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i r
incident ray
reflected ray
normal
IIIa Wave Motion: Light (Optics)
1 Laws of reflection
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i
r
incident ray
refracted ray
normal 2 Laws of refraction
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3 A light ray hits medium from vacuum at angle of incidence v. Angle of
refraction is m. Refractive index n of medium:
n =
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4 Refractive index n is also related to the speed of light c in a vacuum and v in the medium:
n =
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5 General form of Snell’s law:
θ1
n1 < n2
n2
n1
θ1
n1 > n2
n2
n1
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6 Conditions for total internal reflection:
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7 Critical angle C of a medium and its refractive index n:
C =
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8 Construction rules for lenses
Convex lenses Concave lenses
FF' FF'
FF' FF'
FF' FF'
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9 Focal plane:
Convex: Concave:
FF'
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10 Position and nature of images for convex lenses
Object position Image
position Nature of
image
At infinity
Beyond 2F'
At 2F'
Between F' and 2F'
At F'
Within F'
IF'
2F
2F'
F
O
F'
2F
2F'
F
I
O
F'
2F
2F'
F
I
O
F'
2F
2F'
F
I
O
F'
2F
2F'
F
O
F'
2F
2F'
FI
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11 Position and nature of images for concave lenses
Object position Image
position Nature of
image
At infinity
Between lens and infinity
I
F' F
IF' F
O
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12 Object height and image height are ho and hi. Object distance and image distance are u and v. Magnification m of image:
m =
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13 Focal length of lens is f. Lens formula:
u: positive
v: for real, for virtual
f: for convex, for concave
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14 Graphs (for convex lens)
1
u
1
v
u
v
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i r
incident ray
reflected ray
normal
IIIa Wave Motion: Light (Optics)
1 Laws of reflection
(1) Incident ray, reflected rayand normal on same plane
(2) r = i
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i
r
incident ray
refracted ray
normal 2 Laws of refraction
(1) Incident ray, refracted rayand normal on same plane
(2) sin i
sin r = constant (Snell’s law)
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3 A light ray hits medium from vacuum at angle of incidence v. Angle of
refraction is m. Refractive index n of medium:
n = sin 𝜃v
sin 𝜃m(≥ 1)
no unit
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4 Refractive index n is also related to the speed of light c in a vacuum and v in the medium:
n = c
v(≥ 1)
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5 General form of Snell’s law: n1 sin θ1 = n2 sin θ2
θ1
n1 < n2
θ2
n2
n1 θ1
n1 > n2
θ2 n2
n1
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6 Conditions for total internal reflection:
(1) Light travels from larger n to smaller n.
(2) i > C
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7 Critical angle C of a medium and its refractive index n:
C = sin–1 (1
n)
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8 Construction rules for lenses
Convex lenses Concave lenses
FF' FF'
FF' FF'
FF' FF'
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9 Focal plane:
Convex: Concave:
F
Rays do NOT stop here!
F'
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10 Position and nature of images for convex lenses
Object position Image
position Nature of
image
At infinity
on focal plane
real, inverted, diminished
Beyond 2F'
between F and 2F
real, inverted, diminished
At 2F'
at 2F real, inverted, same size
Between F' and 2F'
beyond 2F
real, inverted, magnified
At F'
at infinity
no image
Within F'
same side as object
virtual, erect, magnified
IF'
2F
2F'
F
O
F'
2F
2F'
F
I
O
F'
2F
2F'
F
I
O
F'
2F
2F'
F
I
O
F'
2F
2F'
F
O
F'
2F
2F'
FI
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11 Position and nature of images for concave lenses
Object position Image
position Nature of
image
At infinity
on focal plane on the same side as object
virtual, erect, diminished
Between lens and infinity
within F' on the same side as object
virtual, erect, diminished
I
F' F
IF' F
O
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12 Object height and image height are ho and hi. Object distance and image distance are u and v. Magnification m of image:
m = ℎ𝑖
ℎ𝑜=
v
u
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13 Focal length of lens is f. Lens formula:
1
f=
1
u +
1
v
u: positive
v: positive for real, negative for virtual
f: positive for convex, negative for concave
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14 Graphs (for convex lens)
1
u
1
v
1
f
slope = –1
u
v
2f
2f
u = v = 2f
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