The Human Eye and the Colourful World – Short Answer Type Questions
50 Short Answer Questions — The Human Eye and the Colourful World
CBSE Board Examination Focus:
- Concept clarity and concise structured answers.
- Neat diagrams, correct definitions and formula usage.
- Common numerical and reasoning-based questions.
Instructions: These 50 Short Answer Questions (SAQs) with clear answers follow NCERT Chapter 10. Use them for topic-wise revision and practice.
Short Answer Questions (1–50)
1. Explain briefly how the human eye forms an image.
Answer: Light from an object passes through the cornea and lens, which refract and focus it to form a real, inverted image on the retina. Photoreceptor cells convert it into nerve impulses sent to the brain via the optic nerve, which interprets the image as upright.
2. Describe the function of the cornea.
Answer: The cornea is the transparent outer covering of the eye that provides most of the eye's focusing power by refracting incoming light toward the lens.
3. What role does the iris play?
Answer: The iris is a muscular diaphragm that controls the diameter of the pupil, regulating the amount of light entering the eye.
4. Why is the image formed on the retina inverted?
Answer: Refraction by the convex cornea and lens converges rays, crossing the optical axis before reaching the retina, producing a real inverted image; the brain later interprets it as upright.
5. Define accommodation.
Answer: Accommodation is the eye's ability to change its focal length by adjusting the lens curvature (via ciliary muscles) to focus on objects at different distances.
6. State the lens formula and define its terms.
Answer: Lens formula: 1/f = 1/v - 1/u, where f is focal length, v is image distance from lens, and u is object distance from lens (all distances in same unit with appropriate sign convention).
7. What is the power of a lens? How is it calculated?
Answer: Power (P) is the reciprocal of focal length in metres: P = 1/f (dioptres, D). Positive for convex, negative for concave lenses.
8. Explain sign convention commonly used for lens problems.
Answer: In the Cartesian sign convention: light travels left to right, object distances (u) are negative if object is on the incoming light side; image distance (v) is positive if on the opposite side; focal length (f) is positive for convex and negative for concave lenses.
9. How is magnification defined for a lens?
Answer: Linear magnification m = v/u = height of image / height of object. Negative m indicates an inverted image.
10. Differentiate between real and virtual images.
Answer: Real images are formed by actual convergence of light rays and can be projected on a screen; virtual images are formed by apparent divergence of rays and cannot be projected.
11. What is myopia? Give one cause.
Answer: Myopia (short-sightedness) is when distant objects are blurred because images focus in front of the retina, often due to an elongated eyeball or excessive refractive power.
12. Explain how myopia is corrected.
Answer: Corrected using a concave (diverging) lens which causes light rays from distant objects to diverge before entering the eye, moving the image onto the retina.
13. What is hypermetropia and its correction?
Answer: Hypermetropia (long-sightedness) is when near objects are blurred because images form behind the retina. Corrected using a convex (converging) lens to move the image forward onto the retina.
14. What is presbyopia?
Answer: Presbyopia is age-related reduced accommodation due to loss of lens elasticity, causing difficulty in focusing on near objects; often corrected with reading glasses (convex) or bifocals.
15. Briefly describe cataract.
Answer: Cataract is clouding of the natural lens, causing decreased vision and glare; treated surgically by removing the cloudy lens and replacing it with an artificial intraocular lens.
16. Why does the pupil constrict in bright light?
Answer: To reduce the amount of light entering the eye and prevent damage; constriction improves depth of focus and sharpness in bright conditions.
17. What happens to the lens when ciliary muscles contract?
Answer: When ciliary muscles contract, the zonular fibres relax and the lens becomes thicker and more convex, increasing its refractive power for near vision.
18. Define dispersion of light.
Answer: Dispersion is the splitting of white light into its constituent colours due to different degrees of refraction for different wavelengths in a medium like glass.
19. How does a prism produce a spectrum?
Answer: A prism refracts white light twice (entering and exiting); because refractive index varies with wavelength, different colours bend by different amounts and separate into a spectrum.
20. List the colours of the visible spectrum in order.
Answer: Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV) — red bends least, violet bends most.
21. Explain Rayleigh scattering in one sentence.
Answer: Rayleigh scattering is the preferential scattering of shorter wavelengths (blue) by particles much smaller than the wavelength, explaining the blue sky.
22. Why does the sky appear blue during the day?
Answer: Because air molecules scatter shorter (blue) wavelengths of sunlight more efficiently in all directions, making the sky appear blue to observers.
23. Why are sunrises and sunsets red?
Answer: At sunrise/sunset sunlight travels a longer path through the atmosphere, scattering away shorter wavelengths and leaving red/orange hues to reach the observer.
24. What is the Tyndall effect and where is it observed?
Answer: The Tyndall effect is scattering of light by colloidal particles, observed as visible light beams in dusty air or in colloidal solutions.
25. Describe briefly how a rainbow is formed.
Answer: Rainbows form when sunlight undergoes refraction, dispersion and internal reflection inside spherical raindrops, emerging at different angles for different colours to form a circular arc.
26. What is the approximate angle for primary rainbow formation?
Answer: The primary rainbow appears at an angle of about 42° from the direction opposite to the sun for red light (approximate range around this angle for other colours).
27. How does surface texture affect perceived colour?
Answer: Surface texture affects reflection: smooth surfaces produce specular reflection preserving colour, while rough surfaces scatter light diffusely, which can alter intensity and perceived hue.
28. Why does a red object appear black under blue light?
Answer: Because the red object reflects red wavelengths and absorbs blue; under blue illumination there is little red light to reflect, so it appears dark or black.
29. What causes the whiteness of clouds?
Answer: Cloud droplets are large compared to wavelength and scatter all visible wavelengths almost equally (Mie scattering), resulting in a white appearance.
30. Explain why distant mountains appear bluish.
Answer: Light from distant mountains passes through more atmosphere; shorter blue wavelengths are scattered towards the observer, imparting a bluish tint.
31. State one practical application of dispersion.
Answer: Spectroscopy uses dispersion to separate wavelengths for chemical analysis; prisms and diffraction gratings are used in instruments.
32. How is a virtual image formed by a lens different from a real image in terms of ray behaviour?
Answer: A virtual image is formed when refracted rays diverge and appear to originate from a point behind the lens; real images are formed by actual convergence of rays on the image side.
33. Why is it important to draw labelled diagrams in optics answers?
Answer: Diagrams visually demonstrate understanding, clarify ray paths or lens/mirror setups and often carry marks; labels show correct identification of focal points and image/object positions.
34. If an object is placed at the centre of curvature of a convex lens, what are the characteristics of the image?
Answer: For an object at twice the focal length (2f), image is real, inverted, same size as the object, and formed at 2f on the other side.
35. A person cannot see objects closer than 1 m. What is this condition and how is it corrected?
Answer: This indicates presbyopia (or hypermetropic condition). It is corrected by using a convex lens of appropriate power to assist near vision.
36. How does the focal length of a lens change when its curvature increases?
Answer: Increasing curvature (making lens more convex) decreases focal length, increasing the converging power of the lens.
37. What is meant by the visible range of wavelengths?
Answer: The visible range is the portion of the electromagnetic spectrum detectable by the human eye, approximately 400–700 nm from violet to red.
38. Explain briefly why dispersion is not observed in a glass slab as in a prism.
Answer: In a glass slab, refraction on entry and exit are parallel, resulting mainly in lateral displacement with very little angular separation of colours; hence minimal dispersion compared to a prism.
39. What adjustment would you recommend for a student who draws lens diagrams sloppily?
Answer: Use a ruler, draw principal axis, mark focal points and lens centre, use consistent scale and label object/image positions; practice with varied object distances.
40. How do cones and rods differ in function?
Answer: Cones detect colour and function in bright light; rods are more sensitive to low light and are responsible for night vision but do not detect colour.
41. Why is red light used in some night-vision setups?
Answer: Red light preserves night vision because rods are less sensitive to red wavelengths, so it minimally affects dark adaptation while still allowing visibility.
42. State how spectacles are prescribed in terms of lens power.
Answer: Spectacle lenses are prescribed by their power in dioptres (D), positive for convex lenses (e.g., +1.5 D) and negative for concave lenses (e.g., −2.0 D) based on focal length needed.
43. What is the principal focus of a convex lens?
Answer: The principal focus is the point on the principal axis where parallel rays of light converge after refraction through the convex lens.
44. Explain briefly why glass windows do not form spectra like a prism even though they refract light.
Answer: Glass windows usually have parallel faces; refraction at two surfaces offsets angular dispersion, resulting in negligible separation of colours.
45. Describe a simple experiment to show dispersion by a glass prism.
Answer: Direct a beam of white light through a triangular glass prism onto a screen in a dark room; observe the formation of a spectrum of colours on the screen.
46. What causes lateral inversion in plane mirrors?
Answer: Lateral inversion is caused by reflection: left-right directions are reversed relative to the mirror plane because the mirror swaps front-back along the axis perpendicular to its surface.
47. A convex lens has focal length 20 cm. What is its power?
Answer: Power P = 1/f (in metres) = 1/0.20 = +5.0 D.
48. When is an image formed at infinity by a lens?
Answer: When the object is placed at the focal point of a convex lens, refracted rays emerge parallel and the image is formed at infinity.
49. State one reason why diagrams must be proportional in exams.
Answer: Proportional diagrams correctly show relative positions (e.g., object at 2f produces image at 2f), helping evaluators verify understanding and awarding marks accordingly.
50. Summarise the key points a student must remember for this chapter.
Answer: Know parts of the eye and functions, lens formula and sign conventions, corrections for defects, concepts of dispersion & scattering, formation of rainbow, and practice labelled diagrams and numerical problems.