1. Discover PHYSICS for GCE ‘O’ Level Science
Unit 10 Light
Reflection of Light

2. 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.

3. 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.

4. 10.1 What is Light?
How does light travel?
Light travels in straight lines.

5. 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.

6. 10.1 What is Light? How do we see?
Figure 10.3 shows how our eyes see objects.

7. 10.1 What is Light?
Key Ideas
Light is a form of energy.
Light travels in straight lines called rays.

8. 10.1 What is Light? Test Yourself 10.1 Figure 10.4
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 The light rays are reflected from the apple to the eye.

9. 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. 10.2 Reflection of Light Laws of Reflection: 1st Law of Reflection
The incident ray, the reflected ray and the normal to the reflecting surface all lie in the same plane.
2nd Law of Reflection
The angle of incidence is equal to the angle of reflection
 i =  r

11. 10.2 Reflection of Light
Table 10.1 Characteristics of regular and irregular reflection

12. 10.2 Reflection of Light
Characteristics of the image formed by a
plane mirror:
The image is of the same size as the object.
It undergoes lateral inversion.
It is upright.
It is virtual.
The distance of the image from the mirror is equal to the distance of the object from the mirror.

13. 10.2 Reflection of Light Key Ideas 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
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

14. 10.2 Reflection of Light
Test Yourself 10.2
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: i r
incident ray
reflected ray
normal
mirror
The angle of incidence is equal to the angle of reflection.

15. 10.2 Reflection of Light Test Yourself 10.2
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

16. 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.

17. 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

18. 10.3 Constructing Ray Diagrams
Step 1: Image distance from mirror = object distance from mirror
Measure accurately the perpendicular distance between object O and the mirror surface.
Mark off the same distance behind the mirror to locate image I
Fig 10.16

19. 10.3 Constructing Ray Diagrams
Step 2: Draw the light rays from the image to
the eye
Use dotted lines behind the mirror
Use bold lines in front of the mirror surface.
Fig 10.17

20. 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

21. 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

22. 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

23. 10.3 Applications of Mirrors
Optical testing
Figure Plane mirrors are used to reduce the distance required for optical testing

24. 10.3 Applications of Mirrors
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 A blind corner mirror

25. 10.3 Applications of Mirrors
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 A voltmeter with a mirror below the scale

26. 10.3 Applications of Mirrors
Periscope
Using two plane mirrors inclined at 45°, a periscope allows a person to look over a high wall or obstacles.
Figure A periscope

27. 10.3 Applications of Mirrors
Other uses
Mirrors can be found in many optical instruments such as telescopes, overhead projectors and CD players.

28. 10.3 Constructing Ray Diagrams
Key Ideas
Ray diagrams are used to locate the position of a mirror image.
Some applications of mirrors include the periscope, for observing blind corners and avoiding parallax error on instrument scales.

29. 10.3 Constructing Ray Diagrams
Test Yourself 10.3
Figure shows an extended object. Using ray diagrams, locate the image of the arrow. Describe the image formed.
Answer:

30. 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.
Image
3.0 m
Chart
mirror
Patient’s
eye H
K E
B N A
3.5 m
3.5 m