33 Years NEET-AIPMT Chapterwise Solutions - Physics 2020

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86 NEET-AIPMT Chapterwise Topicwise Solutions Physics
⎛ A + δ ⎞
⎛ A + δ ⎞
sin
⎝
⎜
⎠
⎟
sin
50. (d) : As µ =
2
A ⎝
⎜
or cot
2 ⎠
⎟
=
⎛ A ⎞ 2 ⎛ A ⎞
sin
⎝
⎜
⎠
⎟
sin
2
⎝
⎜
2 ⎠
⎟
⎛ A ⎞ ⎛ A + δ ⎞
cos sin
⎝
⎜
2 ⎠
⎟
⎝
⎜
2 ⎠
⎟
=
⎛ A ⎞ ⎛ A ⎞
sin sin
⎝
⎜
2 ⎠
⎟
⎝
⎜
2 ⎠
⎟
π A A δ
− = +
2 2 2 2
\ d = p – 2A = 180° – 2A
⎛ π A⎞
δ
or sin − sin ;
⎝
⎜
2 2 ⎠
⎟ = ⎛ A
⎝
⎜
2 + ⎞
2⎠
⎟
51. (b) : On reflection from the silvered surface, the
incident ray will retrace its path, if it falls normally on
the surface.
By geometry, r = A
Applying Snell’s law at surface PQ,
1sini = msinr
sini
sin2A
µ= = = 2cos
A
sinr
sin A
⎛ A + δ ⎞
sin
m
⎝
⎜
⎠
⎟
52. (b) : As µ =
2
⎛ A ⎞
sin
⎝
⎜
⎠
⎟
2
⎛ A+
A⎞
⎛ A⎞
⎛ A⎞
sin
⎝
⎜
⎠
⎟
2sin
µ=
2 sin A ⎝
⎜
⎠
⎟ cos
⎝
⎜
⎠
⎟
2 2
= =
⎛ A ⎞ ⎛ A ⎞ ⎛ A ⎞
sin
⎝
⎜
⎠
⎟ sin
⎝
⎜
⎠
⎟ sin
2 2
⎝
⎜
⎠
⎟
2
⎛ ⎞
= 2cos
⎝
⎜
A
2 ⎠
⎟
As d = i + e – A
At minimum deviation, d = d m , i = e
\ d m = 2i – A or 2i = d m + A
δm
+ A A+
A
i = = = A ( δm
= A (given))
2 2
i min = 0° ⇒ A min = 0°
Then, m max = 2cos0° = 2
π
π
imax
= ⇒ Amax
=
2 2
1
Then, µ min = 2cos
45 = 2× = 2
2
So refractive index lies between 2 and 2.
53. (b) : Angle of prism,
A = r 1 + r 2
For minimum deviation
r 1 = r 2 = r \ A = 2r
Given, A = 60°
A 60° Hence, r = = = 30 °
2 2
54. (a) : A = 60° , µ = 3, δm
= ?
µ =
⎛ A + δm
⎞
sin
⎝
⎜ 2 ⎠
⎟
A
sin
2
∴ δm
= 60°
55. (d) : An observer can see a rainbow only when his
back is towards the sun.
56. (b) : Red light of the visible spectrum is having a
maximum wavelength of about 650 nm.
57. (a) : The condition for dispersion without deviation
is given as (m – 1)A = (m′ – 1)A′
Given m = 1.42, A = 10°, m′ = 1.7, A′ = ?
\ (1.42 – 1) × 10 = (1.7 – 1)A′
(0.42) × 10 = 0.7 × A′
042 . × 10
or A′= = 6
07 .
58. (a) : The reddish appearance of the sun at sunrise
and sunset is due to the scattering of light.
59. (c) : For dispersion without deviation
d 1 + d 2 = 0
(m 1 – 1)A 1 + (m 2 – 1) A 2 = 0
( µ 1−1)
A1
A2
=−
( µ 2 −1)
Substituting the given values, we get
(. 15− 115 ) °
A 2 =− =− 10°
(. 175−1)
Negative sign shows that two prisms must be joined in
opposition.
60. (b) : The rainbow is an example of the dispersion of
sunlight by the water drops in the atmosphere. This is a
phenomenon due to a combination of the refraction of
sunlight by spherical water droplets and of internal (not
total) reflection.
61. (a) : According to Rayleigh, the amount of
scattering is inversely proportional to the fourth power
of the wavelength.
62. (b) : Here, v = – 400 cm = – 4 m, u = ∞, f = ?
Using lensformula, 1 − 1 =
1
v u f
1 1 1
or or m
−4
− ∞ = f
f
=−4
Lens should be concave.
1 1
Poweroflens= = =−025
. D
f − 4