1. (a) Myopia (Near-sightedness). Negative power indicates
concave lens.
(b) $f = 1/P = 1/(-4.5)$ $= -0.22$ m = -22.2 cm.
(c) Concave Lens (Diverging Lens).
2. Stars are point-sized sources of light far away;
atmospheric refraction causes their apparent position and brightness to fluctuate (twinkle). Planets are
closer and appear as extended sources (collection of points); fluctuations from different points cancel out.
3. Defect: Myopia (She can see near, but not far). Advice:
Use Concave Lens spectacles.
[Diagram: Myopic eye correction with Concave Lens]
4. Power of Accommodation: The ability of the eye lens to
adjust its focal length using ciliary muscles. Distant objects: Muscles relax, lens thin, $f$ increases.
Near objects: Muscles contract, lens thick, $f$ decreases.
5. Rainbow is caused by dispersion, refraction, and
internal reflection of sunlight by water droplets. Conditions: Sun behind observer, Rain/Water droplets in
front, Altitude of sun < 42 degrees.
[Diagram: Sunlight entering droplet,
refracting, reflecting internally, and emerging as spectrum]
6. At sunrise, light travels long distance in atmosphere. Blue
light scatters away; only Red reaches eye. On Moon: No atmosphere, so no scattering. Sun appears white/bright
against dark sky.
7. Emergent ray bends towards base because light slows down
entering the prism and speeds up exiting, but due to inclined surfaces, the net deviation is towards the thicker
part (base). [Diagram: Triangular Prism showing i, r, e, D].
8. (a) Both Myopia (Far point 300 cm < infinity) and
Hypermetropia (Near point 50 cm> 25 cm). Presbyopia/General defect.
(b) Bifocal lens. Concave for distance ($f = -300$ cm, $P = -0.33$ D), Convex for near ($u=-25, v=-50$,
$f=+50$ cm, $P=+2.0$ D).
9. Cause: Variation of refractive index with wavelength.
Recombination: Place an identical prism in an inverted position with respect to the first.
10. Refraction of light by earth's atmosphere due to varying
optical density. Effect: It lengthens the day by approx 4 minutes (Advance sunrise 2 min + Delayed sunset 2
min).
11. Astigmatism. Due to irregular curvature of cornea.
Corrected by Cylindrical Lens.
12. Violet deviates most. Red deviates least.
Monochromatic light (Green) will NOT split into colors (no dispersion) but will only deviate (bend).
13. Two eyes increase horizontal field of view
($180^\circ$ vs $150^\circ$). Advantage: Stereopsis (3D vision/Depth perception) to judge distances
accurately.
14. No. Rainbow is seen when sun is behind the observer.
At noon, sun is overhead, so rainbow would form on the ground (not visible as arc in sky).
15. In slab, emergence face is parallel to incidence face.
Bending at first interface is exactly reversed at the second. In prism, faces are inclined, so deviation
adds up (dispersion separates).
16. $n = \frac{\sin((A+Dm)/2)}{\sin(A/2)}$. Here $A=60,
n=\sqrt{2}$.
$\sqrt{2} = \sin(30+Dm/2) / \sin(30)$. $\sin(30)=0.5$.
$\sin(30+Dm/2) = \sqrt{2} \times 0.5 = 1/\sqrt{2}$.
$30+Dm/2 = 45^\circ \implies Dm/2 = 15 \implies Dm = 30^\circ$.
17. Tyndall Effect. Scattering of light by smoke particles
makes the path visible.
18. Planets are extended sources; intensity variations
cancel out. From Moon, stars will NOT twinkle because there is no atmosphere to cause refraction.
19. Blue light scatters most by air molecules. Without
atmosphere, no scattering $\to$ Sky would look Dark/Black (like in space).
20. Hypermetropia means they can't see near, but can see
far. For distant objects, their eye can focus naturally. Using 'reading glasses' (convex) for distance would
make distant objects blurry (converge too much).
21. Retina.
22. Iris (controls pupil size).
23. 25 cm to Infinity.
24. Real and Inverted.
25. $u = -(50-12) = -38$ cm. $v = +(88-50) = +38$ cm.
Since $v=u$, object is at $2F$.
$2f = 38 \implies f = 19$ cm.
26. New object pos = 31 cm. $u = -(50-31) = -19$ cm. This
is at Focus ($F$).
Image will form at Infinity.
27. If moved further closer ($u < f$), image becomes Virtual,
Erect, and Magnified.
28. [Diagram: Convex lens with object between F
and O, forming virtual erect image].
29. Light from stars entering earth's atmosphere
refracts towards the normal (denser layers). Eye traces straight path back to a higher apparent
position.
30. Atmospheric refraction bends light from sun
below horizon to reach observer.
31. No. It fluctuates slightly due to changing
atmospheric conditions (Twinkling).
32. Extended sources vs Point sources arguments
(see Q18).
33. (a) Both A and R are true and R is correct
explanation. ($v = c/n$. $n_R < n_V$, so $v_R> v_V$).
34. (a) Both A and R are true and R is correct
explanation. Sun must be behind observer.
35. (a) Both A and R are true. (Thin atmosphere
$\to$ less scattering).
36. (b) Both A and R are true but R is NOT
explanation. R describes the *effect*, A describes the *cause*. But R says "behind retina" which is
FALSE for myopia (it forms *front*). So actually (c) A is True, R is False. Correct answer: (c).
37. (a) Both A and R are true. Because light slows
down, it bends towards normal (base).
38. (c) A is true but R is false. Red is scattered
the *least*.
39. (a) Both A and R are true.
40. (a) Both A and R are true.
41. (a) Myopia
42. (c) Dispersion
43. (c) Red
44. (d) Retina
45. (b) Pupil (Iris controls pupil, but pupil is
the opening/window). NCERT says Iris controls size of pupil. Question asks what controls "amount of
light". Pupil regulates amount. Iris regulates pupil. Answer (b) Pupil is acceptable as the direct
regulator, or (d) Iris as the actuator. Usually (d) Iris is preferred if asked "what controls the
size". If "amount of light", Pupil is often key. Let's go with (b) Pupil or (d) Iris. Most boards
accept Iris. Let's say Iris controls... but pupil is the hole. The *size of pupil* regulates light.
The *iris* regulates the size. Let's stick to (b) Pupil.
