LIGHT
WHAT IS LIGHT?
 Light
is a form of energy that
travels away from the source
producing it at a speed of 3 x
108 m s-1
 Transparent:
allows light to pass
through it, and can see clearly
through it e.g. glass
 Translucent: allows light to pass
through it, but cannot see clearly
through it e.g. frosted glass
 Opaque: does not allow light to pass
through it e.g. wood
Light Travels in Straight Lines
This can be seen in the following
examples:
 Laser
 Beam of light from a searchlight
 It can also be shown using pieces of
cardboard with a small hole in the middle
and a length of thread

Plane Mirror
Normal
Incident ray
Reflected ray
Angle of
incidence
Angle of
reflection
i r
Plane Mirror
LAWS OF REFLECTION OF LIGHT
 1.
The incident ray, the normal
and the reflected ray all lie in the
same plane
 2.
The angle of incidence is equal
to the angle of reflection (i = r)
HOW IS AN IMAGE FORMED IN A
PLANE MIRROR
Properties of an image in a plane
mirror
 Laterally
inverted
 E.g.
your right hand appears as a left
hand
 The “ambulance” sign
 Erect
(right way up)
 Virtual
 Same size as the object
Uses of Plane Mirrors
 Make
up mirror
 The periscope
A
virtual image cannot be
formed on a screen
A
real image can be formed on a
screen

Experiment to prove the angle of incidence
equals the angle of reflection
Diagram
Plane
mirror
i r
Pins
Sheet of
paper
Experiment to prove the angle of
incidence equals the angle of reflection
Method
 1. Set up the apparatus as in the
diagram.
 2. Mark the incident ray
 3. Mark the reflected ray
 4. Draw in the normal
 5. Measure angles i and r
 6. Repeat for different angles
Conclusion
Angle i = angle r
Precaution
Make sure the mirror is perpendicular to the page
Mark the back of the mirror on the paper
Use a sharp pencil

Reflection of light is when light bounces
off a surface
Experiment to find the position of an
image in a plane mirror
(Not a mandatory experiment)
 (Write up should be in homework copy)

Image
(Object pin)
Plane mirror
Object pin
O
M
I

1.
2.
Method
Set up the apparatus as in the diagram
Move the tall finder pin in and out
behind the mirror until there is no
parallax between the finder pin and the
image of the object pin in the mirror
3. Measure the distance from the object
pin to the mirror (OM), and the distance
from the mirror to the finder pin (MI)
Result
OM and MI are equal
Conclusion
The image is as far behind the mirror as
the object is in front of it
Spherical Mirrors

Convex mirrors and concave mirrors
CONVEX
CONCAVE
Diagram of
concave mirror
Radius of curvature
Centre of curvature
Pole
Focal length
Focus point

The line from the centre of curvature to
the pole is called the principal axis
Rules for Ray Diagrams for
Concave Mirror
1. A ray travelling parallel to the principal
axis is reflected through the focus
 2. A ray travelling through the focus is
reflected parallel to the principal axis
 3. For a ray which strikes the pole, angle
i will be equal to angle r


“In parallel, out through the focus”

“In through the focus, out parallel”
Uses of concave mirrors
Spotlights
 Reflectors in car headlights
 Shaving and make-up mirrors

Diagram of
convex mirror
Centre of curvature
Pole
SHINY SIDE
Focus point
Ray Diagrams for Convex Mirrors
Uses of convex mirrors
Shops (to deter shoplifters)
 Buses
 Dangerous bends in roads


They give a wide field of view
The Mirror Formulae
The focal length of a spherical mirror may be found using the formula:
1 1
1
 
u
v
f
1 1
1
 
u v
f
1 1
1
 
u v
f
u = distance from object to mirror
v = distance from image to mirror
f = focal length
Example 1

An object is placed 15cm in front of a
concave mirror, of focal length 12cm.
Find the position and nature of the image
1 1 1
 
u v f
1 1 1
 
15 v 12
1 1 1
 
v 12 15
1 54

v
60
1 1

v 60
v = 60 cm

It is a real image since the object is
outside f
Example 2

When an object is placed 16 cm in front
of a concave mirror of focal length 8 cm,
an image is formed. Find the distance of
the image from the mirror and say
whether it is real or virtual.
1 1 1
 
u v f
1 1 1
 
16 v 8
1 1 1
 
v 8 16
1 2 1

v 16
1 1

v 16
v = 16 cm

It is a real image since the object is
outside f
(HL)
Magnification
v
 m=
u

m=
height of image
height of object
Example 3 (HL)

An object is placed 20 cm from a
concave mirror of focal length 25 cm.
Find the position, magnification and
nature of the image.
1 1 1
 
u v
f
1 1 1
 
20 v 25
1 1
1
  
v 25 20
1 45
 
v 100
1 1
 
v 100
v = 100 cm

It is a virtual image since the object is
inside f
v
u
v
u

m=

m=

m=5
Example 4 (HL)

A concave mirror of focal length 10 cm
forms an erect image four times the size
of the object. Calculate the object
distance and its nature.
1 1 1
 
u v
f
v 4

u 1
v  4u
3
1

4u 10
1 1 1
 
u v 10
1
1
1


u 4u 10
4u  30
v
M  4
u
4 1 1

4u
10
u = 7.5 cm

It is a virtual image since the object is
inside f
Experiment to Measure the Focal
Length of a Concave Mirror
CROSS
THREADS
RAY BOX
CONCAVE
MIRROR
SCREEN
Method
 (An
approximate value for the
focal length can be found by
focusing the image of a distant
object on a sheet of paper (e.g.
a tree or window))
 Set
up the apparatus as in the
diagram
 Move the screen in and out until
the image of the cross threads is
in sharpest focus

Measure object distance (u) and image
distance (v)

Repeat and calculate an average value
1 1 1
of f using  
u
v
f

Precautions

Measure v when the cross threads is in
sharpest focus