Chapter 10: The Human Eye and the Colourful World
[IMAGE PLACEHOLDER: Human Eye Structure & Vision Defects Overview]
1-Mark Questions & Solutions
Q1 | 1 Mark
A person cannot see objects distinctly kept beyond $2\text{ m}$. This defect can be corrected by using a lens of power:
(a) $+0.5\text{ D}$ (b) $-0.5\text{ D}$ (c) $+0.2\text{ D}$ (d) $-0.2\text{ D}$
(a) $+0.5\text{ D}$ (b) $-0.5\text{ D}$ (c) $+0.2\text{ D}$ (d) $-0.2\text{ D}$
Answer: (b) $-0.5\text{ D}$
Calculation: For far point $v = -2\text{ m}$, focal length $f = -2\text{ m}$. Power $$P = \frac{1}{f} = \frac{1}{-2\text{ m}} = -0.5\text{ D}$$
Calculation: For far point $v = -2\text{ m}$, focal length $f = -2\text{ m}$. Power $$P = \frac{1}{f} = \frac{1}{-2\text{ m}} = -0.5\text{ D}$$
Q2 | 1 Mark
At noon the sun appears white as:
(a) light is least scattered (b) all the colours of white light are scattered away (c) blue colour is scattered most (d) red colour is scattered most
(a) light is least scattered (b) all the colours of white light are scattered away (c) blue colour is scattered most (d) red colour is scattered most
Answer: (a) light is least scattered (travels shorter distance through atmosphere).
Q3 | 1 Mark
When light rays enter the eye, most of the refraction occurs at the:
(a) crystalline lens (b) outer surface of the cornea (c) iris (d) pupil
(a) crystalline lens (b) outer surface of the cornea (c) iris (d) pupil
Answer: (b) outer surface of the cornea
Q4 | 1 Mark
The phenomenon of light responsible for the working of the human eye is:
(a) Reflection (b) Refraction (c) Power of accommodation (d) Phototropism
(a) Reflection (b) Refraction (c) Power of accommodation (d) Phototropism
Answer: (b) Refraction
Q5 | 1 Mark
The focal length of the eye lens increases when eye muscles:
(a) are relaxed & lens becomes thinner (b) contract & lens becomes thicker (c) are relaxed & lens becomes thicker (d) contract & lens becomes thinner
(a) are relaxed & lens becomes thinner (b) contract & lens becomes thicker (c) are relaxed & lens becomes thicker (d) contract & lens becomes thinner
Answer: (a) are relaxed & lens becoming thinner
[IMAGE PLACEHOLDER: Prism Light Dispersion & Rainbow Formation]
Q6 | 1 Mark
Twinkling of stars is due to atmospheric:
(a) Dispersion of light by water droplets (b) Refraction of light by different layers of varying refractive indices (c) Scattering of light by dust particles (d) Internal reflection of light by clouds
(a) Dispersion of light by water droplets (b) Refraction of light by different layers of varying refractive indices (c) Scattering of light by dust particles (d) Internal reflection of light by clouds
Answer: (b) refraction of light by different layers of varying refractive indices
Q7 | 1 Mark
Which of the following phenomenon of light is involved in the formation of a rainbow?
(a) reflection, refraction & dispersion (b) refraction, dispersion & total internal reflection (c) refraction, dispersion & internal reflection (d) dispersion, scattering & total internal reflection
(a) reflection, refraction & dispersion (b) refraction, dispersion & total internal reflection (c) refraction, dispersion & internal reflection (d) dispersion, scattering & total internal reflection
Answer: (b) refraction, dispersion & total internal reflection (or internal reflection)
Q8 | 1 Mark
In an experiment to trace the path of a ray of light through a triangular glass prism, a student would observe that the emergent ray:
(a) is parallel to incident ray (b) is along same direction (c) bends towards thinner part (d) gets deviated & bends towards thicker part (base) of prism
(a) is parallel to incident ray (b) is along same direction (c) bends towards thinner part (d) gets deviated & bends towards thicker part (base) of prism
Answer: (d) gets deviated & bends towards the thicker part (base) of the prism
Q9 | 1 Mark
The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to:
(a) presbyopia (b) accommodation (c) nearsightedness (d) farsightedness
(a) presbyopia (b) accommodation (c) nearsightedness (d) farsightedness
Answer: (b) accommodation
Q10 | 1 Mark
The clear sky appears blue because:
(a) Blue light absorbed in atmosphere (b) UV radiations absorbed (c) Violet & blue lights get scattered more than light of all other colours by atmosphere (d) All other colours scattered more
(a) Blue light absorbed in atmosphere (b) UV radiations absorbed (c) Violet & blue lights get scattered more than light of all other colours by atmosphere (d) All other colours scattered more
Answer: (c) violet & blue lights get scattered more than light of all other colours by the atmosphere (since scattering intensity $\propto \frac{1}{\lambda^4}$)
Q11 - Q15 | Additional 1-Mark Questions
• Q11: Function of Iris: Controls the size of the pupil and regulates the amount of light entering the eye.
• Q12: Function of Crystalline Lens: Focuses light rays onto the retina to form a real, inverted image.
• Q14: Light Speed in Air: (c) All colors of white light move with approximately the same speed in air ($\approx 3 \times 10^8\text{ m/s}$).
• Q15: Assertion (A): Higher the refractive index of prism material, higher is the angle of deviation. Reason (R): Angle of deviation is directly proportional to angle of prism. Answer: (b) Both A and R are true, but R is not the correct explanation of A.
• Q11: Function of Iris: Controls the size of the pupil and regulates the amount of light entering the eye.
• Q12: Function of Crystalline Lens: Focuses light rays onto the retina to form a real, inverted image.
• Q14: Light Speed in Air: (c) All colors of white light move with approximately the same speed in air ($\approx 3 \times 10^8\text{ m/s}$).
• Q15: Assertion (A): Higher the refractive index of prism material, higher is the angle of deviation. Reason (R): Angle of deviation is directly proportional to angle of prism. Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Q18 - Q30 | Conceptual MCQs & Assertions
• Q18: Light travelling from hot air (rarer) to cold air (denser) bends towards the normal.
• Q19: Flattened appearance of sun at sunrise/sunset is due to atmospheric refraction.
• Q20: To an astronaut in space, the sky/earth surrounding appears black (no atmosphere to scatter light).
• Q21: Image formed on the retina is real and inverted.
• Q22: A person suffering from cataract has an opaque / cloudy eye lens.
• Q24: Danger signals are red because red light is scattered least by smoke or fog due to longer wavelength ($\lambda$).
• Q25: Colour deviated least by prism: Red.
• Q26: Sky looks dark in space because there is no atmosphere to scatter light. (Both A and R true, R explains A).
• Q28: Myopic eye can see nearby objects clearly; eyeball is elongated. (Both A and R true, R explains A).
• Q30: Rainbow seen with observer's back to sun due to internal reflection/dispersion. (Both A and R true, R explains A).
• Q18: Light travelling from hot air (rarer) to cold air (denser) bends towards the normal.
• Q19: Flattened appearance of sun at sunrise/sunset is due to atmospheric refraction.
• Q20: To an astronaut in space, the sky/earth surrounding appears black (no atmosphere to scatter light).
• Q21: Image formed on the retina is real and inverted.
• Q22: A person suffering from cataract has an opaque / cloudy eye lens.
• Q24: Danger signals are red because red light is scattered least by smoke or fog due to longer wavelength ($\lambda$).
• Q25: Colour deviated least by prism: Red.
• Q26: Sky looks dark in space because there is no atmosphere to scatter light. (Both A and R true, R explains A).
• Q28: Myopic eye can see nearby objects clearly; eyeball is elongated. (Both A and R true, R explains A).
• Q30: Rainbow seen with observer's back to sun due to internal reflection/dispersion. (Both A and R true, R explains A).
[IMAGE PLACEHOLDER: Myopia & Hypermetropia Ray Diagrams]
2-Mark & 3-Mark Questions & Solutions
Q32 | 2 Marks
Write the function of each part of the human eye: (i) cornea (ii) iris (iii) crystalline lens (iv) ciliary muscle.
Answer:
(i) Cornea: Protects eye and provides maximum refraction to incoming light rays.
(ii) Iris: Controls pupil size to adjust light entering the eye.
(iii) Crystalline Lens: Provides finer focal length adjustment to focus sharp images on retina.
(iv) Ciliary Muscles: Modifies eye lens curvature to change focal length for accommodation.
(i) Cornea: Protects eye and provides maximum refraction to incoming light rays.
(ii) Iris: Controls pupil size to adjust light entering the eye.
(iii) Crystalline Lens: Provides finer focal length adjustment to focus sharp images on retina.
(iv) Ciliary Muscles: Modifies eye lens curvature to change focal length for accommodation.
Q33 | 2 Marks
What is meant by power of a lens? What does its sign (+ve or -ve) indicate? State its SI unit and relation to focal length.
Answer:
• Power of a lens: Measure of convergence or divergence produced by a lens, defined as reciprocal of focal length in meters: $$P = \frac{1}{f \text{ (in m)}}$$
• SI Unit: Dioptre ($\text{D}$).
• Sign: Positive sign ($+P$) indicates a **convex (converging)** lens; negative sign ($-P$) indicates a **concave (diverging)** lens.
• Power of a lens: Measure of convergence or divergence produced by a lens, defined as reciprocal of focal length in meters: $$P = \frac{1}{f \text{ (in m)}}$$
• SI Unit: Dioptre ($\text{D}$).
• Sign: Positive sign ($+P$) indicates a **convex (converging)** lens; negative sign ($-P$) indicates a **concave (diverging)** lens.
Q36 | 2 Marks
A person suffers from both myopia and hypermetropia. (a) Condition name? (b) When does it happen? (c) Type of lens required?
Answer:
(a) **Presbyopia**.
(b) Occurs with aging due to weakening of ciliary muscles and diminishing flexibility of the eye lens.
(c) **Bifocal lenses** (upper part is concave for distant vision, lower part is convex for reading/near vision).
(a) **Presbyopia**.
(b) Occurs with aging due to weakening of ciliary muscles and diminishing flexibility of the eye lens.
(c) **Bifocal lenses** (upper part is concave for distant vision, lower part is convex for reading/near vision).
Q40 | 3 Marks
A person needs $+3\text{ D}$ lens for near vision correction and $-3\text{ D}$ for distant vision. Calculate focal lengths required for both lenses.
Answer:
• For near vision correction ($P_1 = +3\text{ D}$): $$f_1 = \frac{1}{P_1} = \frac{1}{+3} = +0.33\text{ m} = +33.3\text{ cm} \quad (\text{Convex Lens})$$
• For distant vision correction ($P_2 = -3\text{ D}$): $$f_2 = \frac{1}{P_2} = \frac{1}{-3} = -0.33\text{ m} = -33.3\text{ cm} \quad (\text{Concave Lens})$$
• For near vision correction ($P_1 = +3\text{ D}$): $$f_1 = \frac{1}{P_1} = \frac{1}{+3} = +0.33\text{ m} = +33.3\text{ cm} \quad (\text{Convex Lens})$$
• For distant vision correction ($P_2 = -3\text{ D}$): $$f_2 = \frac{1}{P_2} = \frac{1}{-3} = -0.33\text{ m} = -33.3\text{ cm} \quad (\text{Concave Lens})$$
Q43 | 3 Marks
Define power of accommodation. Write the modification in curvature of eye lens enabling us to see nearby objects clearly.
Answer:
• Power of Accommodation: Ability of the eye lens to adjust its focal length to focus clearly on objects at varying distances.
• Curvature Modification: To view nearby objects, ciliary muscles contract $\rightarrow$ eye lens becomes thicker $\rightarrow$ curvature increases $\rightarrow$ focal length decreases, forming a sharp image on the retina.
• Power of Accommodation: Ability of the eye lens to adjust its focal length to focus clearly on objects at varying distances.
• Curvature Modification: To view nearby objects, ciliary muscles contract $\rightarrow$ eye lens becomes thicker $\rightarrow$ curvature increases $\rightarrow$ focal length decreases, forming a sharp image on the retina.
[IMAGE PLACEHOLDER: Atmospheric Refraction Phenomena]
4-Mark Passage & 5-Mark Questions & Solutions
Q49 - Q50 | 4 Marks
Passages on Dispersion of Light and Atmospheric Refraction:
Q49 (Prism Dispersion): i) (b) Dispersion of light | ii) (a) Formation of rainbow | iii) (a) White light consists of seven colors | iv) (c) All colors travel with different speeds in glass medium.
Q50 (Atmospheric Refraction): i) (a) Increases apparent day length by $\approx 4\text{ minutes}$ | ii) (a) Atmospheric refraction | iii) (c) Atmospheric refraction | iv) (c) Atmospheric refraction | v) (b) Black (in space/without atmosphere).
Q50 (Atmospheric Refraction): i) (a) Increases apparent day length by $\approx 4\text{ minutes}$ | ii) (a) Atmospheric refraction | iii) (c) Atmospheric refraction | iv) (c) Atmospheric refraction | v) (b) Black (in space/without atmosphere).
Q55 | 5 Marks
(a) Define power of accommodation and state component responsible. (b) Student at back bench has difficulty reading blackboard. State defect, two causes, and correction method with ray diagram.
Answer:
(a) Power of accommodation is the ability of the eye lens to adjust its focal length. Ciliary muscles are responsible.
(b) Defect: Myopia (Nearsightedness).
Two Causes:
1) Excessive curvature of the eye lens.
2) Elongation of the eyeball.
Correction Method: Corrected using a **concave lens** of suitable power, which diverges incoming parallel rays so they focus precisely on the retina.
(a) Power of accommodation is the ability of the eye lens to adjust its focal length. Ciliary muscles are responsible.
(b) Defect: Myopia (Nearsightedness).
Two Causes:
1) Excessive curvature of the eye lens.
2) Elongation of the eyeball.
Correction Method: Corrected using a **concave lens** of suitable power, which diverges incoming parallel rays so they focus precisely on the retina.
Q56 | 5 Marks
What is Myopia? List two causes and show correction using ray diagrams for defective eye and corrected eye.
Answer: Myopia is a vision defect where a person can see nearby objects clearly but cannot see distant objects distinctly. Corrected using concave lens. Far point shifts from infinity to a finite distance $d$.
$$\text{Formula for lens power: } P = \frac{1}{-d}$$
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