ICSE Class 10 Biology: Sense Organs — Eye and Ear Notes 2026

Structure of the Human Eye

• Sclera (white): tough outer layer; Cornea: transparent front; focuses light; main site of refraction
• Choroid: dark, pigmented middle layer; prevents internal reflection; contains blood vessels (nutrition)
• Retina: inner layer; contains rods (low light, no colour) and cones (colour, bright light)

Image Formation in Eye

• Light path: cornea → aqueous humour → lens → vitreous humour → retina
• Lens: changes shape for accommodation; ciliary muscles control; elastic lens flattens for far, thickens for near
• Image on retina: inverted and real; brain interprets as upright; binocular vision enables depth perception

Defects of Vision

• Myopia (shortsightedness): image forms in front of retina; eyeball too long or lens too curved; corrected by concave lens
• Hypermetropia (longsightedness): image forms behind retina; eyeball too short; corrected by convex lens
• Presbyopia: old-age; lens loses elasticity; cannot accommodate; bifocal lenses used

Structure of the Human Ear

• Outer ear: pinna + ear canal → collects and channels sound
• Middle ear: tympanic membrane (eardrum) + ossicles (malleus, incus, stapes) → amplify and transmit vibrations to inner ear
• Inner ear: cochlea (hearing) + semicircular canals (balance) + vestibule (static balance)

Mechanism of Hearing

• Sound waves → eardrum vibrates → ossicles amplify and transmit → oval window → fluid in cochlea waves
• Cochlea: organ of Corti; hair cells (receptor cells) detect fluid movement; convert to nerve impulses
• Auditory nerve → brain (temporal lobe) → sound perceived and interpreted

Role of Ear in Balance

• Semicircular canals: detect angular acceleration (rotational movement); 3 planes; filled with endolymph
• Utricle and saccule: detect linear acceleration and gravity orientation; hair cells with otoliths (calcium carbonate crystals)
• If inner ear damaged: vertigo (spinning sensation), loss of balance; motion sickness related to conflicting signals

ICSE Exam Focus

• Diagrams: labelled diagram of eye and ear (vertical section); both among most tested ICSE biology diagrams
• Compare rods and cones: location, function, type of light sensitivity, pigment
• Defects of vision: cause, effect, corrective lens type; drawn with ray diagram for ICSE higher marks