👁️ The Human Eye and the Colourful World
Correction of Eye Defects Using Lenses
लेंसों द्वारा दृष्टि दोषों का सुधार
Class 10 Science | CBSE + Foundation + Competitive
👓 Correction of Eye Defects Using Lenses
1. Introduction – परिचय
Why are Correcting Lenses Required?
A normal eye forms a sharp image of an object on the retina. In some people, due to changes in the shape of the eyeball, optical power of the eye or age-related changes, the image is not formed properly on the retina.
ऐसे मामलों में उपयुक्त correcting lens का उपयोग करके प्रकाश किरणों का मार्ग बदला जाता है ताकि आँख की optical system द्वारा स्पष्ट छवि retina पर बनाई जा सके।
Eye Defect
→
Correcting Lens
→
Modified Rays
→
Retina
2. Basic Principle of Lens Correction – मूल सिद्धांत
The correcting lens placed in front of the eye changes the convergence or divergence of incoming light rays.
आँख के सामने लगाया गया correcting lens आने वाली प्रकाश किरणों के convergence या divergence को बदलता है।
| Defect | Problem | Correcting Lens | Action |
|---|---|---|---|
| Myopia | Image in front of retina | Concave lens | Diverges rays |
| Hypermetropia | Image behind retina | Convex lens | Converges rays |
| Presbyopia | Reduced accommodation | Suitable near correction; multifocal if required | Provides required optical power |
3. Correction of Myopia – मायोपिया का सुधार
Defect
Distant objects appear blurred.
दूर की वस्तुएँ धुंधली दिखाई देती हैं।
Image Position
Image of a distant object forms in front of retina.
छवि रेटिना के सामने बनती है।
Correction
A concave lens is used.
अवतल लेंस का उपयोग किया जाता है।
How Does Concave Lens Correct Myopia?
A concave lens diverges the incoming parallel rays before they enter the eye. This reduces the effective convergence of the eye's optical system and allows the rays to be focused on the retina.
अवतल लेंस प्रकाश किरणों को आँख में प्रवेश करने से पहले अपसारित करता है। इससे आँख की optical system द्वारा किरणों का focus पीछे की ओर shift होता है और image retina पर बन सकती है।
Myopia → Concave Lens → Divergence → Focus shifted backward → Retina
Myopia Correction Using Concave Lens
4. Correction of Hypermetropia – हाइपरमेट्रोपिया का सुधार
Defect
Nearby objects appear blurred.
पास की वस्तुएँ स्पष्ट नहीं दिखाई देतीं।
Image Position
The image of a nearby object tends to form behind retina.
छवि रेटिना के पीछे बनने की प्रवृत्ति होती है।
Correction
A convex lens is used.
उत्तल लेंस का उपयोग किया जाता है।
How Does Convex Lens Correct Hypermetropia?
A convex lens converges the incoming rays before they enter the eye. This increases the effective convergence of the optical system and allows the image to be formed on the retina.
उत्तल लेंस प्रकाश किरणों को आँख में प्रवेश करने से पहले अभिसारित करता है। इससे focus आगे की ओर shift होता है और image retina पर बन सकती है।
Hypermetropia → Convex Lens → Convergence → Focus shifted forward → Retina
Hypermetropia Correction Using Convex Lens
5. Correction of Presbyopia – प्रेसबायोपिया का सुधार
Presbyopia occurs mainly due to age-related reduction in accommodation. The eye becomes less able to focus on nearby objects.
Presbyopia में उम्र बढ़ने के साथ lens flexibility तथा accommodation कम हो सकती है, जिसके कारण पास की वस्तुओं को focus करना कठिन हो जाता है।
Correction
- Suitable convex lens is commonly used for near vision.
- If both near and distant vision require correction, bifocal or progressive lenses may be prescribed.
Presbyopia → Accommodation ↓ → Near Focus Difficult → Near Convex Correction
Important: Presbyopia is an age-related accommodation problem. It is not simply the same as hypermetropia, although the symptoms of difficulty in near vision can overlap.
6. Bifocal Lens – द्विफोकसी लेंस
A bifocal lens has two different optical regions. One region is used for distant vision and the other for near vision.
यदि किसी व्यक्ति को दूर और पास दोनों प्रकार की दृष्टि के लिए अलग-अलग correction की आवश्यकता हो, तो bifocal lens उपयोगी हो सकता है।
| Part | Typical Function |
|---|---|
| Upper region | Distance vision correction |
| Lower region | Near vision correction |
The exact lens powers are determined by an eye-care professional according to the person's refractive error.
7. Combined Vision Problems – संयुक्त दृष्टि समस्याएँ
A person may have more than one refractive or vision problem. For example, an older person may have a pre-existing distance correction requirement and also need additional near correction.
ऐसी स्थिति में एक ही lens system में अलग-अलग optical powers की आवश्यकता हो सकती है। Bifocal या progressive lenses ऐसे मामलों में prescribed किए जा सकते हैं।
8. Power of Correcting Lens – सुधारक लेंस की शक्ति
P = 1/f
Where:
- P = power of lens in dioptre (D)
- f = focal length in metre
| Lens | Focal Length | Power | Nature |
|---|---|---|---|
| Convex | Positive | Positive | Converging |
| Concave | Negative | Negative | Diverging |
9. Important Numericals – महत्वपूर्ण संख्यात्मक प्रश्न
Numerical 1 – Myopia
A myopic person is prescribed a concave lens of focal length –50 cm. Find its power.
f = -50 cm = -0.50 m
P = 1/f
P = 1/(-0.50)
P = -2 D
P = 1/f
P = 1/(-0.50)
P = -2 D
Answer: –2 D
Numerical 2 – Hypermetropia
A person is prescribed a convex lens of focal length +25 cm. Find its power.
f = +25 cm = +0.25 m
P = 1/0.25
P = +4 D
P = 1/0.25
P = +4 D
Answer: +4 D
Numerical 3
A correcting lens has power –5 D. Calculate its focal length.
P = 1/f
f = 1/P
f = 1/(-5)
f = -0.20 m
f = -20 cm
f = 1/P
f = 1/(-5)
f = -0.20 m
f = -20 cm
Negative focal length shows that the lens is concave.
Numerical 4
A convex correcting lens has power +2.5 D. Find its focal length.
f = 1/P
f = 1/2.5
f = 0.4 m
f = 40 cm
f = 1/2.5
f = 0.4 m
f = 40 cm
10. Defect vs Correcting Lens – सबसे महत्वपूर्ण तुलना
| Point | Myopia | Hypermetropia | Presbyopia |
|---|---|---|---|
| Vision difficulty | Distance | Near | Usually near |
| Main reason | Excess convergence / long eyeball | Insufficient convergence / short eyeball | Age-related reduced accommodation |
| Image problem | In front of retina | Behind retina | Near focus becomes difficult |
| Correction | Concave | Convex | Near convex; multifocal if needed |
| Lens power | Negative | Positive | Near correction usually positive |
11. 30 MCQs – Multiple Choice Questions
Q1. Myopia is corrected by:
(A) Concave lens
(B) Convex lens
(C) Plane glass
(D) Prism only
Answer: A
Q2. Hypermetropia is corrected by:
(A) Concave lens
(B) Convex lens
(C) Plane mirror
(D) Concave mirror
Answer: B
Q3. A concave lens mainly:
(A) Converges rays
(B) Reflects rays
(C) Diverges rays
(D) Absorbs rays
Answer: C
Q4. A convex lens mainly:
(A) Diverges rays
(B) Reflects rays
(C) Absorbs rays
(D) Converges rays
Answer: D
Q5. In myopia, the image of a distant object forms:
(A) In front of retina
(B) Behind retina
(C) On iris
(D) On cornea
Answer: A
Q6. In hypermetropia, the image of a nearby object tends to form:
(A) In front of retina
(B) Behind retina
(C) On pupil
(D) On cornea
Answer: B
Q7. Presbyopia is mainly related to:
(A) Corneal colour
(B) Retina colour
(C) Ageing
(D) Iris shape only
Answer: C
Q8. Power of a lens is:
(A) P=f
(B) P=f²
(C) P=2f
(D) P=1/f
Answer: D
Q9. The power of a concave lens is generally:
(A) Negative
(B) Positive
(C) Zero
(D) Infinite
Answer: A
Q10. The power of a convex lens is generally:
(A) Negative
(B) Positive
(C) Zero
(D) Infinite
Answer: B
Q11. A lens of focal length –0.5 m has power:
(A) +2 D
(B) +0.5 D
(C) –2 D
(D) –0.5 D
Answer: C
Q12. A lens of focal length +0.25 m has power:
(A) –4 D
(B) +0.25 D
(C) –0.25 D
(D) +4 D
Answer: D
Q13. The unit of lens power is:
(A) Dioptre
(B) Metre
(C) Joule
(D) Watt
Answer: A
Q14. The focal length used in P=1/f must be expressed in:
(A) cm
(B) m
(C) km
(D) mm
Answer: B
Q15. A myopic eye has excessive:
(A) Divergence
(B) Reflection
(C) Convergence
(D) Scattering
Answer: C
Q16. A hypermetropic eye requires additional:
(A) Divergence
(B) Reflection
(C) Scattering
(D) Convergence
Answer: D
Q17. A person unable to see distant objects clearly may have:
(A) Myopia
(B) Hypermetropia
(C) Presbyopia only
(D) Normal vision
Answer: A
Q18. A person unable to see nearby objects clearly may have:
(A) Myopia only
(B) Hypermetropia
(C) Normal vision
(D) No optical problem
Answer: B
Q19. Presbyopia is associated with reduced:
(A) Scattering
(B) Reflection
(C) Accommodation
(D) Dispersion
Answer: C
Q20. Bifocal lenses can provide:
(A) Only colour correction
(B) Only retinal correction
(C) Only night vision
(D) Different powers for near and distance vision
Answer: D
Q21. Which lens has negative focal length?
(A) Concave lens
(B) Convex lens
(C) Plane glass
(D) None
Answer: A
Q22. Which lens has positive focal length?
(A) Concave lens
(B) Convex lens
(C) Plane glass
(D) None
Answer: B
Q23. If focal length decreases, magnitude of power:
(A) Decreases
(B) Remains same
(C) Increases
(D) Becomes zero
Answer: C
Q24. A concave correcting lens shifts the effective focus:
(A) Forward
(B) Nowhere
(C) Randomly
(D) Backward
Answer: D
Q25. A convex correcting lens helps shift the effective focus:
(A) Forward
(B) Backward
(C) Sideways
(D) Nowhere
Answer: A
Q26. A lens of power –4 D is:
(A) Convex
(B) Concave
(C) Plane
(D) Cylindrical mirror
Answer: B
Q27. A lens of power +5 D is:
(A) Concave
(B) Plane
(C) Convex
(D) Mirror
Answer: C
Q28. Near correction in presbyopia is commonly provided by:
(A) Concave lens
(B) Plane glass
(C) Concave mirror
(D) Suitable convex lens
Answer: D
Q29. The normal eye forms a clear image on the:
(A) Retina
(B) Iris
(C) Cornea
(D) Pupil
Answer: A
Q30. The correcting lens is placed:
(A) Behind the retina
(B) In front of the eye
(C) Inside the optic nerve
(D) Inside the brain
Answer: B
12. 30 Subjective Questions with Answers
2 Marks
Q1. Which lens corrects myopia?
Myopia is corrected by a concave lens. It diverges the incoming rays and helps the eye focus them on the retina.
2 Marks
Q2. Which lens corrects hypermetropia?
Hypermetropia is corrected by a convex lens, which converges the incoming rays before they enter the eye.
2 Marks
Q3. What is the function of a correcting lens?
It changes the convergence or divergence of light entering the eye so that the image can be focused on the retina.
2 Marks
Q4. What is the sign of power of a concave lens?
The power of a concave lens is negative.
2 Marks
Q5. What is the unit of lens power?
The unit of lens power is dioptre (D).
3 Marks
Q6. Explain how a concave lens corrects myopia.
A concave lens diverges incoming rays before they enter the eye. This reduces the effective convergence and shifts the focus backward so that the image is formed on the retina.
3 Marks
Q7. Explain how a convex lens corrects hypermetropia.
A convex lens converges incoming rays before they enter the eye. This increases the effective convergence and shifts the focus forward onto the retina.
3 Marks
Q8. What is the relation between focal length and power?
Power is inversely proportional to focal length:
P = 1/f
where f is in metres.
P = 1/f
where f is in metres.
3 Marks
Q9. Why is the power of a concave lens negative?
Under the standard Cartesian sign convention, the focal length of a concave lens is negative. Since P = 1/f, its power is also negative.
3 Marks
Q10. Why are bifocal lenses used?
Bifocal lenses provide two optical powers, generally one for distance vision and another for near vision. They may be prescribed when both corrections are needed.
4 Marks
Q11. Explain the correction of myopia with a ray diagram.
In myopia, distant-object rays are focused in front of the retina. A concave lens is placed before the eye. It diverges the rays slightly, reducing the effective convergence and enabling the eye to form the image on the retina.
4 Marks
Q12. Explain the correction of hypermetropia with a ray diagram.
In hypermetropia, rays from a nearby object tend to focus behind the retina. A convex lens is placed before the eye. It converges the rays before they enter the eye, allowing the image to be formed on the retina.
4 Marks
Q13. Explain the correction of presbyopia.
Presbyopia results mainly from reduced accommodation with age. Suitable convex near correction can help focus nearby objects. If both near and distance corrections are required, bifocal or progressive lenses may be prescribed.
4 Marks
Q14. Differentiate between concave and convex correcting lenses.
Concave lenses diverge light, have negative focal length and negative power, and are used to correct myopia. Convex lenses converge light, have positive focal length and positive power, and are used to correct hypermetropia.
4 Marks
Q15. What is meant by power of a lens?
Power indicates the converging or diverging ability of a lens. It is given by P = 1/f, where f is focal length in metres. Its unit is dioptre.
5 Marks
Q16. Describe the complete correction mechanism of myopia.
Myopia causes distant-object images to form in front of the retina. A concave lens is placed in front of the eye. The concave lens diverges the incoming rays. These rays then enter the eye with reduced convergence, allowing the eye lens to focus them on the retina.
5 Marks
Q17. Describe the complete correction mechanism of hypermetropia.
Hypermetropia causes nearby-object images to tend to form behind the retina. A convex lens is placed before the eye. It converges the incoming rays, increasing their effective convergence. The eye then forms the image on the retina.
5 Marks
Q18. Explain the role of correcting lenses in vision.
Correcting lenses modify the direction and convergence of incoming rays before they enter the eye. A concave lens adds divergence for myopia, while a convex lens adds convergence for hypermetropia and suitable near correction. The objective is to allow clear retinal image formation.
5 Marks
Q19. Calculate the power of a concave lens of focal length –25 cm.
f = –25 cm = –0.25 m
P = 1/f
P = 1/–0.25
P = –4 D
P = 1/f
P = 1/–0.25
P = –4 D
5 Marks
Q20. Calculate the power of a convex lens of focal length 40 cm.
f = 40 cm = 0.40 m
P = 1/0.40
P = +2.5 D
P = 1/0.40
P = +2.5 D
6 Marks
Q21. Explain myopia, its cause, image formation and correction.
Myopia is short-sightedness. A person can see nearby objects clearly but distant objects appear blurred. It may occur due to an elongated eyeball or excessive converging power of the eye. The image of a distant object forms in front of the retina. It is corrected with a concave lens, which diverges incoming rays and helps move the effective focus onto the retina.
6 Marks
Q22. Explain hypermetropia, its cause, image formation and correction.
Hypermetropia is long-sightedness. Nearby objects appear blurred. It may occur due to a shorter eyeball or insufficient converging power of the eye. The image of a nearby object tends to form behind the retina. A convex lens is used to converge the rays and allow image formation on the retina.
6 Marks
Q23. Explain presbyopia and its correction methods.
Presbyopia is an age-related reduction in accommodation. It occurs mainly due to reduced flexibility of the eye lens and changes in the ciliary muscle system. Suitable convex correction is used for near vision. If both near and distance corrections are required, bifocal or progressive lenses may be prescribed.
6 Marks
Q24. Compare the correcting lenses used for myopia and hypermetropia.
Myopia is corrected by a concave lens because the eye has excessive effective convergence and the image forms in front of the retina. Hypermetropia is corrected by a convex lens because additional convergence is required and the image tends to form behind the retina.
6 Marks
Q25. Derive the formula for power of a lens.
By definition, power is the reciprocal of focal length when focal length is measured in metres:
P = 1/f
Thus a lens with shorter focal length has greater magnitude of power. Positive f gives positive power and negative f gives negative power.
P = 1/f
Thus a lens with shorter focal length has greater magnitude of power. Positive f gives positive power and negative f gives negative power.
6 Marks
Q26. A person has a lens of power –2.5 D. Find its focal length and identify the lens.
P = –2.5 D
f = 1/P = 1/–2.5 = –0.4 m
f = –40 cm
The negative focal length indicates a concave lens.
f = 1/P = 1/–2.5 = –0.4 m
f = –40 cm
The negative focal length indicates a concave lens.
6 Marks
Q27. A person has a lens of power +4 D. Find its focal length.
P = +4 D
f = 1/P = 1/4 = 0.25 m
f = 25 cm.
The lens is convex.
f = 1/P = 1/4 = 0.25 m
f = 25 cm.
The lens is convex.
6 Marks
Q28. Why can an elderly person require bifocal lenses?
Age-related reduction in accommodation can make near focusing difficult. If the person also requires correction for distance vision, two different optical powers may be required. Bifocal lenses provide separate regions for near and distance vision.
6 Marks
Q29. Explain why the sign of lens power is important in eye-defect correction.
The sign indicates the optical nature of the correcting lens. Negative power represents a concave, diverging lens, generally used for myopia. Positive power represents a convex, converging lens, used for hypermetropia and commonly for near correction in presbyopia.
6 Marks
Q30. Explain the complete flow of correction of a defective eye.
First, the vision defect is identified. A suitable lens is then selected according to the required optical power. The correcting lens modifies the convergence or divergence of incoming rays. These modified rays enter the eye, and the eye lens finally focuses the image on the retina, producing clearer vision.
13. Assertion–Reason Questions
Q1. Assertion: Myopia is corrected using a concave lens.
Reason: A concave lens diverges light rays.
Reason: A concave lens diverges light rays.
Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion.
Q2. Assertion: Hypermetropia is corrected using a convex lens.
Reason: A convex lens converges light rays.
Reason: A convex lens converges light rays.
Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion.
Q3. Assertion: A concave lens has negative power.
Reason: Its focal length is negative under the standard sign convention.
Reason: Its focal length is negative under the standard sign convention.
Answer: Both are true, and Reason correctly explains the Assertion.
Q4. Assertion: Bifocal lenses may be prescribed to some people with presbyopia.
Reason: Such a person may require different optical powers for near and distance vision.
Reason: Such a person may require different optical powers for near and distance vision.
Answer: Both are true, and Reason correctly explains the Assertion.
Q5. Assertion: A lens of shorter focal length has greater magnitude of power.
Reason: P = 1/f.
Reason: P = 1/f.
Answer: Both are true, and Reason correctly explains the Assertion.
14. HOTS / Competency-Based Questions
HOTS 1. A student cannot see the writing on a distant blackboard clearly but can read a book comfortably. Which lens should be prescribed?
The student likely has myopia. A concave lens is used.
HOTS 2. A person can see distant objects clearly but struggles to read a nearby book. What correction may be required?
It may indicate hypermetropia or, in an older person, reduced accommodation due to presbyopia. The exact prescription requires an eye examination. Hypermetropia is corrected with a convex lens.
HOTS 3. A correcting lens has power –3 D. Identify the lens and calculate its focal length.
Negative power means concave lens.
f = 1/P = 1/–3 = –0.333 m ≈ –33.3 cm.
f = 1/P = 1/–3 = –0.333 m ≈ –33.3 cm.
HOTS 4. Why cannot a convex lens be normally used to correct myopia?
A myopic eye already has excessive effective convergence. A convex lens would add further convergence and would not shift the focus in the required direction. A concave lens provides divergence.
HOTS 5. Why might an elderly person need different lens powers for different viewing distances?
Because accommodation may decrease with age. The eye may not change its focal length sufficiently for near objects, while a separate distance correction may also be required. Multifocal lenses can provide different optical powers.
15. Important Diagram Labelling
Myopia Correction Diagram
Incident Rays Concave Lens Eye Lens Retina FocusHypermetropia Correction Diagram
Incident Rays Convex Lens Eye Lens Retina FocusPresbyopia / Bifocal Diagram
Distance Vision Region Near Vision Region Eye Retina
16. CBSE Golden Points – परीक्षा के लिए महत्वपूर्ण बिंदु
- Myopia → Concave Lens.
- Myopia: distant objects blurred.
- Myopia: image forms in front of retina.
- Hypermetropia → Convex Lens.
- Hypermetropia: nearby objects blurred.
- Hypermetropia: image tends to form behind retina.
- Presbyopia → Age-related reduction in accommodation.
- Near correction is commonly provided by a suitable convex lens.
- Bifocal/progressive lenses may be used when both near and distance corrections are needed.
- Concave lens → diverging lens → negative power.
- Convex lens → converging lens → positive power.
- P = 1/f.
- Focal length must be in metres when calculating power in dioptres.
- Shorter focal length → greater magnitude of power.
- The final aim of correction is clear image formation on the retina.
17. Memory Tricks – याद रखने की ट्रिक
👁️ MYOPIA
Myopia → Far Problem → Concave
👁️ HYPERMETROPIA
Hypermetropia → Near Problem → Convex
👓 PRESBYOPIA
Age → Accommodation ↓ → Near Correction
🔬 POWER
Small f → Big P
P = 1/f
18. One-Minute Revision
| Defect | Vision Problem | Image | Correcting Lens |
|---|---|---|---|
| Myopia | Distance vision | In front of retina | Concave |
| Hypermetropia | Near vision | Behind retina | Convex |
| Presbyopia | Reduced near accommodation | Near focusing difficult | Convex near correction / Multifocal if required |
MYOPIA → FRONT → CONCAVE
HYPERMETROPIA → BEHIND → CONVEX
PRESBYOPIA → AGE → ACCOMMODATION ↓
CONCAVE → DIVERGE → NEGATIVE POWER
CONVEX → CONVERGE → POSITIVE POWER
🎯 Final Summary
Eye defects can often be corrected by placing a suitable optical lens in front of the eye. Myopia is corrected with a concave lens because additional divergence is required. Hypermetropia is corrected with a convex lens because additional convergence is required. Presbyopia is an age-related reduction in accommodation and may require suitable near correction or multifocal lenses when both near and distance corrections are needed.
Exam Formula:
P = 1/f
Remember:
Myopia → Concave → Negative Power
Hypermetropia → Convex → Positive Power
Presbyopia → Reduced Accommodation → Near Correction
