CBSE · Class X · Science
The Human Eye and the Colourful World
Chapter-Based Practice Question Paper
General Instructions
- This question paper has five sections: A, B, C, D and E. All sections are compulsory.
- Section A has 12 objective-type questions carrying 1 mark each.
- Section B has 8 very short-answer questions carrying 2 marks each.
- Section C has 8 short-answer questions carrying 3 marks each.
- Section D has 4 long-answer questions carrying 5 marks each.
- Section E has 2 case-based questions carrying 4 marks each, with four sub-parts of 1 mark each.
- Wherever asked, draw neat and labelled diagrams.
- Use \(P = \dfrac{1}{f}\) (f in metres) for all lens power calculations, and show all steps.
| Section | Type | No. of Questions | Marks Each | Total Marks |
|---|---|---|---|---|
| A | Multiple Choice Questions | 12 | 1 | 12 |
| B | Very Short Answer | 8 | 2 | 16 |
| C | Short Answer | 8 | 3 | 24 |
| D | Long Answer | 4 | 5 | 20 |
| E | Case-Based | 2 | 4 | 8 |
| Grand Total | 80 | |||
SECTION A — Multiple Choice Questions 12 × 1 = 12 marks
A1.The image of an object is formed on the ______ of the eye.1
A2.The eye lens forms:1
A3.The least distance of distinct vision for a normal young adult is:1
A4.A person can read a book held close but cannot see the blackboard clearly. This person is most likely suffering from:1
A5.Hypermetropia is corrected using a:1
A6.The angle between the incident ray and the emergent ray of a prism is called the angle of:1
A7.Which colour of white light deviates the most on passing through a prism?1
A8.The scattering of light by colloidal particles, which makes the path of a light beam visible, is called:1
A9.The clear sky appears blue because air molecules scatter:1
A10.Danger signal lights are red in colour because:1
A11.The change in curvature of the eye lens during accommodation is brought about by the:1
A12.Cataract is caused due to:1
SECTION B — Very Short Answer Questions 8 × 2 = 16 marks
B1.Define the power of accommodation of the eye.2
B2.What is meant by the near point and the far point of a normal human eye?2
B3.Why do stars twinkle in the night sky? (State the reason briefly.)2
B4.Name the refractive defect of vision that occurs mainly due to (i) ageing, and (ii) elongation of the eyeball.2
B5.What is meant by dispersion of light? Which colour of white light bends the least?2
B6.Why does the sky appear dark instead of blue to an astronaut in space?2
B7.What is the Tyndall effect? Give one everyday example.2
B8.Why must donated eyes be removed within 4–6 hours after death?2
SECTION C — Short Answer Questions 8 × 3 = 24 marks
C1.Draw a labelled ray diagram showing the human eye forming an image of a distant object on the retina, and briefly describe the path light takes.3
C2.What is myopia? State two possible causes and the correction used for this defect.3
C3.What is hypermetropia? State two possible causes and the correction used for this defect.3
C4.Distinguish between myopia and hypermetropia under the heads: image formation, cause, and corrective lens used.3
C5.A person needs a lens of power \(-2.5\) D to correct his distant vision. Name the defect, and find the nature and focal length of the lens.3
C6.Explain, with reason, why planets do not twinkle although they shine by reflected sunlight just like stars.3
C7.Explain why the Sun appears reddish at sunrise and sunset, but whitish at noon.3
C8.What is presbyopia? Why does an elderly person often suffer from both myopia and hypermetropia at once, and how is this corrected?3
SECTION D — Long Answer Questions 4 × 5 = 20 marks
D1.(a) What is meant by the power of accommodation of the eye? (2)
(b) A person's near point has receded to 1 m. Assuming the normal near point is 25 cm, calculate the power and nature of the corrective lens required. (3)5
(b) A person's near point has receded to 1 m. Assuming the normal near point is 25 cm, calculate the power and nature of the corrective lens required. (3)5
D2.(a) Define myopia. Draw ray diagrams to show (i) the defect and (ii) its correction using a suitable lens. (3)
(b) The far point of a myopic person is 1.5 m from the eye. What is the power of the lens required to enable him to see distant objects clearly? (2)5
(b) The far point of a myopic person is 1.5 m from the eye. What is the power of the lens required to enable him to see distant objects clearly? (2)5
D3.(a) State the cause of dispersion of white light through a glass prism. (2)
(b) With the help of a labelled diagram, explain the formation of a rainbow in the sky. (3)5
(b) With the help of a labelled diagram, explain the formation of a rainbow in the sky. (3)5
D4.(a) Explain, giving reasons, the phenomenon of twinkling of stars. (3)
(b) A person needs a lens of power \(+1.5\) D to correct his near vision. Find the focal length of the lens. Is the person suffering from myopia or hypermetropia? (2)5
(b) A person needs a lens of power \(+1.5\) D to correct his near vision. Find the focal length of the lens. Is the person suffering from myopia or hypermetropia? (2)5
SECTION E — Case-Based Questions 2 × 4 = 8 marks
E1.Read the passage and answer the questions that follow.4
Ramesh, a Class 10 student, always sits in the last row of his classroom. He finds it very difficult to read what his teacher writes on the blackboard, though he has no trouble at all reading his textbook held close to his eyes. Worried, his parents took him to an eye specialist, who measured his far point to be 2 m in front of his eye and recommended spectacles.
(i) Name the refractive defect Ramesh is suffering from. (1)
(ii) Which type of lens will the doctor prescribe for Ramesh? (1)
(iii) Calculate the power of the lens needed to correct his vision. (1)
(iv) State one possible cause of this defect. (1)
E2.Read the passage and answer the questions that follow.4
At their school science exhibition, a group of students set up two stalls side by side. At the first stall, they shine a narrow beam of white sunlight through a glass prism and project a beautiful band of colours onto a screen. At the second stall, they display a poster about eye donation, explaining how donated eyes can restore sight to the blind.
(i) Name the phenomenon responsible for the colourful band of light seen at the first stall. (1)
(ii) Write the acronym used to remember the correct order of colours in this band. (1)
(iii) Which colour bends the least, and why do different colours bend by different amounts? (1)
(iv) How many corneal blind people can be given vision by one donor's pair of eyes? (1)
— End of Question Paper —
Detailed Answer Key & Marking Scheme
SECTION A — Answers 12 marks
A1. (c) Retina — the retina is the light-sensitive screen where the eye lens forms the image.
A2. (b) A real, inverted image — like any convex lens, the eye lens forms a real, inverted image on the retina; the brain then interprets it as upright.
A3. (b) 25 cm — the standard near point for a young adult with normal vision.
A4. (b) Myopia — near vision is fine but the far point is nearer than infinity.
A5. (b) Convex lens — provides the extra converging power needed to bring the image forward onto the retina.
A6. (c) Deviation — \(\angle D\) is the angle between the incident and emergent rays.
A7. (c) Violet — violet light bends the most; red bends the least.
A8. (b) Tyndall effect — scattering by colloidal particles that makes a light beam's path visible.
A9. (b) Blue light more strongly — shorter wavelengths scatter more effectively than longer (red) wavelengths.
A10. (b) Red is scattered the least — so it remains visible over long distances through fog/smoke.
A11. (b) Ciliary muscles — contracting/relaxing changes the lens's curvature and thickness.
A12. (c) The eye lens becoming milky and cloudy — treatable through cataract surgery.
Marking: 1 mark for each correct option. No marks for justification unless specifically asked.
SECTION B — Answers 16 marks
B1. The power of accommodation is the ability of the eye lens to adjust (increase or decrease) its focal length, using the ciliary muscles, so that objects at different distances can be focused clearly on the retina.
Marking: 1 mark for "ability to adjust focal length"; 1 mark for mentioning ciliary muscles / focusing objects at different distances.
B2. Near point: the closest distance at which an object can be seen clearly without strain (about 25 cm for a normal adult). Far point: the farthest distance up to which the eye can see clearly (infinity for a normal eye).
Marking: 1 mark each for correct definition.
B3. Starlight undergoes continuous refraction through the Earth's atmosphere, whose refractive index keeps fluctuating slightly. This makes the star's apparent position and brightness fluctuate, so it appears to twinkle.
Marking: 1 mark for mentioning atmospheric refraction; 1 mark for explaining fluctuating apparent position/brightness.
B4. (i) Ageing → Presbyopia. (ii) Elongation of the eyeball → Myopia.
Marking: 1 mark each.
B5. Dispersion is the splitting of white light into its seven component colours (VIBGYOR) as it passes through a prism, because each colour bends by a different amount. Red bends the least.
Marking: 1 mark for definition; 1 mark for naming red.
B6. Above the Earth's atmosphere, there are no air molecules to scatter sunlight, so no scattered blue light reaches the astronaut's eyes — hence the sky looks black even during the "day".
Marking: 1 mark for "no atmosphere to scatter light"; 1 mark for explaining the resulting black sky.
B7. The Tyndall effect is the scattering of light by colloidal-sized particles suspended in a medium, which makes the beam's path visible. Example: a beam of sunlight becoming visible when it enters a smoke-filled room through a small hole (or light through a dense forest canopy).
Marking: 1 mark for definition; 1 mark for a valid example.
B8. Corneal tissue starts to deteriorate soon after death, so the cornea must be removed within 4–6 hours while it is still healthy and transparent enough to be transplanted successfully.
Marking: 2 marks for a reasonable explanation involving tissue viability/deterioration.
SECTION C — Answers 24 marks
C1. Light from a distant object enters through the cornea (which does most of the refraction), passes through the pupil (regulated by the iris), is fine-focused by the eye lens, and forms a real, inverted image on the retina, which is then relayed via the optic nerve to the brain.
Fig: Light from a distant object refracted by the cornea and lens, forming an inverted real image on the retina.
Marking: 2 marks for a correctly labelled diagram (cornea, lens, retina, ray paths); 1 mark for the description.
C2. Myopia (near-sightedness): a defect where distant objects appear blurred but nearby objects are seen clearly; the image of a distant object forms in front of the retina. Causes: (i) excessive curvature of the eye lens, (ii) elongation of the eyeball. Correction: a concave (diverging) lens of suitable power, which diverges the rays slightly before they enter the eye so the image shifts back onto the retina.
Marking: 1 mark for definition, 1 mark for two causes, 1 mark for correction.
C3. Hypermetropia (far-sightedness): a defect where nearby objects appear blurred but distant objects are seen clearly; the image of a nearby object forms behind the retina. Causes: (i) the eyeball becomes too small, (ii) the focal length of the eye lens becomes too long. Correction: a convex (converging) lens of suitable power, which adds the extra focusing power needed to form the image on the retina.
Marking: 1 mark for definition, 1 mark for two causes, 1 mark for correction.
C4.
| Aspect | Myopia | Hypermetropia |
|---|---|---|
| Image formation | In front of the retina | Behind the retina |
| Cause | Eyeball too long / lens too curved | Eyeball too small / focal length too long |
| Corrective lens | Concave lens | Convex lens |
Marking: 1 mark for each correctly filled row.
C5. Since the lens power is negative, this is myopia, and a concave lens is required.
\[ f = \dfrac{1}{P} = \dfrac{1}{-2.5\ \text{D}} = -0.4\ \text{m} = -40\ \text{cm} \]
\[ f = \dfrac{1}{P} = \dfrac{1}{-2.5\ \text{D}} = -0.4\ \text{m} = -40\ \text{cm} \]
Marking: 1 mark for naming the defect, 1 mark for identifying the lens as concave, 1 mark for correctly calculating f = -0.4 m (-40 cm).
C6. Planets are much closer to Earth than stars, so they are seen as extended sources (small discs), not point sources. Each planet can be thought of as a large collection of point-sized sources of light; the small variations in light from each of these points average out over the whole disc, cancelling the twinkling effect that a true point source would show.
Marking: 1 mark for "extended source, not a point source"; 2 marks for explaining the averaging-out of the variation.
C7. At sunrise and sunset, sunlight travels through a much greater thickness of the atmosphere to reach an observer than at noon. Most of the blue (short-wavelength) light gets scattered away along this longer path, leaving mainly the red and orange (long-wavelength) light to reach the eye, which is scattered the least. At noon, the path through the atmosphere is much shorter, so the Sun appears whitish.
Marking: 1 mark for "longer path through atmosphere at sunrise/sunset"; 1 mark for "blue scattered away, red remains"; 1 mark for contrasting with the shorter path at noon.
C8. Presbyopia is the age-related defect where the eye gradually loses its power of accommodation because the ciliary muscles weaken and the eye lens loses flexibility, causing the near point to recede. An elderly person may suffer from both myopia and hypermetropia together (needing both distant and near correction), and this combination is corrected using bi-focal lenses, where the upper (concave) part corrects distant vision and the lower (convex) part corrects near vision.
Marking: 1 mark for definition of presbyopia, 1 mark for explaining why both defects can occur together, 1 mark for bi-focal lens correction.
SECTION D — Answers 20 marks
D1.
(a) The power of accommodation is the eye lens's ability to change its focal length — becoming thinner (longer focal length) for distant objects and thicker (shorter focal length) for near objects — using the ciliary muscles, so that a clear image always forms on the retina.
(b) Here \( u = -25\ \text{cm} = -0.25\ \text{m} \) (object placed at the normal near point) and \( v = -1\ \text{m} \) (image must form, virtually, at the person's actual near point).
Using the lens formula: \[ \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{-1} - \dfrac{1}{-0.25} = -1 + 4 = 3 \] \[ f = \dfrac{1}{3} \approx 0.33\ \text{m}, \qquad P = \dfrac{1}{f} = +3\ \text{D} \] The lens required is a convex lens of power +3 D.
(b) Here \( u = -25\ \text{cm} = -0.25\ \text{m} \) (object placed at the normal near point) and \( v = -1\ \text{m} \) (image must form, virtually, at the person's actual near point).
Using the lens formula: \[ \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{-1} - \dfrac{1}{-0.25} = -1 + 4 = 3 \] \[ f = \dfrac{1}{3} \approx 0.33\ \text{m}, \qquad P = \dfrac{1}{f} = +3\ \text{D} \] The lens required is a convex lens of power +3 D.
Marking: (a) 2 marks for a complete definition. (b) 1 mark for correctly identifying u and v, 1 mark for correct substitution/calculation, 1 mark for the final answer with correct sign and nature of lens.
D2.
(a) Myopia is the defect in which a person can see nearby objects clearly, but distant objects appear blurred because the image forms in front of the retina (the far point is nearer than infinity).
Fig: Myopic eye (top) — image before the retina; corrected eye (bottom) — a concave lens diverges rays so the image lands on the retina.
(b) Far point \( = 1.5\ \text{m} \), so \( f = -1.5\ \text{m} \) (concave lens).
\[ P = \dfrac{1}{f} = \dfrac{1}{-1.5} \approx -0.67\ \text{D} \]
\[ P = \dfrac{1}{f} = \dfrac{1}{-1.5} \approx -0.67\ \text{D} \]
Marking: (a) 1 mark for definition, 2 marks for correctly labelled diagrams (defect + correction). (b) 1 mark for method, 1 mark for correct final value with sign.
D3.
(a) Different colours of white light travel at slightly different speeds inside glass and therefore bend (refract) through different angles as they pass through a prism — violet bends the most and red the least. This unequal bending separates white light into its spectrum of colours, which is called dispersion.
(b) Raindrops in the atmosphere act like tiny prisms. Sunlight entering a raindrop is first refracted and dispersed into its component colours, then internally reflected off the back of the drop, and refracted again as it exits — separating the colours further. Because a rainbow always forms in the direction opposite to the Sun, millions of droplets together send different colours to the observer's eye at slightly different angles, producing the familiar arc of colours (VIBGYOR).
(b) Raindrops in the atmosphere act like tiny prisms. Sunlight entering a raindrop is first refracted and dispersed into its component colours, then internally reflected off the back of the drop, and refracted again as it exits — separating the colours further. Because a rainbow always forms in the direction opposite to the Sun, millions of droplets together send different colours to the observer's eye at slightly different angles, producing the familiar arc of colours (VIBGYOR).
Fig: Dispersion of white light through a glass prism into VIBGYOR — red deviates least, violet deviates most.
Marking: (a) 2 marks for explaining unequal bending of colours. (b) 1 mark for refraction+dispersion, 1 mark for internal reflection, 1 mark for a correctly labelled diagram/explanation of the resulting arc.
D4.
(a) Starlight undergoes continuous refraction as it passes through the Earth's atmosphere, which has a gradually and irregularly changing refractive index (since air density and temperature keep fluctuating). Because stars are extremely distant, they behave as point sources of light. As the atmosphere bends the path of light from the star slightly differently moment to moment, the star's apparent position keeps shifting a little, and the amount of light reaching the eye fluctuates — so the star appears to twinkle, alternately brighter and fainter.
(b) \[ f = \dfrac{1}{P} = \dfrac{1}{1.5} \approx 0.67\ \text{m} = 66.7\ \text{cm} \] Since the power is positive (a convex lens is needed) and it is used to correct near vision, the person is suffering from hypermetropia.
(b) \[ f = \dfrac{1}{P} = \dfrac{1}{1.5} \approx 0.67\ \text{m} = 66.7\ \text{cm} \] Since the power is positive (a convex lens is needed) and it is used to correct near vision, the person is suffering from hypermetropia.
Marking: (a) 1 mark for atmospheric refraction with changing refractive index, 1 mark for "point source", 1 mark for explaining fluctuating apparent position/brightness. (b) 1 mark for correct focal length, 1 mark for correctly identifying hypermetropia.
SECTION E — Answers 8 marks
E1.
(i) Myopia (near-sightedness) — 1 mark.
(ii) A concave (diverging) lens — 1 mark.
(iii) \( f = -2\ \text{m} \), so \( P = \dfrac{1}{f} = \dfrac{1}{-2} = -0.5\ \text{D} \) — 1 mark.
(iv) Any one valid cause: excessive curvature of the eye lens, or elongation of the eyeball — 1 mark.
(ii) A concave (diverging) lens — 1 mark.
(iii) \( f = -2\ \text{m} \), so \( P = \dfrac{1}{f} = \dfrac{1}{-2} = -0.5\ \text{D} \) — 1 mark.
(iv) Any one valid cause: excessive curvature of the eye lens, or elongation of the eyeball — 1 mark.
Marking: 1 mark for each correct sub-part; part (iii) requires the correct sign and value.
E2.
(i) Dispersion of white light — 1 mark.
(ii) VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red) — 1 mark.
(iii) Red bends the least; different colours bend by different amounts because each colour travels at a slightly different speed inside the glass of the prism — 1 mark.
(iv) Up to four corneal blind people — 1 mark.
(ii) VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red) — 1 mark.
(iii) Red bends the least; different colours bend by different amounts because each colour travels at a slightly different speed inside the glass of the prism — 1 mark.
(iv) Up to four corneal blind people — 1 mark.
Marking: 1 mark for each correct sub-part.
— End of Answer Key —
