimulus react to the

stimulus response

• Each sensory organ has receptors that sensitive to

stimuli

• Stimulus receptors nerve impulses

Brain nerve impulses nerves nerve

impulses nerves effectors response

The pathway from stimulus to Response…

(Stimulates) (in sensory organ) (electrical signals)

(Interprets)

(muscles & glands) (sense)

The sense of Touch… • Structure of the human skin

The sense of Touch… • Structure of the human skin

The sense of Touch…

• Sensitivity of the

skin

• The skin on

different parts of

the body has

different degrees

of sensitivity.

Sensitivity of the skin

Number of receptors present Thickness of the epidermis

Receptor number ,

sensitivity of skin

Thickness of epidermis ,

Sensitivity of skin

The sense of Smell…

(Nerve to brain)

The sense of Smell…

• The nose is the sensory organ that gives the sense of

smell.

• The nostrils are the holes in the nose.

• The nostrils open into a hollow space called nasal cavity.

• The roof of the nasal cavity has many sensory cells/

smell receptors to detect smell.

• The nasal cavity is also lined with mucus.

• Mucus moistens and warms air current before it enters

the lungs.

• Hairs in the nostrils trap dust and dirt in the air so that

only clean air enters the lungs.

The sense of Smell…

• Chemicals (e.g.: food & flowers) present in

inhaled air chemical particles enter

the nasal cavity chemicals dissolve

in the mucus lining Smell receptors

stimulated and send out nerve impulses to

the brain The brain identifies the smell

Detection of Smell…

The sense of Taste…

• The tongue is the sensory organ that gives the sense of taste.

• The basic tastes are sweet, salty, sour, and bitter.

• The surface of tongue is covered with taste buds.

• Each taste bud contains many taste receptors.

• Taste receptors are sensitive to chemicals in food.

• These chemicals must dissolved in saliva before they can stimulate the taste receptors.

• The tongue has 4 types of taste receptors: sweet receptors, sour receptors, salty receptors and bitter receptors.

• The receptors are located at different parts of the tongue.

The sense of Taste…

• Chemicals (e.g.: food & flowers) present in

inhaled air chemical particles enter

the nasal cavity chemicals dissolve

in the mucus lining Smell receptors

stimulated and send out nerve impulses to

the brain The brain identifies the smell

Detection of Taste by the Taste Receptors…

The sense of Hearing… • The ear is the sensory organ that gives the sense of

hearing that is sensitive to sound stimuli.

• The human ear can be divided into 3 parts: the outer

ear, the middle ear and the inner ear.

The sense of Hearing…

• Pinna collects and directs sound waves into auditory canal.

• Auditory canal directs sound waves to the eardrum.

• Eardrum vibrates at the same frequency with the sound waves that hit it and transfer the vibration to the ossicles.

• Ossicles amplify the vibration and transfer them from the eardrum to the oval window.

• Oval window transfers the vibrations from the ossicles to the inner ear.

• Vibrations of the oval window cause the fluid in the cochlea to vibrate.

• Vibration/movement of the fluid stimulates the receptors in the cochlea. Nerve impulses are generated.

• Auditory nerves send nerve impulses to the brain which interprets the impulses as sound.

• Semicircular canals help the body maintain its balance (not involved in the mechanism of hearing).

• Eustachian tube helps balance the air pressure on both sides of the eardrum (not involved in the mechanism of hearing).

The sense of Sight… • The eyes are the sensory organs of sight which are sensitive to

light.

• The wall of the eyeball is made up of 3 layers: sclera (outer layer), choroid (middle layer) and retina (innermost layer). It has many photoreceptors called cones and rods that are sensitive to light.

The sense of Sight…

• Ciliary muscle (body) contracts and relaxes to change the thickness of the lens. This changes the focal length of the lens.

• Suspensory ligaments hold the lens in place and connect it to the ciliary body.

• Aqueous humour helps to refract light and focus the image onto the retina. It maintains the shape of the eye and the pressure in the eye.

• Conjunctiva, the thin, transparent membrane that protects the cornea.

• Cornea, the curved, transparent layer at the front of the eye. It is a continuation of the sclera.

• Cornea allows the light to enter the eye. Helps to focus the light onto the retina by bending the light rays passing through it.

• Iris controls the size of the pupils and hence the amount of light entering the eye.

• Vitreous humour, the jelly-like material found between the lens and the retina. It helps keep the shape of the eye spherical. It helps refract light onto the retina.

• Sclera protects and shapes the eye.

• Choroid, the capillaries supply nutrients and oxygen to the eye.

The black pigments absorb light and prevent reflection of the light.

The sense of Sight…

• Retina detects light stimuli and sends nerve impulses to the brain.

• Suspensory ligaments hold the lens in place and connect it to the ciliary body.

• Yellow spot (fovea), the part of the retina most sensitive to the light. It detects the images of objects formed and changes them into nerve impulses.

• Blind spot, this point on the retina is not sensitive to the light. Images falling on this spot cannot be detected because there is no receptor cells.

• It is the spot where the optic nerves leave the eyeball.

• Optic nerves, send nerve impulses from the retina to the brain to be interpreted.

• The brain interprets the impulses and converts the inverted image to an upright image.

How we see…

• Light rays travel from an object and enter the eye through the pupil.

• The light rays are refracted (bent) by the cornea, aqueous humour, eye lens and vitreous humour.

• An inverted image smaller than the actual image is formed on the retina.

• The image stimulates the photoreceptors in the retina to produce nerve impulses.

• The nerve impulses are sent by the optic nerves to the brain.

• The brain interprets the impulses and converts the inverted image to an upright image.

Light and Sight… Properties of Light

• Light is a form of energy that travel at the speed of 3.0 x 108 ms-1.

• Light travels in straight lines in the same medium.

• Light is reflected when it hits an opaque surface.

• Light is refracted when it travels from one transparent medium to another.

Reflection of Light

• Light which falls on an opaque object maybe absorbed or reflected.

• Reflection takes place when light rays which fall on the surface of an opaque object bounces off it.

• The amount and direction of the reflected light depend on the type of the surface the light falls on.

• Light is refracted when it travels from one transparent medium to another.

• The image on the a plane mirror is upright, laterally inverted, same size and same distance from the mirror as the object.

Light and Sight… Reflection of Light

• Smooth and flat surfaces such as mirrors, glass and shiny metals will reflect light regularly (Regular reflection).

• Rough and uneven surfaces such as clothes and wood will cause the light reflected is scattered (Diffused reflection).

normal

P Q

Incident ray Reflected ray

The ray of light

that approaches

the surface

The ray of light that

bounces off the

surface of an object

Regular Reflection Diffused Reflection

Application of Reflection of light in daily life… Reflection of Light

• Plane mirrors are used in bathrooms or bedrooms.

• Side mirrors and rear-view mirrors are used in cars.

• Convex mirrors give a wider scope of view and are used as security mirrors in shops and fish-eye mirrors at road bends.

• Concave mirrors magnify images and are used in microscopes and telescopes.

• Periscopes are used to see above the water surface in the submarines. A periscope consists of 2 plane mirrors which reflect and change the direction of light.

• Kaleidoscopes form colourful images. They are made from 2 more more plane mirrors which reflect the light from objects placed between the mirrors.

Reflection of Light …

Reflection of Light …

Concave mirror Periscope

Refraction of Light … • Light can travel through transparent media such as air, water

and glass.

• Light travels at different speeds in different media.

• When light travels from one medium into another, the change in the speed of light causes the light to bend or change direction.

• The refraction of light is the bending of light as light travels through two types of media that have different densities.

• The speed of light increases when light enters a less dense medium.

• The speed of light decreases when light enters a denser medium.

• Refraction of light does not occur when the light ray is directly perpendicular to the surface of the medium. The light will pass through the medium in a straight line.

• When light travels from a less dense medium (air) to a denser medium (glass), its speed decreases and the light ray is refracted towards the normal.

• When light travels from a denser medium (glass) to a less dense medium (air), its speed increases and the light ray is refracted away from the normal.

Refraction of Light …

Phenomena of the Refraction of light… Pond appears to be shallower than it really is

• Light from the bottom of the pond is refracted away from the normal when light moves out of the water into the air.

• The pond appears to be shallower than the actual depth of the pond.

Drinking straw in the glass of water appears to be bent

• Light from the straw is refracted away from the normal as it travels from the water to the air.

• The image of the straw is somewhere above of the bottom of the glass.

• Thus, the straw appears bent.

Vision Defects and Ways to Correct Them… • An image is focused exactly on the retina for a person with normal

vision.

• The common defects of vision are:

(a) short-sightedness (myopia)

(b) long-sightedness (hypermetropia)

(c) astigmatism

(d) colour blindness

(e) presbyopia

Focus on retina