Light

1
Light
2
Light is a form of energy
Crookes 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?
3
Light travels in straight lines
How are shadows formed?
4
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.

5
Reflection
Angle of incidence Angle of reflection
Normal
Reflected ray
Incident ray
Angle of reflection
Angle of incidence
Mirror
6
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.

7
Reflection

• Laws of Reflection Animation 1
• Laws of Reflection Animation 2

8
How is an image formed in a plane mirror?
9
Virtual Image

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

d
d
10
(No Transcript)
11
Curved Mirrors

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

12
Real Image

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

13
All ray diagrams in curved mirrors and lens are
drawn using the same set of rays.
Concave Mirror
Object
F
Principal Axis
14
F
15
You can draw any ray diagram by combining 2 of
these rays The only difference is where the
object is based.
F
16
Ray Diagrams- Object outside 2F
1/. Inverted 2/. Smaller 3/. Real
F
2F
The images can be formed on a screen so they are
real.
17
Object at 2F
1/. Inverted 2/. Same Size 3/. Real
The image is at 2F
18
Object between 2F and F
1/. Inverted 2/. Magnified 3/. Real
The image is outside 2F
19
Object at F
The image is at infinity
20
Object inside F
1/. Upright 2/. Magnified 3/. Virtual
The image is behind the mirror
21
Convex Mirror
The image is behind the mirror
1/. Upright 2/. Smaller 3/. Virtual
F
22
Convex Mirror only one ray diagram
F
The image is behind the mirror
23
Uses of curved mirrors

• Concave Mirrors
• Dentists Mirrors
• Make up mirrors

• Convex Mirror
• Security Mirrors
• Rear view mirrors

24
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.

25
Calculations
ffocal length uobject distance vimage distance

• Use the formula

26
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 f10 and u20

Using the formula
V20cm real
27
Magnification

• What is the magnification in the last question?
• Well u20 and v20
• As

• m1
• Image is same size

28
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 f30 and u20

Using the formula
V60cm Virtual
29
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 u30

The minus is Because the Mirror is convex
Using the formula
V60/5cm 12cm Virtual
30
MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE
MIRROR 
31
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.
32
Refraction
The fisherman sees the fish and tries to spear it
Fisherman use a trident as light is bent at the
surface
33
(No Transcript)
34
Refraction into glass or water
AIR
WATER
35
Refraction out of glass or water
36
Refraction through a glass block
Light slows down but is not bent, due to entering
along the normal
37
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

38
Proving Snells Law
i
r
Repeat for different angles of incidence
39
Real and Apparent Depth

• A pool appears shallower

40
(No Transcript)
41
MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID
42
Finding No Parallax Looking Down
Pin at bottom
Pin reflection in mirror
No Parallax
Parallax
43
Refractive Index(n) in terms of relative speeds

•  

44
Refractive Index

• Ratio of speeds

45
Refraction out of glass or water
46
Finding the Critical Angle
1) Ray gets refracted
2) Ray still gets refracted
4) Total Internal Reflection
3) Ray still gets refracted (just!)
47
Critical Angle

• Varies according to refractive index

48
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?

49
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.
50
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.
51

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

2) Binoculars and periscopes (using reflecting
prisms)
52
Mirages
53
Lenses
Two types of lenses
Converging Lens Diverging Lens
54
Ray Diagrams
Optical Centre
2F
F
F
55
(No Transcript)
56
Converging Lens- Object outside 2F
Image is 1/. Real 2/. Inverted 3/. Smaller
57
Object at 2F
Image is 1/. Real 2/. Inverted 3/. Same size
58
Object between 2F and F
Image is 1/. Real 2/. Inverted 3/. Magnified
59
Object at F
Image is at infinity
60
Object inside F
Image is 1/. Virtual 2/. Erect 3/. Magnified
61
Calculations
ffocal length uobject distance vimage distance

• Use the formula

62
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 f40 and u30

Using the formula
V120cm virtual
63
Magnification

• What is the magnification in the last question?
• Well u30 and v120
• As

• Image is larger

64
MEASUREMENT OF THE FOCAL LENGTH OF A CONVERGING
LENS Show on OPTICAL BENCH
65
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.
66
The Eye
67
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.
68
Why is not a good idea to water plants on a sunny
day?

• The water forms droplets on the leaves.
• These droplets act as converging lenses and
  focus the sun onto the leaves, burning them.
• As a result the leaves will have brown spots.

69
Why cant 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.

70
Diverging Lens
Image is 1/. Virtual 2/. Upright 3/. Smaller
F
F
71
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 u30

The minus is Because the Diverging lens
Using the formula
V60/5cm 12cm Virtual
72
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 u30

Using the formula
V20cm virtual
73
Magnification

• What is the magnification in the last question?
• Well u30 and v20
• As

• Image is smaller

74
Myopia (Short Sighted)
Image is formed in front of the retina.
Correct with diverging lens.
75
Hyper-Myopia (Long-Sighted)
Image is formed behind the retina.
Correct with a converging lens
76

• Power of Lens
• Opticians use power to describe lenses.
• P
•  
• So a focal length of 10cm 0.1m is written as
  P10m-1
•  
• A diverging lens with a negative focal length
  f-40cm-0.4m
• Has a power of P -2.5m-1

77

• 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
• PtotalP1 P2