Light

By Neil Bronks

Light is a form of energy

Crooke’s Radiometer proves light has energy

Turns in sunlight as the light heats the black side

Light travels in straight lines

Reflection-Light bouncing off object

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

Angle of incidence = Angle of reflection

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

FPrincipal Axis

Pole

Concave Mirror

Object

All ray diagrams in curved mirrors and lens are drawn using the same set of rays.

F

F

You can draw any ray diagram by combining 2 of these rays

The only difference is where the object is based.

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 2F1/. Inverted

2/. Same Size

3/. Real

The image is at 2F

F2F

Object between 2F and F1/. Inverted

2/. Magnified

3/. Real

The image is outside 2F

F2F

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

Convex Mirror – only one ray diagram

The image is behind the mirror

F

Uses of curved mirrors

Concave Mirrors Dentists MirrorsMake –up mirrors

• Convex MirrorSecurity Mirrors

Rear view mirrors

Calculations

Use the formula

F

vuf

111

u

v

f=focal length

u=object distance

v=image distance

ExampleAn object is placed 20cm from a concave

mirror of focal length 10cm find the position of the image formed. What is the nature of the image?

Collect info f=10 and u=20

20

1

10

11

v

Using the formula

vuf

111

vuf

111

10 20V=20cm real20

11v

u

vm

20

20

Magnification

What is the magnification in the last question?

Well u=20 and v=20As

u

vm

u

vm

2

2

• m=1• Image is same

size

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

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

Questions

An object 2cm high is placed 40cm in front of a concave mirror of focal length 10cm find the image position and height.

An image in a concave mirror focal length 25cm is 10cm high if the object is 2cm high find the distance the object is from the mirror.

MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE MIRROR 

u

v

Lamp-box

Crosswire

Screen

Concave mirror

Approximate focal length by focusing image of window onto sheet of paper.

Place the lamp-box well outside the approximate focal length

Move the screen until a clear inverted image of the crosswire is obtained.

Measure the distance u from the crosswire to the mirror, using the metre stick.

Measure the distance v from the screen to the mirror. Repeat this procedure for different values of u. Calculate f each time and then find an average value.

 Precautions The largest errors are in measuring with

the meter rule and finding the exact position of the sharpest image.

Refraction

Fisherman use a trident as light is bent at the surface

The fisherman sees the fish and tries to spear it

Refraction into glass or water

Light bends towards the normal due to entering a

more dense mediumAIR

WATER

Refraction out of glass or water

Light bends away from the normal due to entering a less

dense medium

Refraction through a glass block

Light bends towards the normal due to entering a

more dense medium

Light bends away from the normal due to entering a less

dense medium

Light slows down but is not bent, due to

entering along the normal

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 = 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.

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

H/W

LC Ord 2006 Q2

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

WaterReal depth

MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID

Finding No Parallax – Looking Down

Pin atbottom

Pin reflectionin mirror

Parallax No Parallax

     Set up the apparatus as shown.

    Adjust the height of the pin in the cork above the mirror until there is no parallax between its image in the mirror and the image of the pin in the water.

    Measure the distance from the pin in the cork to the back of the mirror – this is the apparent depth.

    Measure the depth of the container – this is the real depth.

    Calculate the refractive index n= Real/Apparent

Repeat using different size containers and get an average value for n.

Refraction out of glass or water

Light stays in denser medium

Reflected like a mirror

Angle i = angle r

Snell’s Window

Finding the Critical Angle…1) Ray gets refracted

4) Total Internal Reflection3) Ray still gets refracted (just!)

2) Ray still gets refracted

THE CRITICAL ANGLE

Semi-Circular Block Expt and on the internet click here

Mirages

Critical Angle

Varies according to refractive index n

C1

sin

n

145sin

n

17071.0

7071.0

1n 41.1n

Uses of Total Internal Reflection

Optical fibres:

An optical fibre is a long, thin, transparent rod made of glass or plastic. Light is internally reflected from one end to the other, making it possible to send large chunks of information

Optical fibres can be used for communications by sending e-m signals through the cable. The main advantage of this is a reduced signal loss. Also no magnetic interference.

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.

1) Endoscopes (a medical device used to see inside the body):

2) Binoculars and periscopes (using “reflecting prisms”)

Now is a good time to get out the light demo kit

H/W

LC Ord 2003 Q7

Focal Point

Focal Point

Lenses Two types of lenses

Converging LensDiverging Lens

Focal Length=f

Focal Length=f

2FF F

Optical Centre

Ray Diagrams

2FF F

2FF F2F

Converging Lens- Object outside 2F Image is

1/. Real

2/. Inverted

3/. Smaller

2FF F2F

Object at 2F Image is

1/. Real

2/. Inverted

3/. Same size

2FF F2F

Object between 2F and F

Image is

1/. Real

2/. Inverted

3/. Magnified

FF

Object at F

Image is at infinity

FF

Object inside F Image is

1/. Virtual

2/. Erect

3/. Magnified

H/W

Draw the 5 ray diagrams for the converging lens and the diagram for the diverging lens .

Write 3 characteristics of each image.

Calculations

Use the formula

vuf

111

u

v

f=focal length

u=object distance

v=image distance

2FF F2F

vuf

111 =

-120

ExampleAn object is placed 30cm from a

converging lens of focal length 40cm find the position of the image formed. What is the nature of the image?

Collect info f=40 and u=30

Using the formula

vuf

111

vuf

111

40 30 vuf

111

v 3040- V=120cm

virtual

u

vm

120

30

Magnification

What is the magnification in the last question?

Well u=30 and v=120As

u

vm

u

vm

4

1• Image is larger

u v

Lamp-box with crosswire Lens Screen

MEASUREMENT OF THE FOCAL LENGTH OF A CONVERGING LENS

Show on OPTICAL BENCH

1.      Place the lamp-box well outside the approximate focal length 2.    Move the screen until a clear inverted image of the crosswire is obtained.3.    Measure the distance u from the crosswire to the lens, using the metre stick.4. Measure the distance v from the screen to the lens. 5. Calculate the focal length of the lens using

  6. Repeat this procedure for different values of u. 7. Calculate f each time and then find the average value.

vuf

111

H/W

LC Ord 2002 Q3

Accommodation

The width of the lens is controlled by the ciliary muscles.

For distant objects the lens is stretched.

For close up objects the muscles relax.

Accommodation internet

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

vuf

111 =

-20

ExampleAn object is placed 30cm from a diverging

lens of focal length 60cm find the position of the image formed. What is the nature of the image? (Remember f must be negative)

Collect info f=-60 and u=30Using the formula

vuf

111

vuf

111

-60 30 vuf

111

v 30-60- V=20cm

virtual

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

H/W

LC Higher 2002 Q12 (b)LC Higher 2003 Q3