Chapter 5 Light PHYSICS FORM 4 Cikgu Desikan Compiled by
Chapter 5
Light
PHYSICSFORM 4
Cikgu DesikanCompiled by
1. Understanding reflection of light
2. Understanding refraction of light
3. Understanding total internal reflection of light
4. Understanding lenses
Chapter 5
LightP
RE
SP
M P
HY
SIC
S
2016
Learning Objectives :
Dear students,
The two basic processes of education are knowing and valuing.
2007 2008 2009 2010 2011 2012 2013 2014 2015
P1 5 5 5 4 5 5 5 4
P2
A 1 1 1 1 1 1 1 1
B - - - - - - - -
C - 1 - - - - - -
P3A - - - - - - - -
B 1 - - - 1 - 1 -
Analysis of Past Year Questions
Chapter 5
Light
Concept Map
Dear students,
Light
LensesReflection of
Light
Refraction of
Light
Laws of
Reflection
Laws of
Refraction
Refractive
index
Convex
Lens
Ray
diagram
Concave
lens
Mirror
Ray Diagram
Positions and
characteristics of
image
v
1
u
1
f
1
Positions and
characteristics of image
Total internal
Reflection
sinc
1n
r sin
i sinn
medium in light of v
vacuum in light of vn
d
Dn
Plane
Convex
Concave
Great dreams of great dreamers are always transcended.(by Dr. Abdul Kalam)
5.1 Understanding Reflection Of Light
Reflection
1. Mirror works because it reflects light..
2. The light ray that strikes the surface of the
mirror is called incident ray.
3. The light ray that bounces off from the
surface of the mirror is called reflected ray.
4. The normal is a line perpendicular to the
mirror surface where the reflection occurs.
5. The angle between the incident ray and the
normal is called the angle of incidence, i
6. The angle between the reflected ray and the
normal is called the angle of reflection, r.
Laws of Reflection
NBA
Mirror O
AO = Incident ray
OB = Reflected ray
i = Angle of incident
r = Angle of reflection
i r
4
Draw ray diagrams to show the positioning and characteristics of the image formed by a
plane mirror.
Characteristics of the image formed by
reflection of light.
Notes:
Real image : Image that can be seen on a
screen
Virtual image : Image that cannot be seen
on a screen.
Plane Mirror Student
Object
5
Great dreams of great dreamers are always
transcended. “
” APJ Abdul Kalam
Concave Mirror
Reflection of light on curved mirror
Convex Mirror
Common terminology of curved mirrors
Centre of curvature, C
The center of sphere of the mirror
Principle axis
The connecting line from the centre of curvature to point P
Radius of curvature, CP
The distance between the centre of curvature and the surface
of the mirror.
Focal point, F
The focal point of a concave mirror is the point on the principle
axis where all the reflected rays meet and converge.
The focal point of convex mirror is the point on the principle
axis where all the reflected rays appear to diverge from
behind the mirror.
6
C FP
CFP
Differences
Common terminology of curved mirrors
Focal length, f
The distance between the focal point and the surface of the mirror. (FP or ½ CP)
Object distance, u
The distance between the object and the surface of the mirror.
Image distance, v
The distance between the image and the surface of the mirror.
Concave Mirror Convex Mirror
Rays travelling parallel to the principal axis
converge to a point, called the focal point on
the principal axis.
Rays travelling parallel to the principal axis
appear to diverge from a point behind the
mirror, called the focal point on the principal
axis.
7
Concave Mirror Convex Mirror
Rule 1
A ray parallel to the principal axis is reflected
through F.
A ray parallel to the principal axis is reflected as
if it comes from F.
Rule 2
A ray passing through F is reflected parallel to
the principal axis
A ray directed towards F is reflected parallel to
the principal axis.
Construction Rules for Concave Mirror and Convex Mirror
Object
F
Object
F
Object
F
Object
F
8
Construction Rules for Concave Mirror and Convex Mirror
Concave Mirror Convex Mirror
Rule 3
A ray passing through C is reflected back along
the same path through C.
A ray is directed towards C is reflected back
along the same path away from C.
If an egg is broken by an outside force….
A life ends.
If an egg breaks from within...... .
Life begins.
Great things always begin from within .
“
”
Object
F C
Object
F C
9
O
C F
u > 2f u = 2f or u = c
f < u < 2f or f < u < c u = f
O C F
O
F
O
C F
10
Ray Diagrams of concave mirror
u < f Object
distance
Characteristics of the
image:
u > 2f
u = 2f
f < u < 2f
u = f
u < f
Ray Diagram of concave mirror
Excellent ! We can attack in
any direction.
O
C F
11
Sir, we are surrounded from all
sides by enemies!
f < u < 2f
Ray Diagram of convex mirror
u < f
O
C F F C
O
C F F C
Object distance Characteristics of the image:
u > 2f
• Diminished, upright, virtual
• Image formed within 0 < v < f
u = 2f
f < u < 2f
u = f
u < f
12
Application of Reflection of Light
13
Anti-parallax Mirror in Ammeters or Voltmeter
1. A parallax error occurs when the scale is viewed at
an improper angle (the eye sees both the pointer and
its image).
2. Some meters provide a mirror within the display, so
that a user can easily determine the correct viewing
angle by checking the needle's reflection.
3. The proper angle is achieved when the needle's
reflection is not visible to the user's eye.
Periscope
mirror
strip
pointer
pointer’s image
1. A periscope can be used to see over the top
of high obstacles such as a wall.
2. It is also used inside a submarine to observe
the surrounding above water surface.
3. Consist of 2 plane mirror inclined at an
angle of 45°.
4. The final image appears upright.
ray from a
far object
45°
45°
mirror
14
Ambulance
• Why is the word ‘AMBULANCE’
purposely inverted laterally on
an ambulance car?
• Images seen through the rear
mirror of a car is laterally
inverted.
Make-up Mirror
Concave mirrors with
long focal lengths,
produce virtual,
magnified and upright
images
• The light bulb is fixed in position at the focal
point of the concave mirror to produce a
beam of parallel light rays.
• The beam of parallel light rays will maintain
a uniform intensity for a greater distance.
• Other applications are the headlight of motor
vehicles and the lamp of slide projectors.
Reflector of torchlight
parallel
light rays
bulb
F
Field of vision
a) Plane mirror
Wider field of vision
b) Convex mirror
15
Widening the field of vision
• When a convex mirror is used, the field of vision is larger than a plane mirror
• Convex mirrors are used as rear view mirrors in motor vehicles to give drivers a wide-angle view
of vehicles behind them.
• It is also used as shop security mirrors.
A concave parabolic surface is
used to focus the radio wave
signals.
Transmission of radio waves and signals
5.2 Understanding Refraction Of Light
Refraction of light
Angle of
incidence, i
the angle between the incident ray
and the normal.
Angle of
refraction, r
the angle between the refracted ray
and the normal
i > r the ray bent towards the normal, and
the speed of light decreases.
r < i the ray bent away from the normal
and the speed of light increases.
NADenser
medium
O
i
r
Less Dense medium
NALess Dense
medium
O
i
r
Denser medium
B
B
AO = Incident ray
OB = Refracted ray
ON = Normal line
i = Angle of incident
r = Angle of refraction 16
When a light ray travels from
less dense medium to denser
medium
When a ray of light travels
from denser medium to less
dense medium.
When light ray is incident
normally on the boundary
between the two medium.
The light ray is refracted
towards the normal.
The speed of light decreases.
The light ray is refracted away
from the normal.
The speed of light increases.
The light ray is does not bend.
3 ways in which a ray of light can travel through two medium
The Laws of Refraction
17
NADenser
medium
O
i
r
Less Dense mediumB
NALess Dense
medium
O
i
r
Denser mediumB
nconstantr sin
i sin
Snell’s Law
• The refraction of light is caused by the
change in velocity of light when it
passes from a medium to another
medium.
• The refractive index has no units.
• It is an indication of the light-bending
ability of the medium as the ray of light
enters its surface from the air.
Refractive Index, n
1
medim in light of velocity
vacuum in light of velocityn
2
Real Depth and Apparent Depth
1. Rays of light coming from the real fish, O
travels from water (more dense) to air
(less dense)
2. The rays are refracted away from the
normal as they leave the water.
3. When the light reaches the eye of the
person, it appears to come from a virtual
fish, I which is above the real fish O.
h
Hn 3
18
Velocity of light in medium
h = Aparent depth
H = Real depth
Air
O
H
hWater
I
Normal
19
a) Draw a ray diagram from point P to the eye to show how the legs appear shorter.
b) The depth of water is 0.4 m. Calculate the distance of the image of the foot at point P from
the surface of the water. [Refractive index of water = 1.33]
Exercise 5.2
1.
3. A light ray is incident normally on a glass prism which has a
refractive index of 1.50.
a) complete the ray diagram.
b) Find the incident angle and the refractive angle
20
2. The light ray travels from air to medium x. Find the:
a) incident angle
b) refracted angle
c) refractive index 60° Medium X
45°
Air
30°
60°
5.3 Total Internal Reflection Of Light
1. When light travels from a denser medium to a less
dense, it bends away from normal.
2. A small part of the incident ray is reflected inside the
glass.
3. The angle of refraction is larger than the angle of
incidence, r > i
i < c
1. When the angle of incidence, i keeps on increasing, r
too increases and the refracted ray moves further
away from the normal – and thus approaches the
glass – air boundary.
2. The refracted ray travels along the glass-air boundary.
3. This is the limit of the light ray that can be refracted in
air as the angle in air cannot be any larger than 90°.
4. The angle of incidence in the denser medium at this
limit is called the critical angle, c.
i = c
21
c
Glass
Air
Normal
Incident
ray Weak
reflected
ray
r = 90°Refracted
ray
i = c c
i
Glass
Air
Normal
Incident
ray Weak
reflected
ray
Refracted
rayr
1. If the angle of incidence is increased further so that it
is greater than the critical angle, the light is not
refracted anymore, but is internally reflected.
2. This phenomenon is called total internal reflection.
i > c
The two conditions for total internal reflection to occur are:
1. light ray enters from a denser medium towards a less dense medium
2. the angle of incidence in the denser medium is greater than the critical angle of the medium.
Total internal reflection
Conditions
22
Glass
Air
Normal
Incident
ray
Strong
reflected
ray
i > cc
Figure shows a light ray strikes the surface of a prism. The
refractive index of glass is 1.5. Find the critical angle. Complete
the path of the light ray that passes into and out of the prism.
Calculate the critical angle, c [ Refractive index of water = 1.33 ].
Exercise 5.3
23
45°
c
Water
Air
1.
2.
Natural Phenomenon involving Total Internal Reflection
1. Mirage is caused by refraction
and total internal reflection.
2. Mirage normally occur in the
daytime when the weather is hot.
3. The air above the road surface
consists of many layers.
4. The layers of air nearest the road are hot and the layers get cooler and denser towards the
upper layers.
5. The refractive index of air depends on its density. The lower or hotter layers have a lower
refractive index than the layers above them.
Mirage
Sunset 1. The Sun is visible above the horizon
even though it has set below the
horizon.
2. Light entering the atmosphere is
refracted by layers of air of different
densities producing an apparent shift
in the position of the Sun.
24
Applications of Total Internal Reflection
1. The periscope is built using two right-angled
prisms.
2. The critical angle of the glass prisms is 42°.
3. Total internal reflection occurs when the light rays
strike the inside face of a 45°angles with an angle
of incidence, I, greater than the critical angle, c,.
4. The image produced is upright and has the same
size as the object.
Advantage of the prisms periscope compared to a
mirror periscope:
a) the image is brighter because all the light energy
is reflected.
b) the image is clearer because there are no multiple
images as formed in a mirror periscope.
Prism Periscope
25
object
image
prism
prism
45°
45°
refraction
Total internal reflectionFish sees outside
world inside 96˚
cone
1. A fish is able to see an object above the water surface because the rays of light from the object
are refracted to the eyes of the fish or diver.
2. Due to total internal reflection, part of the water surface acts as a perfect mirror, which allows
the fish and diver to see objects in the water and the objects around obstacles.
3. A fish sees the outside world inside a 96° cone. Outside the 96°cone, total internal reflection
occurs and the fish sees light reflected from the bottom of the pond. The water surface looks
like a mirror reflecting light below the surface.
Fish’s Eye View
26
1. A pair of binoculars uses two prisms which
are arranged as shown in figure.
2. Light rays will be totally reflected internally
two times in a pair of binoculars.
3. The benefits of using prisms in binoculars:
a) an upright image is produced.
b) The distance between the objective lens
and the eyepiece is reduced. This make
the binoculars shorter as compared to a
telescope which has the same
magnifying power.
Prism Binoculars
27
Prism A
Prism BObjective
lens
Eyepiece
lens
Object
Image
45°
45°
1. Fiber optics consists of a tubular rod
which is made from glass and other
transparent material.
2. The external wall of a fiber optic is less
dense than the internal wall.
3. When light rays travel from a denser
internal wall to a less dense external wall
at an angle that exceeds the critical angle,
total internal reflection occurs repeatedly.
4. This will continue until the light rays enter
the observer’s eye.
5. Optical fiber is widely used in
telecommunication cables to transmit
signal through laser. It can transmit signal
faster and through long distance with high
fidelity.
6. Optical fiber is also used in an endoscope
for medical emerging.
Advantage of using optical fibres cables over copper cables:
(a) much thinner and lighter
(b) a large number of signals can be sent through them at one time.
(c) transmit signals with very little loss over great distances.
(d) signals are safe and free of electrical interference
(e) can carry data for computer and TV programmes.
Optical fibers
28
Internal wall
external wall
5.4 Lenses
Lenses are made of transparent material such as glass or clear plastics. They have two faces, of
which at least one is curved.
Convex Lens
Concave Lens
Convex lenses @
converging lenses
- thicker at the centre
Concave lenses @
diverging lenses
- thinner at the centre
29
Biconvex Plano-convex Concavo - convex
Biconcave Plano-concave Concavo - concave
Focal Point and Focal Length of a Lens
A point on the principle axis to which incident
rays of light traveling parallel to the axis
converge after refraction through a convex
lens.
Focal Point @ the principal focus, F
Distance between the focal point , F and
optical centre, C on the lens.
30
Focal length, f
Distance between the focal point, F and the
optical centre , C
Focal Point @ principal focus, F
A point on the principal axis to which incident
rays of light traveling parallel to the axis
appear to diverge after refraction through a
concave lens.
Focal length, f
CF F
Light rays
Principal axis
Focal
point
Optical center
CFPrincipal axis
Light rays
Focal
point
f
f
The ray parallel to the
principal axis is refracted
through the focus point, F.
A ray passing through the
focus point is refracted
parallel to the principal axis.
A ray passing through the
optical centre travels straight
without bending.
The ray parallel to the
principal axis is refracted as if
it appears coming from focus
point, F which is located at
the same side of the incident
ray.
A ray passing the focus point
is refracted parallel to the
principle axis.
A ray passing through the
optical centre travels straight
on without bending.
Concave Lens
Rules for Ray Diagrams
Convex Lens
31
OF FOF F
O
FF
OF F OF FO FF
1 2 3
1 2 3
Ray Diagrams of convex lens
OF2F F
u < f
OF2F F
u = f
OF2F F
f < u < 2f
2F
32
OF
F
u =
OF2F F
u = 2f
2FO
F2F F
u > 2f
Object
distance
Characteristics of the image:
u = Diminished, inverted, real
u > 2f Diminished, inverted, real
u = 2f Same size, inverted, real
f < u < 2f Magnified, inverted, real
u = f Magnified, upright, virtual
u < f Magnified, upright, virtual
33
Ray Diagrams of concave lens
OF2F
f < u < 2f
Object
Image
R1
R3
34
Ray Diagrams of concave lens
OF2F
u = 2f
Object
distance Characteristics of the image:
u =
u > 2f
u = 2f
f < u < 2f
u = f
u < f
OF2F
u < f
35
Power of Lenses
1. The power of a lens is a measure of its ability to
converge or to diverge an incident beam of light.
2. SI unit = m-1 or Diopter (D).
3. Power for a convex lens is positive. Power for a
concave lens is negative.
f in m f in cm
Find the power:
a) convex lens, f = 20 cm,
b) concave lens, f = -5 cm.
Example 1
Lens Formula
1
2
3
f = focal length
u = object distance
v = image distance
m = Linear magnification
hI = size of image
h0 = size of object
m < 1
m = 1
m > 1
36
1. An object is placed in front of a convex lens with focal length of 10 cm. Find the nature, position
and magnification of the image formed when the object distance is 15 cm.
37
Exercise 5.4
2. An object is placed 20 cm from a concave lens of focal length 15 cm. Calculate the image
distance. State the characteristics of the image formed.
38
3. A convex lens with focus length of 15 cm formed an image which is real, inverted and same
size with the object. What is the object distance from the lens?
39
4. When an object of height 3.0 cm is placed 20 cm from a concave lens of focal length 30cm,
what is the height of the image formed?
40
Applications of Lenses
Simple Microscopes
Application : to magnified the image
Lens : a convex lens
Object distance: less than the focal length
of the lens, u < f
Characteristics of image: virtual, upright,
magnified
The magnifying power increases if the focal
length of the lens is shorter.
41
OF F
object
eye
Application : view very distant objects like the planets and the stars.
Made up of two convex lenses :Objective lens and eyepiece lens
Focal length fo for objective lens is longer than the focal length for eyepiece lens, fe The objective lens converges the parallel rays from a distant object and forms a real,
inverted and diminished image at its focal point.
The eyepiece lens is used as a magnifying glass to form a virtual, upright and magnified
image.
At normal adjustment the final image is formed at infinity.
This is done by adjusting the position of the eyepiece lens so that the first real image
becomes the object at the focal point, Fe of the eyepiece lens.
Normal adjustment: The distance between the lenses is f0 + fe 42
Telescope
FoFo
Fe Fe
Light ray
from distant
object
Objective
lens Eyepiece
lensfefo
Final image
formed at infinity
u1 =
u2 = fe
Compound Microscope
43
Application: to view very small objects like microorganisms
Uses 2 powerful convex lenses (Objective lens, Eyepiece lens ) of
short focal lengths.
Focal length fo for objective lens is shorter than the focal length for
eyepiece lens, fe Object to observed must be placed between F0 and 2F0
Characteristics of 1st image: real, inverted, magnified.
object
Objective
lens
Eyepiece
lens
Final image
Fe
Fe
Fo
fo fe
Fo2Fo 1st image
fo < u1 < 2fo
u2 < fe
The eyepiece lens is used as a magnifying glass to magnify the first image formed by the
objective lens.
The eyepiece lens must be positioned so that the first image is between the lens and Fe, the
focal point of the eyepiece lens.
Characteristics of final image formed by the eyepiece lens: virtual, upright and magnified.
Normal Adjustment: The distance between the lenses is greater than the sum of their individual
focal length (fo + fe)
44