Glossary of Terms Related to The Human Eye
- Cornea
A transparent, tough front layer of the eye that protects it and allows light to enter. - Aqueous Humour
A clear liquid between the cornea and the lens that maintains the eye’s shape and provides nutrients. - Iris
The coloured, muscular part of the eye that controls the size of the pupil and the amount of light entering the eye. - Pupil
The adjustable, central opening in the iris that appears black and regulates the amount of light entering the eye. - Ciliary Muscles
Muscles that adjust the shape (curvature) of the eye lens to help focus images on the retina. - Eye Lens
A transparent, flexible, convex lens inside the eye that focuses light onto the retina to form clear images. - Retina
The light-sensitive inner layer of the eye where images are formed. It contains rods and cones. - Rods
Photoreceptor cells in the retina that are sensitive to dim light and help with night vision. - Cones
Photoreceptor cells in the retina that detect colour and function best in bright light. - Optic Nerve
The nerve that carries electrical signals from the retina to the brain for image processing. - Blind Spot
The area on the retina where the optic nerve exits the eye; it has no rods or cones, thus no vision. - Persistence of Vision
The phenomenon where an image stays on the retina for about 1/16th of a second after the object is removed. - Power of Accommodation
The ability of the eye lens to change its focal length to focus on objects at various distances. - Near Point
The minimum distance at which an object can be seen clearly without strain; about 25 cm for a normal eye. - Far Point
The maximum distance up to which an eye can see clearly; at infinity for a normal eye. - Range of Vision
The distance between the near point and the far point where the eye can see clearly. - Myopia (Near-sightedness)
A defect where distant objects appear blurry because the image forms in front of the retina; corrected by concave lenses. - Hypermetropia (Far-sightedness)
A defect where nearby objects appear blurry because the image forms behind the retina; corrected by convex lenses. - Cataract
An eye condition where the lens becomes cloudy, leading to impaired vision; often corrected by surgery. - Colour Blindness
A genetic condition where a person cannot distinguish between certain colors due to defective cone cells. - Night Blindness
A condition where a person has difficulty seeing in low light due to the malfunctioning of rod cells. - Photoreceptors
Specialized cells in the retina (rods and cones) that detect light and send signals to the brain. - Colour Perception
The ability of the eye (cones) to differentiate various colours under bright light conditions. - Vitreous Humour
The transparent, gel-like substance filling the space between the lens and the retina, maintaining the eye’s shape. - Optical Device
A tool or structure that uses lenses or mirrors to bend light; the human eye is a natural optical device. - Cinematography
The technique of capturing moving images, which relies on the persistence of vision to create the illusion of motion. - Sclera
The white outer layer of the eyeball that provides protection and structure. - Choroid
A dark, vascular layer between the retina and sclera that provides nutrients and absorbs stray light. - Fovea
A small pit in the retina where cones are concentrated, providing the sharpest vision. - Adaptation (to light)
The eye’s adjustment to different levels of brightness, controlled by the pupil and photoreceptors.
Something To Know
A. Fill in the blanks.
- The ____________ of the human eye determines the colour of a person s eye.
Answer – Iris
- The light sensitive screen of the human eye is called ____________ .
Answer – Retina
- The cone-shaped cells, on the retina, respond to the____________ .
Answer – Colours
- The defect of vision, in which eyeball becomes smaller than its normal size, is called ____________.
Answer – Hypermetropia
- A ____________lens is used to correct myopia and a ____________lens is used to correct hypermetropia.
Answer – Concave, Convex
B. State True or False for the following statements.
- Iris is the muscular assembly that controls the size of the pupil.
Answer – True
- The eye lens forms a virtual, erect and diminished image of an object, on the retina.
Answer – False
(Correct explanation: The eye lens forms a real and inverted image on the retina.)
- The retina of the human eye has a large number of cone-shaped and rod-shaped cells. The rods can respond even in dim light.
Answer – True
- Cinematography makes use of the ‘persistence of vision’ of the human eye.
Answer – True
- For a normal human eye, the range of vision is from (nearly) 25 cm to infinity.
Answer – True
- Myopia is caused due to excessive curvature of the cornea.
Answer – True
C. Tick (✓) the correct option.
- The change in focal length of an eye lens, to focus the image of objects at varying distances, is brought about by the action of—
pupil
iris
retina
ciliary muscles
Answer – ciliary muscles
- The impression of the image formed on the retina of the human eye remains there for about—
1/16th second
1/24th second
1/16 second
1/32 second
Answer – 1/16th second
- The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This ability of the eye is known as—
persistence of vision
far sightedness
near sightedness
power of accommodation
Answer – power of accommodation
- A near sighted person will face difficulty while reading—
a book held in hands.
a text message on the mobile phone.
a street signboard across the street.
an e-mail, on the computer screen.
Answer – a street signboard across the street.
- The defect of the human eye, in which the eye lens of a person gets progressively cloudy, resulting in blurred vision, is called—
near sightedness
far sightedness
cataract
nightblindness
Answer – cataract
- The Braille system, for visually challenged persons, is a—
Visual aid
auditory aid
tactual aid
electronic aid
Answer – tactual aid
D. Answer the following questions in brief:
- Name the part of the human eye which acts as a protective layer for the eye.
Answer – Cornea
- The pupil appears black. Why?
Answer – The pupil appears black because no light is reflected back from inside the eye.
- Differentiate between:
(a) rod-shaped and cone-shaped cells
Answer – Rod-shaped cells are sensitive to dim light and help in night vision.
Cone-shaped cells are sensitive to bright light and help in colour vision.
(b) near point and far point of human eye
Answer – Near point is the minimum distance at which an object can be seen clearly (about 25 cm for a normal eye).
Far point is the maximum distance up to which the eye can see objects clearly (infinity for a normal eye).
- State the difference between:
(a) myopia and hypermetropia
Answer – Myopia (near-sightedness) is a defect in which distant objects appear blurred.
Hypermetropia (far-sightedness) is a defect in which nearby objects appear blurred.
(b) tactual aids and auditory aids
Answer – Tactual aids help through the sense of touch (e.g., Braille system).
Auditory aids help through the sense of hearing (e.g., audio books).
- Define the terms:
(a) Blind spot on the retina
Answer – It is the spot on the retina where the optic nerve leaves the eye; no vision is possible at this point.
(b) Persistence of vision of human eye
Answer – It is the ability of the eye to retain the image of an object for about 1/16th of a second after the object is removed.
- Give the meaning of the terms:
(a) Power of accommodation of the human eye
Answer – It is the ability of the eye to adjust the focal length of its lens to focus on objects at varying distances.
(b) Least distance of distinct vision
Answer – It is the minimum distance from the eye at which an object can be seen clearly without strain (about 25 cm for a normal eye).
(c) Range of vision
Answer – It is the distance between the near point and the far point of the human eye (from 25 cm to infinity for a normal eye).
- What happens to the image distance in the eye when the distance of the object from the eye increases?
Answer – The image distance in the eye remains the same because the eye lens adjusts its focal length to keep the image formed on the retina.
- When Sahil tried to read a book by keeping it at a distance of 25 cm, the text print appeared to be blurred.
(a) Is the image of the text getting formed before the retina, or beyond the retina of his eye?
Answer – Beyond the retina.
(b) What conclusion can be drawn about the ‘least distance of distinct vision’ of Sahil’s eye?
Answer – Sahil’s least distance of distinct vision is more than 25 cm; he might be suffering from hypermetropia.
- A person, with a myopic eye, cannot see objects beyond 1.2 m distinctly. What can be said about the likely size of his eyeball? State the nature of the corrective lens that is needed to restore his proper vision?
Answer – The person’s eyeball is likely to be longer than normal. A concave (diverging) lens is needed to correct his vision.
E. Answer the following questions.
- Draw a labelled sketch of the human eye. Explain the function of the following parts of the human eye—
(a) cornea
(b) iris
(c) pupil
(d) eye lens
(e) ciliary muscles
(f) retina
(g) optic nerves
Answer – Explanation of the functions:
(a) Cornea:
Transparent, curved layer that protects the eye and helps in focusing light onto the retina.
(b) Iris:
Coloured, muscular part of the eye that controls the size of the pupil and thus regulates the amount of light entering the eye.
(c) Pupil:
A small opening in the center of the iris that allows light to enter the eye.
(d) Eye lens:
A transparent, flexible structure that changes its shape to focus light onto the retina, forming a clear image.
(e) Ciliary muscles:
Muscles that change the shape of the lens to adjust focus for near or distant objects (power of accommodation).
(f) Retina:
Light-sensitive screen at the back of the eye containing rod and cone cells; it captures the image and sends signals to the brain.
(g) Optic nerve:
Nerve that carries visual information from the retina to the brain for interpretation.
- Explain how we perceive objects as they are.
Answer – When light from an object enters our eyes, it passes through the cornea, aqueous humor, pupil, lens, and vitreous humour before reaching the retina. The lens focuses the light to form a real, inverted, and diminished image on the retina. The retina then converts this image into electrical signals which are sent to the brain via the optic nerve. The brain processes these signals and interprets the object in its correct orientation and size, allowing us to perceive objects as they actually are.
- State any five precautions that are needed to take care of our eyes.
Answer – Read in proper light — neither too bright nor too dim.
Maintain a proper distance while reading or working on a screen (about 25–30 cm).
Wash your eyes regularly with clean water to keep them free from dust and infection.
Avoid touching or rubbing your eyes with dirty hands.
Eat a balanced diet rich in Vitamin A to maintain healthy vision.
- Explain how optical and non-optical aids help visually challenged persons to develop their skills to a higher level.
Answer – Optical aids like magnifying glasses, special spectacles, and powerful lenses help partially sighted people to improve their vision.
Non-optical aids like Braille books, audio recordings, screen-reading software, and mobility canes help people who are totally blind to read, learn, move independently, and communicate.
Both types of aids allow visually challenged persons to access education, develop their talents, and lead independent, successful lives.
- What is the Braille system? How does it help the visually challenged persons?
Answer – The Braille system is a method of reading and writing for visually challenged persons, using patterns of raised dots that can be felt with the fingers.
It helps them by enabling independent reading, writing, and communication, thus allowing access to education, literature, and information, and fostering self-reliance and confidence.
Value Based Question
When Sumit joined his new school, he soon developed a very good understanding and friendship with Amit. Some miscreant students did not like this and succeeded in creating misunderstanding between them. Their Physics teacher observed their resulting ‘indifferent towards each other’ behaviour.
She called both of them to her room and told them about a (generally) age related defect of the eye. She explained to them that, in this defect, an opaque layer comes over the eye lens and blurs the vision. In the same way, she went on, misunderstanding can blur our thinking and weaken our mutual friendship and trust. She advised them to have a clear and open discussion with each other as this would help in clearing their misunderstandings and reviving their friendship.
- What values are displayed by the class teacher in the above situation?
Answer – The class teacher displayed values of care, understanding, empathy, wisdom, problem-solving attitude, and guidance towards open communication and mutual trust. She emphasized the importance of clearing misunderstandings through healthy conversation, much like treating a physical defect to restore clarity.
- Name the defect of vision caused by the formation of an opaque layer over the eye lens. How is it corrected?
Answer – The defect of vision is called Cataract.
It is corrected by surgical removal of the opaque lens and replacement with an artificial lens.
- Have a group discussion on the importance of interpersonal understanding and trust in friendship.
Answer – Interpersonal understanding and trust are the foundations of any strong friendship. Trust ensures that friends feel safe, supported, and respected. Open communication helps avoid misunderstandings and resolves conflicts quickly. Good understanding allows friends to accept each other’s differences and stand by each other in times of need. Without trust and understanding, even small issues can cause relationships to break down. Therefore, developing empathy, patience, and honest communication is essential for maintaining lifelong friendships.
Something To Do
- Make a list of the students of Classes VII and VIII who use spectacles. Find out the nature of lens used and type of defect they have.
Answer – Ananya (Class VII) – Concave lens – Myopia (near-sightedness)
Rohan (Class VIII) – Convex lens – Hypermetropia (far-sightedness)
Priya (Class VII) – Concave lens – Myopia
Arjun (Class VIII) – Convex lens – Hypermetropia
- The human eyes can play tricks on brain. What we see is not always there, or is it so? There are several ‘optical illusions’ that demonstrate this point. Try to create figures/diagrams which give an impression of the optical illusions.
Which line is larger?
In which case, is the distance between the two boxes more?
Answer – Optical Illusion: The two lines below may appear to be of different lengths, but they are actually the same size.
___________
___________
Explanation:
This is similar to the Müller-Lyer illusion, where arrows or context make lines seem unequal.
The brain misinterprets depth cues, making one line appear longer.
Conclusion: Both lines are equal in length.
- In Which Case is the Distance Between the Two Boxes More?
Optical Illusion: The gaps between the boxes below may seem different, but they are identical.
[■] [■] [■] [■]Explanation:
Surrounding shapes or patterns (like converging lines) can distort perceived distance.
The brain uses context to judge space, leading to misjudgment.
Conclusion: The distance between the boxes is the same in both cases.
- Survey your neighbourhood. Is there any registered eye bank? Find out how this eye bank functions and helps visually impaired persons.
Answer – Yes, there is a registered eye bank named Vision Care Eye Bank near my area.
It functions by collecting donated eyes (corneas) from deceased donors.
After testing and processing, the healthy corneas are transplanted to visually impaired persons suffering from corneal blindness.
They also spread awareness about eye donation.
- Make friends with some visually challenged students (It is a known fact that a person, with a sensory handicap, is gifted with some talent). Enquire from them how they master their disability and lead a fairly independent life.
Answer – Visually challenged students learn to use Braille, screen reader software, and mobility aids like white canes.
They master their disability by developing keen listening skills, strong memory, and touch sensitivity.
They also take part in special coaching for independent living and education support to lead confident and independent lives.
- Solve the puzzle with the help of clues given below:
Across -»
- farsightedness.
- an opening through which light enters the eye.
- the ‘white’ of the eye.
- eye lens becomes opaque due to development of a membrane over it.
- sends the electrical signal to the brain.
- dark coloured muscular diaphragm.
Down
- light sensitive cells.
- defect corrected by an appropriate concave lens.
- the screen of the eye.
- system which has 63 dot patterns.
- sensory nerve cell which gets activated only in bright light.
- focusses the light rays on the retina.
Answer – Across –
Hypermetropia
Pupil
Sclera
Cataract
Optic nerve
Iris
Down –
Rods
Myopia
Retina
Braille
Cones
Eye lens
Additional Questions and Answers – 01
Short Answer Questions (1-2 marks each)
Question – What is the function of the cornea in the human eye?
Answer – The cornea acts as a protective transparent layer and helps refract light entering the eye.
Question – How does the iris regulate light entering the eye?
Answer – The iris adjusts the size of the pupil: it expands in dim light to allow more light and contracts in bright light to reduce light entry.
Question – What are rod and cone cells? State their functions.
Answer – Rods: Detect brightness/dim light (night vision).
Cones: Detect colors (active in bright light).
Question – Define “blind spot.” Why does it occur?
Answer – The blind spot is a region on the retina without photoreceptor cells (rods/cones) where the optic nerve exits the eye, making it insensitive to light.
Question – What is the “range of vision” for a normal human eye?
Answer – Near point: ~25 cm; Far point: Infinity.
Question – How does the eye lens adjust to see nearby and distant objects?
Answer – Ciliary muscles change the lens curvature:
Relaxed: For distant objects (lens becomes thin).
Contracted: For nearby objects (lens thickens).
Question – What causes myopia? How is it corrected?
Answer – Cause: Excessive curvature of the cornea or elongated eyeball.
Correction: Concave lens.
Question – Why do older people often need reading glasses?
Answer – Due to hypermetropia (farsightedness), caused by reduced lens flexibility or shortened eyeball, corrected with a convex lens.
Long Answer Questions (3-5 marks each)
Question – Explain the structure and function of the human eye with a labelled diagram.
Key parts: Cornea, iris, pupil, lens, retina, optic nerve.
Answer – Functions:
Cornea: Refracts light.
Lens: Focuses light onto the retina.
Retina: Converts light to electrical signals.
Question – Describe how the eye forms an image of an object.
Answer – Light enters → Cornea refracts → Pupil adjusts size → Lens focuses inverted image on retina → Photoreceptors generate signals → Brain interprets as erect.
Question – Compare myopia and hypermetropia. How are they corrected?
Answer – Myopia: Distant objects blurry (image forms before retina); corrected with concave lens.
Hypermetropia: Near objects blurry (image forms behind retina); corrected with convex lens.
Question – What is the Braille system? How does it help visually challenged individuals?
Answer – Braille: Tactile writing system using raised dots for alphabets/numbers.
Uses: Reading/writing via touch; aids education and communication.
Multiple-Choice Questions (MCQs)
Question – The cornea is part of:
- a) Retina
- b) Outer protective layer (Correct)
- c) Optic nerve
Question – Rod cells are responsible for:
- a) Colour vision
- b) Vision in dim light (Correct)
Question – Myopia is corrected using:
- a) Convex lens
- b) Concave lens (Correct)
Question – The persistence of vision lasts approximately:
- a) 1/16th second (Correct)
- b) 1 second
Question – Braille was developed by:
- a) Louis Braille (Correct)
- b) Helen Keller
True/False Questions
Question – The pupil appears black because it absorbs all light.
True (No light is reflected back).
Question –
False (Rods are active in dim light).
Question – Cataract is caused by vitamin A deficiency.
False (It’s lens opacity; night blindness is due to vitamin A deficiency).
Question – Hypermetropia occurs when the eyeball is too long.
False (Eyeball is too short or lens focal length too long).
Question – The far point of a normal eye is infinity.
True.
Additional Questions and Answers 02
Q1. What is the function of the cornea?
Answer: The cornea refracts (bends) incoming light towards the lens of the eye.
Q2. What is the function of the pupil?
Answer: The pupil controls the amount of light entering the eye by adjusting its size.
Q3. Name the transparent, flexible structure that focuses light onto the retina.
Answer: Eye lens.
Q4. What happens to the pupil in dim light?
Answer: The pupil enlarges (dilates) in dim light to allow more light to enter the eye.
Q5. What is the shape of a concave lens?
Answer: A concave lens is thinner at the center and thicker at the edges.
Q6. What is cataract?
Answer: Cataract is a defect in which the lens becomes cloudy, leading to blurred vision.
Q7. Name two defects of vision other than cataract.
Answer: Myopia (near-sightedness) and Hypermetropia (far-sightedness).
Q8. What causes myopia?
Answer: Myopia is caused when the eyeball is too long or the eye lens is too curved.
Long Questions and Answers
Q1. Explain the working of the human eye in detail.
Answer: The human eye works like a camera. Light enters through the cornea, passes through the aqueous humour, pupil, and lens, and finally falls on the retina. The lens focuses light rays to form a real and inverted image on the retina. Retina converts the light into electrical signals, which are carried by the optic nerve to the brain. The brain interprets the signals to recognize the object.
Q2. What is the power of accommodation of the human eye? How does it help us?
Answer: Power of accommodation is the ability of the human eye to adjust the focal length of the lens to focus objects at various distances.
When we see nearby objects, the ciliary muscles contract, making the lens thicker to focus the object.
When we see distant objects, the muscles relax, making the lens thinner.
This helps us to see both near and far objects clearly.
Q3. Differentiate between myopia and hypermetropia. Write their causes and corrections.
Answer:
Myopia | Hypermetropia |
Near objects are seen clearly, but distant objects are blurred. | Distant objects are seen clearly, but near objects are blurred. |
Caused by elongated eyeball or over-curved lens. | Caused by shortened eyeball or flat lens. |
Corrected by concave lens. | Corrected by convex lens. |
Q4. Describe any five important care tips to keep our eyes healthy.
Answer: Do not read in dim light or very bright light.
Maintain a proper distance while reading (around 25–30 cm).
Take regular breaks while using computers or mobiles.
Eat a balanced diet rich in Vitamin A.
Get your eyes checked regularly by an eye specialist.
MCQs (Multiple Choice Questions)
- The transparent front part of the eye is called—
(a) Iris
(b) Cornea
(c) Lens
(d) Retina
Answer: (b) Cornea
- The retina of the eye contains—
(a) only rod cells
(b) only cone cells
(c) both rod and cone cells
(d) only nerve endings
Answer: (c) both rod and cone cells
- Which part of the human eye controls the size of the pupil?
(a) Cornea
(b) Lens
(c) Iris
(d) Retina
Answer: (c) Iris
- Myopia can be corrected using—
(a) Convex lens
(b) Concave lens
(c) Plane mirror
(d) Prism
Answer: (b) Concave lens
- The least distance of distinct vision for a normal human eye is—
(a) 25 cm
(b) 50 cm
(c) 1 m
(d) 10 cm
Answer: (a) 25 cm
- Which of the following is NOT a cause of hypermetropia?
(a) Shorter eyeball
(b) Flattened lens
(c) Lengthened eyeball
(d) Loss of lens elasticity with age
Answer: (c) Lengthened eyeball
True or False
- The retina acts as a screen where images are formed.
Answer: True
- Iris changes the focal length of the eye lens.
Answer: False (Ciliary muscles do this)
- A convex lens is used to correct near-sightedness.
Answer: False
- Cataract can lead to complete blindness if not treated.
Answer: True
- The pupil appears white in color because of the lens reflection.
Answer: False (It appears black)
- The power of accommodation decreases with age.
Answer: True