Light

By Neil Bronks

Laws of Reflection

The angle of incidence ,i, is always equal to the angle of reflection, r.

The incident ray, reflected ray and the normal all lie on the same plane.

Virtual Image

An image that is formed by the eyeCan not appear on a screen

d d

Real ImageRays really meetCan be formed on a screen

F2F

Ray Diagrams- Object outside 2F

1/. Inverted

2/. Smaller

3/. RealF

The images can be formed on a screen so they are real.

2F

Object at F

The image is at infinity

F2F

Object inside F

1/. Upright

2/. Magnified

3/. Virtual The image is behind the mirror

F

Convex Mirror

1/. Upright

2/. Smaller

3/. Virtual

The image is behind the mirror

F

Uses of curved mirrors

Concave Mirrors Dentists MirrorsMake –up mirrors

•Convex MirrorSecurity Mirrors

Rear view mirrors

60

1

ExampleAn object is placed 20cm from a concave

mirror of focal length 30cm find the position of the image formed. What is the nature of the image?

Collect info f=30 and u=20

Using the formula

vuf

111

v

1

20

1

30

1

20

1

30

11

vV=60cm Virtual

u

vm

20

20

Magnification

What is the magnification in the last question?

Well u=20 and v=60As

20

60

u

vm

u

vm

6

2

• m=3• Image is magnified

60

5

ExampleAn object is placed 30cm from a convex

mirror of focal length 20cm find the position of the image formed. What is the nature of the image?

Collect info f=-20 and u=30

Using the formula

vuf

111

v

1

30

1

20

1

20

1

30

11

v

V=60/5cm =12cm VirtualThe minus is

Because theMirror is convex

MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE MIRROR 

u

v

Lamp-box

Crosswire

Screen

Concave mirror

Laws of REFRACTION

The incident ray, refracted ray and normal all lie on the same plane

SNELLS LAW the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for 2 given media.

sin i =constant =n (Refractive Index)

sin r

Proving Snell’s Law

i

r

Sin i

Sin r

A straight line though the origin proves Snell’s law.

The slope is the refractive index.

Laser

Glass Block

Protractor

Refractive Index

Ratio of speeds

5.1/200000000

/300000000sm

sm

c

cn

water

air

Real and Apparent Depth

A pool appears shallower

Apparent

aln

Re

Cork

Pin

MirrorApparent depth

Pin

Image

Water

Real depth

MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID

Finding the Critical Angle…1) Ray gets refracted

4) Ray gets internally reflected3) Ray still gets refracted (just!)

2) Ray still gets refracted

THE CRITICAL ANGLE

Critical Angle

Varies according to refractive index n

C1

sin

n

145sin

n

17071.0

7071.0

1n 41.1n

Practical Fibre Optics

It is important to coat the strand in a material of low n.

This increases Total Internal Reflection

The light can not leak into the next strand.

Focal Point

Focal Point

Lenses Two types of lenses

Converging Lens Diverging Lens

2FF F2F

Converging Lens- Object outside 2F Image is

1/. Real

2/. Inverted

3/. Smaller

FF

Object inside F Image is

1/. Virtual

2/. Erect

3/. Magnified

u

vm

12

30

Magnification

What is the magnification in the last question?

Well u=30 and v=12As

u

vm

u

vm

2

5• Image is smaller

u v

Lamp-box with crosswire Lens Screen

MEASUREMENT OF THE FOCAL LENGTH OF A CONVERGING LENS

Diverging Lens

FF

Image is

1/. Virtual

2/. Upright

3/. Smaller

60

5

ExampleAn object is placed 30cm from a diverging

lens of focal length 20cm find the position of the image formed. What is the nature of the image?

Collect info f=-20 and u=30

Using the formula

vuf

111

v

1

30

1

20

1

20

1

30

11

v

V=60/5cm =12cm VirtualThe minus is

Because theDiverging lens

u

vm

20

30

Magnification

What is the magnification in the last question?

Well u=30 and v=20As

u

vm

u

vm

2

3• Image is smaller

Sign Convention

f Positive

Veither

f Positive

Veither

f negative

Vnegative

f negative

Vnegative

vuf

111

Myopia (Short Sighted)

Image is formed in front of the retina.

Correct with diverging lens.

Hyperopia (Long-Sighted)

Image is formed behind the retina.

Correct with a converging lens

Power of LensOpticians use power to describe lenses.

P= 

So a focal length of 10cm= 0.1m is written as P=10m-1

 A diverging lens with a negative focal

length f=-40cm=-0.4mHas a power of P = -2.5m-1

f

1

Lens in Contact

Most camera lens are made up of two lens joined to prevent dispersion of the light.

The power of the total lens is Ptotal=P1+ P2