Table of Contents
PYQ on The Human Eye and the Colourful World
Section A
- Select the correct statements from the following in the context of the human eye :
(i) Most of the refraction of light rays entering the eye occurs at the crystalline lens.
(iI) The human eyeball has a diameter of approximately 2.3 centimetres.
(iII) The iris is a pigmented, muscular diaphragm that regulates the size of the pupil.
(iV) To focus on objects at varying distances, the ciliary muscles adjust the lens’s position relative to the retina.
(A) (i) and (iV)
(B) (iI) and (iII)
(C) (i), (iI) and (iV)
(D) (iI), (iII) and (iV)
Year 2025
PYQ on The Human Eye and the Colourful World
Section B
- Both A and R are factually correct, and R provides the precise logical explanation for A.
- A is true and R is also true, but R does not logically justify or clarify A.
- Assertion A holds true; however, the reason given in R is factually incorrect.
- Assertion A is not valid, but the statement in Reason R is accurate.
- Assertion (A): Due to scattering of sunlight, the clear sky predominantly appears blue.
Reason (R): The wavelength of blue light is nearly 1:8 times the wavelength of red light.
Year 2025
PYQ on The Human Eye and the Colourful World
Section C
- (i) Define the term power of accommodation of human eye. As an object’s distance increases from 5 meters to 500 meters, what change occurs in the image distance within the eye? Name and explain the role of the part of human eye responsible for it in this case.
(iI) A person cannot clearly see objects farther than 2 meters. Name this vision defect, list two of its possible causes, and identify the type of lens used to correct it.
Year 2025
- (i) Name and explain the phenomenon of light responsible to make the path of a beam of light visible when it enters a smoke/dust-filled room through a small hole. Explain how the color of light we perceive depends on the size of the particles in the medium it passes through.
(iI) For presbyopia requiring bifocal lenses with powers +2.0 D and -0.5 D, which lens corrects distant vision and what is its focal length?
Year 2025
- Define the term ‘power of accommodation’ of human eye. Name the part of the eye responsible for this process and describe its action when focusing on (i) nearby and (ii) distant objects.
Year 2025
- (i) When and where does a rainbow appear in the sky? List, in the correct order, the optical phenomena involved in the formation of a rainbow. Draw a labelled diagram to show the formation of a rainbow.
(iI) List two conditions to observe a rainbow in the sky. Why is a rainbow not visible from the moon’s surface? Explain.
Year 2025
- (i) What is hypermetropia? List its two possible causes. Name the type of lens used to correct this defect and state its role in this case.
(iI) Presbyopia requires correction with bifocal lenses. If he needs two lenses of power + 2:5 D and — 0:25 D, which one of these two lenses is for the correction of distant vision and what is its focal length?
Year 2025
PYQ on The Human Eye and the Colourful World
Answer Key
Section A
1.
Option (B)
Statement (i): Most of the refraction of light rays entering the eye occurs at the crystalline lens.
This is false. Most refraction (about two-thirds) occurs at the cornea, not the lens. The lens fine-tunes the focus.
Statement (iI): The diameter of the eye ball is about 2.3 cm.
This is true. Average axial length of the human eyeball is about 24 mm (2.4 cm), and 2.3 cm is close enough to be considered correct in many general science contexts.
Statement (iII): Iris is a dark muscular diaphragm which controls the size of the pupil.
This is true. Iris adjusts pupil size to control light entering.
Statement (iV): While focusing on objects at different distances, the distance between the lens and the retina is adjusted by the ciliary muscles.
This is false. In the human eye, the distance between lens and retina is fixed. Focusing is done by changing the curvature (and thus focal length) of the lens via ciliary muscles — not by moving the lens toward/away from the retina.
PYQ on The Human Eye and the Colourful World
Section B
1.
Assertion A holds true; however, the reason given in R is factually incorrect.
Assertion (A): Due to scattering of sunlight, the clear sky predominantly appears blue.
This is true. The blue color of the sky is because of Rayleigh scattering, which scatters shorter wavelengths (blue/violet) more effectively.
Reason (R): The wavelength of blue light is nearly 1:8 times the wavelength of red light.
This is false. Actually, blue light has a shorter wavelength than red light.
Approximate wavelengths:
- Blue light: ~450–495 nm
- Red light: ~620–750 nm
Blue wavelength is roughly 0.6–0.7 times red wavelength, not 1.8 times.
So R is factually incorrect.
PYQ on The Human Eye and the Colourful World
Section C
1.
(i) Power of accommodation is the ability of the eye lens to change its focal length (by adjusting its curvature) with the help of ciliary muscles, so that both near and far objects can be seen clearly on the retina.
When the object distance increases from about 5 m to 500 m (or infinity), the image distance in the eye decreases slightly and then becomes constant, because for distant objects, the eye lens becomes thinner (least converging), and the image is formed exactly on the retina without further change.
The ciliary muscles relax in this case, making the suspensory ligaments taut, which flattens the lens and increases its focal length.
(iI)
The person can see objects clearly up to 2 m but not beyond → this is myopia (short-sightedness/near-sightedness).
Two possible causes:
- Excessive curvature of the cornea/eye lens (too much converging power).
- Elongation of the eyeball (image forms in front of the retina for distant objects).
Correction:
Using concave (diverging) lenses of appropriate power, so that parallel rays from infinity appear to come from the far point (2 m) after refraction through the lens, and the eye then focuses them on the retina.
2. (i)
The phenomenon is Tyndall effect (scattering of light by colloidal-sized particles).
When a beam of light enters a smoke or dust-filled room through a small hole, the tiny suspended particles scatter the light in all directions, making the path of light visible.
Dependence of colour on particle size:
- If the scattering particles are very fine (smaller than wavelength of light), shorter wavelengths (blue/violet) are scattered more, giving a blue tint.
- If the particles are larger (comparable to or larger than wavelength), all wavelengths scatter almost equally, so scattered light appears white.
2. (iI)
In presbyopia, bifocal lenses have:
- Upper part for distant vision
- Lower part for near vision
Given powers: +2.0 D and –0.5 D.
- The +2.0 D lens is converging, used for near vision (presbyopia lacks accommodation).
- The –0.5 D lens is diverging, used for distant vision → this implies the person also has some myopia for distance.
So for distant vision: lens power = –0.5 D
Focal length F (in metres) = 1 / Power (in D)
[f = \frac{1}{-0.51} \approx -2 \, \text{m}]
Answer:
Lens for distant vision: –0.5 D, focal length = –2 m (negative sign indicates concave lens).
3. Power of accommodation of the human eye is the ability of the eye lens to change its focal length (and hence converging power) by adjusting its curvature with the help of ciliary muscles, so that sharp images of objects at different distances are formed on the retina.
The part of the eye that plays the major role in this process is the ciliary muscles along with the suspensory ligaments and the eye lens.
(i) When focusing on nearby objects
- Ciliary muscles contract.
- This reduces tension on the suspensory ligaments.
- The eye lens becomes more convex (thicker).
- Focal length decreases, converging power increases.
- Image of the nearby object is formed clearly on the retina.
(iI) When focusing on distant objects
- Ciliary muscles relax.
- Suspensory ligaments become taut.
- The eye lens becomes less convex (thinner/flatter).
- Focal length increases, converging power decreases.
- Image of the distant object is formed clearly on the retina.
4.
i. A rainbow appears in the sky when the sun is shining and it is raining (or there are water droplets) in the opposite direction of the sun from the observer.
It is seen as a circular arc (often semicircle) in the sky opposite to the sun, with the centre of the arc along the line from the sun through the observer’s eye.
Phenomena of light involved (in sequence):
- Refraction again as the light exits the droplet, further separating colours.
- Refraction of sunlight as it enters a water droplet.
- Dispersion of white light into its constituent colours inside the droplet.
- Internal reflection at the inner surface of the droplet.
Labelled diagram of rainbow formation:
II. Two conditions to observe a rainbow:
- The sun must be behind the observer.
- There must be water droplets (rain, mist, spray) in the atmosphere in front of the observer.
Why a rainbow cannot be seen from the surface of the moon:
Without these, the phenomenon of rainbow formation cannot occur.
There is no atmosphere on the moon.
No atmospheric water droplets are present to cause refraction, dispersion, and internal reflection of sunlight.
5.
(i) Hypermetropia (Long-sightedness)
It is a defect of vision in which a person can see distant objects clearly but nearby objects appear blurred, because the image of a nearby object is formed behind the retina.
Two possible causes:
- Eyeball too short (axial length shorter than normal).
- Lens too flat or less converging (focal length too long).
Correction:
- Convex (converging) lens is used.
- Role: It converges incoming light rays more before they enter the eye, so that after refraction by the eye lens, the image of a nearby object is formed exactly on the retina.
(iI)
In presbyopia with bifocal lenses:
- Upper part → for distant vision
- Lower part → for near vision
Given powers: +2.5 D and –0.25 D.
- The +2.5 D lens is for near vision (helps in reading).
- The –0.25 D lens is for distant vision (implies slight myopia along with presbyopia).
Lens for distant vision: –0.25 D
Focal length:
[f = \frac{1}{P} = \frac{1}{-0.25} = -4 \, \text{m}];
Answer:
For distant vision: –0.25 D lens, focal length = –4 m (concave lens).
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