Discover PHYSICS
for GCE ‘O’ Level Science
Unit 10 Light
Reflection of Light
10.1 What is Light?
Learning Outcomes
In this section, you’ll be able to:
• Understand how light travels and how we are
able to see.
10.1 What is Light?
• Light is a form of energy.
• Our eyes detect light in a range of
colours – red, orange, yellow, green,
blue, indigo and violet.
• Light travels at a speed of
3.0 × 108 m s-1 in vacuum.
10.1 What is Light?
How does light travel?
Light travels in straight lines.
10.1 What is Light?
What are luminous and non-luminous
objects?
• Luminous objects are objects which
give off light of their own
e.g. Lighted lamp, images on a TV
screen
• Non-luminous objects are objects that
do not give off light.
10.1 What is Light?
How do we see?
Figure 10.3 shows how our eyes see objects.
10.1 What is Light?
Key Ideas
1. Light is a form of energy.
2. Light travels in straight lines called
rays.
10.1 What is Light?
Test Yourself 10.1
1. A student drew rays on the picture in Figure 10.4 to
show how the eye sees the apple. Explain why the light
rays are drawn incorrectly.
Figure 10.4
Answer: The direction of light rays should be from the
apple to the eye and not as shown in Fig 10.4. The light
rays are reflected from the apple to the eye.
10.2 Reflection of Light
Learning Outcomes
In this section, you’ll be able to:
• Understand the terms used for reflection:
• Normal
• Angle of incidence
• Angle of reflection
• State that, for reflection, the angle of
incidence is equal to the angle of reflection
• Apply the principle of reflection in constructing
ray diagrams and performing calculations
10.2 Reflection of Light
Laws of Reflection:
1. 1st Law of Reflection
The incident ray, the reflected ray and the
normal to the reflecting surface all lie in the
same plane.
2. 2nd Law of Reflection
The angle of incidence is equal to the angle of
reflection
 i =  r
10.2 Reflection of Light
Table 10.1 Characteristics of regular and irregular reflection
10.2 Reflection of Light
Characteristics of the image formed by a
plane mirror:
1. The image is of the same size as the
object.
2. It undergoes lateral inversion.
3. It is upright.
4. It is virtual.
5. The distance of the image from the
mirror is equal to the distance of the
object from the mirror.
10.2 Reflection of Light
Key Ideas
1. The two laws of reflection are:
• The incident ray, reflected ray and the normal at
a point of incidence all lie in the same plane.
• The angle of incidence is equal to the angle of
reflection
 i =  r
2.
•
•
•
•
•
The image formed by a plane mirror is:
Same size as the object
Laterally inverted
Upright
Virtual
Distance of image from mirror is equal to the
distance of object from the mirror
10.2 Reflection of Light
Test Yourself 10.2
1. Draw a diagram to show clearly the incident ray,
normal, reflected ray, angle of incidence and the
angle of reflection. What is the relationship
between angle of incidence and angle of
reflection.
Answer:
mirror
incident ray
i
r
normal
reflected ray
The angle of incidence is equal
to the angle of reflection.
10.2 Reflection of Light
Test Yourself 10.2
2. What are the characteristics of an image
formed in a plane mirror?
Answer:
The image is
• Upright
• Laterally inverted
• Same size as object
• Virtual
• Distance of image from mirror is equal to the
distance of object from mirror
10.3 Constructing Ray Diagrams
Learning Outcome
In this section, you’ll be able to:
• Apply the principle of reflection in
constructions, measurements and
calculations.
10.3 Constructing Ray Diagrams
Ray Diagrams for plane mirrors
By drawing accurate ray diagrams, we can locate
the position of a mirror image.
Figure 10.15
10.3 Constructing Ray Diagrams
Step 1: Image distance from mirror = object distance
from mirror
i) Measure accurately the perpendicular distance
between object O and the mirror surface.
ii) Mark off the same distance behind the mirror to locate
image I
Fig 10.16
10.3 Constructing Ray Diagrams
Step 2: Draw the light rays from the image to
the eye
i) Use dotted lines behind the mirror
ii) Use bold lines in front of the mirror surface.
Fig 10.17
10.3 Constructing Ray Diagrams
Step 3: Draw the incident rays from object O to
the point of incidence on the mirror surface.
• Note that the angle of incidence = angle of reflection
Fig 10.18
10.3 Constructing Ray Diagrams
Drawing ray diagram for extended object
By considering the extended object as a
number of points, we apply the same 3 steps
to locate the image.
Figure 10.19
10.3 Constructing Ray Diagrams
Multiple images in plane mirrors
When two mirrors are placed 90o to each
other, and an object O is placed between
them, 3 images are formed.
Figure 10.20
10.3 Applications of Mirrors
1. Optical testing
Figure 10.23 Plane mirrors
are used to reduce the
distance required for
optical testing
10.3 Applications of Mirrors
2. Mirrors at blind corners
• Such mirrors can be installed in shops to
allow shopkeepers to watch out for
shoplifters
• Can also be installed at blind corners to allow
drivers to make a turn safely
Figure 10.24 A blind corner mirror
10.3 Applications of Mirrors
3. Instrument Scales
By placing a mirror below the pointer of a scale,
parallax error can be avoided when taking
readings. This is done by ensuring that the image
of the pointer is in line with the pointer itself.
Figure 10.25 A voltmeter with
a mirror below the scale
10.3 Applications of Mirrors
4. Periscope
Using two plane mirrors inclined at 45°, a
periscope allows a person to look over a high
wall or obstacles.
Figure 10.26 A periscope
10.3 Applications of Mirrors
5. Other uses
Mirrors can be found in many optical
instruments such as telescopes, overhead
projectors and CD players.
10.3 Constructing Ray Diagrams
Key Ideas
1. Ray diagrams are used to locate the
position of a mirror image.
2. Some applications of mirrors include
the periscope, for observing blind
corners and avoiding parallax error on
instrument scales.
10.3 Constructing Ray Diagrams
Test Yourself 10.3
1. Figure 10.27 shows an extended object.
Using ray diagrams, locate the image of the
arrow. Describe the image formed.
Answer:
10.3 Constructing Ray Diagrams
Test Yourself 10.3
2. An optician’s eye chart is fixed 0.5 m behind the eyes of
a patient looking into a mirror placed 3.0 m in front of
him. Find the distance of the chart as seen by his eyes.
Answer:
The distance of the eye chart is 3.5 m from the mirror.
Hence the image of the eye chart is 3.5 m in the mirror.
Therefore chart’s image is 6.5 m away from the patient’s
eyes.
3.0 m
Chart
Image
mirror
Patient’s
eye
3.5 m
3.5 m