IIIa Wave Motion: Light (Optics)

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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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