Sense Organs

ICSE Class 10 · Biology · Chapter 11

What this chapter covers

Sense organs and receptors
External structure of the eye
Internal structure of the eye
Functions of the parts of the eye and how an image is formed
Stereoscopic vision
Adaptation of the eye
Accommodation of the eye
Defects of the eye and their correction
External structure of the ear
Internal structure of the ear
Functions of the parts of the ear
Course of perception of sound
Role of the ear in maintaining balance

Key definitions

Sense organs are specialised organs that contain receptors able to detect particular changes in the surroundings and pass this information to the nervous system.

A receptor is a cell or group of cells that responds to one particular kind of stimulus and converts it into nerve impulses.

Tears are a watery, slightly salty secretion of the lacrimal glands that keeps the front of the eye moist, washes away dust and contains an enzyme, lysozyme, which kills bacteria.

Conjunctivitis is the inflammation of the conjunctiva, usually caused by a bacterial or viral infection, in which the eyes become red, itchy and watery.

The sclera is the tough, white, outermost layer of the eyeball, made of dense fibrous connective tissue, which protects the inner parts and keeps the shape of the eyeball.

The cornea is the transparent, curved front part of the outer layer of the eye through which light enters the eyeball.

The choroid is the middle layer of the eyeball, rich in blood vessels and dark pigment, which nourishes the retina and absorbs stray light so that it is not reflected inside the eye.

The ciliary body is a ring of muscular tissue at the front of the choroid that holds the lens through the suspensory ligaments and changes the shape of the lens.

The iris is the coloured, muscular, circular curtain in front of the lens that controls the amount of light entering the eye by changing the size of the pupil.

The pupil is the opening at the centre of the iris through which light passes to the lens.

The lens is a transparent, flexible, biconvex structure behind the iris that focuses light rays onto the retina.

Suspensory ligaments are fine fibres that join the edge of the lens to the ciliary body and hold the lens in position.

Key points

Sense organs contain receptors that convert stimuli into nerve impulses; the sensation is produced in the cerebrum.
The eye is protected by the orbit, eyebrows, eyelids, eyelashes, conjunctiva and tears from the lacrimal glands; six muscles move each eyeball.
The wall of the eyeball has three layers: sclera (with the transparent cornea in front), choroid (with ciliary body and iris) and retina.
Rods work in dim light and contain rhodopsin; cones work in bright light, give colour vision and contain iodopsin.
The yellow spot gives the sharpest vision; the blind spot has no receptors.
The aqueous humour lies between the cornea and lens; the vitreous humour fills the chamber behind the lens.
The image on the retina is real, inverted and diminished; the brain interprets it as upright.
Stereoscopic vision comes from two forward-facing eyes; it gives depth perception and judgement of distance.
In light adaptation the pupil constricts and rhodopsin is bleached; in dark adaptation the pupil dilates and rhodopsin is re-formed.
For near objects the ciliary muscles contract, the ligaments slacken and the lens thickens; for distant objects the reverse happens.
Myopia is corrected by concave lenses, hyperopia by convex lenses, presbyopia by convex or bifocal lenses, astigmatism by cylindrical lenses and cataract by surgery.
The ear has external, middle and internal parts; the ossicles are the malleus, incus and stapes.
The Eustachian tube equalises pressure on both sides of the tympanum.
The organ of Corti in the cochlea is the organ of hearing; the auditory area lies in the temporal lobe of the cerebrum.
The utriculus and sacculus detect static balance using otoliths; the semicircular canals detect dynamic balance using the movement of endolymph.

Questions answered

Define accommodation of the eye.

Accommodation is the ability of the eye to focus objects at different distances clearly on the retina by changing the curvature, and hence the focal length, of the lens.

Name the part of the eye: (a) that controls the size of the pupil, (b) where the optic nerve leaves the eye, (c) that is transparent and has no blood vessels, at the front of the eye.

(a) Iris (b) Blind spot (c) Cornea.

Give two differences between rods and cones.

Rods work in dim light and cannot distinguish colours; cones work in bright light and give colour vision. Rods contain rhodopsin; cones contain iodopsin.

Why does the inside of the eyeball have a black pigmented layer?

The pigment of the choroid absorbs stray light, so light is not reflected about inside the eye and the image on the retina stays sharp.

State what happens to the pupil and to rhodopsin when you enter a dark room from bright sunlight.

The radial muscles of the iris contract and the pupil dilates, letting in more light. Rhodopsin is slowly re-formed in the rods, so they become sensitive to dim light again.

Give two advantages of stereoscopic vision.

It gives a three-dimensional view of objects, and it allows the distance and depth of an object to be judged accurately.

A person needs spectacles with concave lenses. Name the defect and state where the image forms without the spectacles.

Myopia (short-sightedness); the image of a distant object forms in front of the retina.

Distinguish between hyperopia and presbyopia with respect to their cause.

Hyperopia is caused by a short eyeball or a flat lens and can occur at any age. Presbyopia is caused by the lens losing its elasticity and the ciliary muscles weakening with age.

Past board exam questions

ICSE 2027 sample paper · 5 marks

Tanya, a ten-year-old girl had a severe earache. Her mother took her to an E.N.T. doctor for a checkup. The doctor noticed that she had a throat infection which had spread to her ear. Answer the following questions: (a) Name the structure that connects the throat and ear. (b) What is the collective term for the three cartilaginous structures present in the middle ear? (c) Which is the sensory organ located in cochlea? (d) What is the significance of the tiny hair present in the auditory canal? (e) What is the function of auditory nerve?

Show answer

(a) Eustachian tube (b) Ear ossicles (malleus, incus and stapes) (c) Organ of Corti (spiral organ) (d) They (with the wax) trap dust and other small foreign particles and prevent them from reaching the ear drum. (e) It carries nerve impulses from the inner ear (cochlea and vestibular apparatus) to the brain for hearing and body balance.

ICSE 2027 sample paper · 2 marks

What is the reason for Hyperopia? How can it be corrected?

Show answer

Hyperopia (long-sightedness) is caused by shortening of the eyeball from front to back, or the lens being too flat (less converging), so the image of near objects forms behind the retina. It is corrected by using a convex lens.

ICSE 2026 sample paper · 1 mark

Aruna went to a theatre to watch a movie on a bright sunny morning. As she came out of the theatre, she felt a dazzling effect and could not open her eyes. After a few minutes her vision became normal. This is due to:

Accommodation of eye
Mutation
Polarisation
Adaptation of eyes

Show answer

Adaptation of eyes

ICSE 2026 sample paper · 1 mark

Aditya who is fond of roller skating. Identify the correct terms which help him to maintain his body balance:

Cerebellum, Semicircular canals, Vestibule
Vestibule, Sclera, Cerebrum
Malleus, Semicircular canals, Utriculus
Sacculus, Stapes, Cochlea

Show answer

Cerebellum, Semicircular canals, Vestibule

ICSE 2026 sample paper · 5 marks

Rajeev, a 10 year old boy, went to an Ophthalmologist to get his eyes tested. He noticed a poster in the clinic with questions related to the parts of human eye. Help Rajeev answer the questions: (a) Which is the fluid that lubricates the eye? (b) Which cells help in colour vision? (c) Which nerve sends impulses from the eyeball to the brain? (d) Name the structure that gives colour to the eyes. (e) Name the part that allows entry of light rays into the eye.

Show answer

(a) Tears (lacrimal fluid) (b) Cones (c) Optic nerve (d) Iris (e) Pupil

ICSE 2026 board paper · 2 marks

Given below is the picture of an eagle. Eagles have binocular vision. What is the advantage of such a vision?

Show answer

In binocular vision both eyes look at the same object together and the brain combines the two slightly different images into one. This gives a three-dimensional view, i.e. perception of depth, so the eagle can judge the distance of its prey accurately (and the overlapping fields also cover the loss of a blind spot).

ICSE 2026 board paper · 3 marks

Tara's grandmother is 70 years old and has a passion for embroidery. She faces difficulty in threading the needle as the eye of the needle appears blurred. The ophthalmologist diagnosed it as an age-related disorder. (a) Name the eye disorder she is suffering from. (b) How can the above defect be corrected? (c) Where is the image formed in the above disorder?

Show answer

(a) Presbyopia (age-related long-sightedness, due to loss of elasticity of the lens / weakening of the ciliary muscles). (b) By wearing spectacles with convex lenses for near vision (bifocal lenses if distant vision is also affected). (c) The image of a near object is formed behind the retina.

ICSE 2026 sample paper · 2 marks

Which part of the ear equalizes air pressure on either side of the ear drum? Where is this structure located?

Show answer

The Eustachian tube. It is located in the middle ear, connecting the middle ear cavity to the pharynx (throat).