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Light. Light is a form of energy. Crooke’s Radiometer proves light has energy. Turns in sunlight as the light heats the black side. Can you think of another example to demonstrate that light is a form of energy?. Light travels in straight lines. How are shadows formed?. Reflection.

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light is a form of energy
Light is a form of energy

Crooke’s Radiometer proves light has energy

Turns in sunlight as the light heats the black side

Can you think of another example to demonstrate that light is a form of energy?

light travels in straight lines
Light travels in straight lines

How are shadows formed?

reflection
Reflection
  • Reflection is the bouncing of light off an object.
  • When light bounces off objects it scatters in all directions – diffuse reflection.
  • Highly polished surfaces (mirror) behave in a more predictable way.
reflection1
Reflection

Angle of incidence = Angle of reflection

Normal

Reflected ray

Incident ray

Angle of reflection

Angle of incidence

Mirror

laws 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.
reflection2
Reflection

Laws of Reflection Animation 1

Laws of Reflection Animation 2

virtual image
Virtual Image
  • An image that is formed by the apparent intersection of light rays
  • Can not appear on a screen

d

d

curved mirrors
Curved Mirrors
  • Curved mirrors consist of a series of small mirrors combined together.
  • Each individual mirror must obey the laws of reflection.
real image

2F

F

Real Image
  • An image that is formed by the actual intersection of light rays.
  • Can be formed on a screen
slide13

Pole

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

Concave Mirror

Object

F

Principal Axis

slide15

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

The only difference is where the object is based.

F

ray diagrams object outside 2f
Ray Diagrams- Object outside 2F

1/. Inverted

2/. Smaller

3/. Real

2F

F

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

object at 2f

2F

F

Object at 2F

1/. Inverted

2/. Same Size

3/. Real

The image is at 2F

object between 2f and f

2F

F

Object between 2F and F

1/. Inverted

2/. Magnified

3/. Real

The image is outside 2F

object at f

2F

F

Object at F

The image is at infinity

object inside f

F

Object inside F

1/. Upright

2/. Magnified

3/. Virtual

The image is behind the mirror

convex mirror
Convex Mirror

The image is behind the mirror

1/. Upright

2/. Smaller

3/. Virtual

F

convex mirror only one ray diagram
Convex Mirror – only one ray diagram

F

The image is behind the mirror

uses of curved mirrors
Uses of curved mirrors
  • Concave Mirrors
      • Dentists Mirrors
      • Make –up mirrors
  • Convex Mirror
  • Security Mirrors
  • Rear view mirrors
ray diagram example
Ray Diagram Example
  • An object 4 cm high is placed at right angles to the axis of a concave mirror and at a distance of 30 cm from the mirror. If the focal length of the mirror is 10 cm find the position, size and nature of the image.
  • This can be done using a diagram or by calculation.
calculations

u

v

F

Calculations

f=focal length

u=object distance

v=image distance

  • Use the formula
example

10

20

Example

An 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

Using the formula

V=20cm real

magnification

20

2

20

2

Magnification
  • What is the magnification in the last question?
  • Well u=20 and v=20
  • As
  • m=1
  • Image is same size
example1
Example

An 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

V=60cm Virtual

example2
Example

An 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

The minus is

Because the

Mirror is

convex

Using the formula

V=60/5cm =12cm Virtual

measurement of the focal length of a concave mirror
MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE MIRROR

Concave mirror

Crosswire

Lamp-box

Screen

u

v

slide31

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
Refraction

The fisherman sees the fish and tries to spear it

Fisherman use a trident as light is bent at the surface

slide34

Light bends towards the normal due to entering a more dense medium

Refraction into glass or water

AIR

WATER

slide36

Light bends towards the normal due to entering a more dense medium

Light bendsaway from the normal due to entering a less dense medium

Refraction through a glass block

Light slows down but is not bent, due to entering along the normal

laws of refraction
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
Proving Snell’s Law

i

r

Repeat for different angles of incidence

real and apparent depth
Real and Apparent Depth
  • A pool appears shallower
slide41

Cork

Pin

Apparent depth

Mirror

Real depth

Water

Image

Pin

MEASUREMENT OF THE

REFRACTIVE INDEX OF A LIQUID

finding no parallax looking down
Finding No Parallax – Looking Down

Pin at

bottom

Pin

reflection

in mirror

No Parallax

Parallax

refractive index
Refractive Index
  • Ratio of speeds
slide45

Light stays in denser medium

Reflected like a mirror

Angle i = angle r

Refraction out of glass or water

finding the critical angle

THE CRITICAL ANGLE

Finding the Critical Angle…

1) Ray gets refracted

2) Ray still gets refracted

4) Total Internal Reflection

3) Ray still gets refracted (just!)

critical angle
Critical Angle
  • Varies according to refractive index
refractive index and critical angle
Refractive Index and Critical Angle
  • Refractive Index is defined in relation to light going from air into that medium (i.e. air to glass or air to water)
  • Ex 1: The critical angle for a certain medium is 500 . Find its refractive index
  • Ex 2: The refractive index of glass is 1.5. What is the critical angle for glass?
uses of total internal reflection
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
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.

slide51

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

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

slide53

Focal Point

Focal Point

Lenses

Two types of lenses

Converging Lens Diverging Lens

slide54

Ray Diagrams

Optical Centre

2F

F

F

slide55

2F

F

F

converging lens object outside 2f

2F

F

2F

F

Converging Lens- Object outside 2F

Image is

1/. Real

2/. Inverted

3/. Smaller

object at 2f1

2F

F

2F

F

Object at 2F

Image is

1/. Real

2/. Inverted

3/. Same size

object between 2f and f1

2F

F

2F

F

Object between 2F and F

Image is

1/. Real

2/. Inverted

3/. Magnified

object at f1

F

F

Object at F

Image is at infinity

object inside f1

F

F

Object inside F

Image is

1/. Virtual

2/. Erect

3/. Magnified

calculations1

u

v

2F

F

2F

F

Calculations

f=focal length

u=object distance

v=image distance

  • Use the formula
example3

-

=

v

40

30

40

30

-120

Example

An 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

V=120cm virtual

magnification1

120

4

30

1

Magnification
  • What is the magnification in the last question?
  • Well u=30 and v=120
  • As
  • Image is larger
slide64

Lamp-box with crosswire

Screen

Lens

v

u

MEASUREMENT OF THE FOCAL LENGTH

OF A CONVERGING LENS

Show on OPTICAL BENCH

slide65

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.

power of accommodation ability to focus a real image of an object on the retina
Power of Accommodation- ability to focus a real image of an object on the retina

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.

why is not a good idea to water plants on a sunny day
Why is not a good idea to water plants on a sunny day?
  • The water forms droplets on the leaves.
  • Thesedroplets act as converging lenses and focus thesun onto the leaves, burning them.
  • As a result theleaves will have brown spots.
why can t we focus clearly under water yet swimming goggles will restore clear focus
Why can’t we focus clearly under water yet swimming goggles will restore clear focus?
  • Hint: your cornea and water have a similar refractive index
  • Light refracts when travelling from air through the cornea of your eye, but water and the cornea have the same refractive index , so light does not refract.
  • By wearing goggles however light which hits your eye is coming from air, so the usual focusing applies and objects appear normal.
diverging lens
Diverging Lens

Image is

1/. Virtual

2/. Upright

3/. Smaller

F

F

example4
Example

An 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

The minus is

Because the

Diverging lens

Using the formula

V=60/5cm =12cm Virtual

example5

-

=

v

-60

30

-60

30

-20

Example

An 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=30

Using the formula

V=20cm virtual

magnification2

20

2

30

3

Magnification
  • What is the magnification in the last question?
  • Well u=30 and v=20
  • As
  • Image is smaller
myopia short sighted
Myopia (Short Sighted)

Image is formed in front of the retina.

Correct with diverging lens.

hyper myopia long sighted
Hyper-Myopia (Long-Sighted)

Image is formed behind the retina.

Correct with a converging lens

slide76
Power of Lens

Opticians 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=-0.4m

Has a power of P = -2.5m-1

slide77
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