Contents are appropriate for IGCSE Physics

1. LIGHT

2. Light : What is Light?
 Light carries energy and travels
as a wave.
 Light travels at 300 000 000
m/s or 300 000 km/s (much faster
than sound).
 Light waves travel in straight
lines.
 Light waves undergo reflection,
refraction diffraction and
interference.
 Light waves are transverse

3. Reflection

4. Reflection
• Laws of Reflection
– The angle of incidence
equals the angle of
reflection [ <i = < r ]
- The incident and
reflected rays and the
normal lie in the same
plane.
Reflection: the bouncing back of light as it strikes a barrier (mirror).

5. Definitions:
1. Incident ray (i): the ray of light that strikes the
mirror (barrier)
2. Normal (N): a line drawn perpendicular to the
mirror drawn at the point where the incident ray
strikes the mirror.
3. Angle of incidence (<i): the angle between the
incident ray and Normal.
4. Reflected ray (r): the ray of light leaving the mirror
surface.
5. Angle of reflection (<r): the angle between the
reflected ray and the Normal.

6. 1. Draw a line and label it ‘mirror’.
2. Draw a dot on top of the line and label it as ‘O’
3. Draw a line (incident ray) from the ‘O’ to the
mirror and label it i1.
4. Draw a ‘Normal’ perpendicular to the point where
i1 meets the mirror and label it N1.
5. Use a protractor to measure the angle of incidence
(<i1).
6. Use the formula: angle of incidence = angle of
reflection, and draw the reflected ray (r1).
7. Label the angle of reflection as <r1
Reflection: Image location by construction

7. 8. Repeat the steps 3 to 7 for i2, <i2, N2, <r2 and r2.
9. Extend r1 and r2 backwards as dotted lines. They
meet at the Image location.
10. Measure the distance from the object to the
mirror.
11. Measure the distance from the image to the
mirror.
(draw diagram on the board)
Question:
What is your conclusion?

8. CONCLUSION
When a plane mirror forms an image:
1. The image is the same size as the object.
2. The image is as far behind the mirror as the
object is in front.
3. A line joining equivalent points on the object
and image passes through the mirror at right
angles.

9. REFRACTION

10. Refraction : Bending light
 The speed of light waves depends on the material they are travelling
through.
 If the light waves enter a different material [e.g. from glass into air]
the speed changes.
 This causes the light to bend [or refract].
Air = Fastest Diamond = slowestGlass = slower
Glass
Refraction: the bending of light as it moves from one medium to another due to
change in wave speed.

11. Refraction
• In both cases the speed of the wave has
decreased. This is indicated by the decrease in
wavelength!

12. Refraction : Investigating Refraction
Angle i
Angle r

13. Definitions:
1. Incident ray (i): the ray of light that strikes the
boundary.
2. Normal (N): a line drawn perpendicular to the
boundary drawn at the point where the incident
ray strikes the boundary.
3. Angle of incidence (<i): the angle between the
incident ray and Normal.
4. Refracted ray (r): the ray of light in the other
medium.
5. Angle of refraction (<r): the angle between the
refracted ray and the Normal.

14. Speed of light
• v is the speed of light in the
new medium.
• c= 3.0 x 108 m/s
• As the index increases the
speed decrease.
n is the absolute index of
refraction, Refractive index.
This is a measure of optical
density. n is defined as the
ratio of the speed of light in
a vacuum to the speed of
light in a new medium.

15. Refractive index
Medium Refractive index
Diamond 2.42
Glass (crown) 1.52
Acrylic plastic (Perspex) 1.49
Water 1.33
Exercise:
Calculate the speed of light in;
(a) Diamond
(b) Glass (crown)
(c) Acrylic plastic (Perspex)
(d) Water

16. -As light moves from one medium to another, it
undergoes refraction.
-The refractive index of a denser medium is greater than
that of a less dense medium.
-When light moves from a denser medium to a less
dense medium, it bends away from the normal.
-When light moves from a less dense medium to a more
dense medium, it bends towards the normal.

17. SNELL’S LAW
• In 1620, Willebrord Snell the Dutch scientist
discovered the link between the two angles: their
sines are always in proportion.
• When light passes from one medium into another:
sin i is a constant
sin r
That is:
refractive index = sin i
sin r

18. Example
1. A ray of light strikes a material at 35o and is
refracted at 15o. What is the refractive index of the
material?
2. In an experiment, it was discovered that light
travels at 1.9 x 1o8m/s in a certain material.
Calculate
a. The refractive index of the material
b. The angle of refraction given the angle of incidence as 38o.

19. Exercise
1. In an experiment, it was discovered that light travels
at 1.7 x 1o8m/s in a certain material. Calculate
a. The refractive index of the material
b. The angle of refraction given the angle of incidence as 38o.
2. Calculate the angle of refraction if light (in air) strikes
water at an angle of incidence of 23o.
3. What is the angle of incidence when light is refracted
at 13o in a Diamond block?

20. Refraction: Determining the refractive index of glass
1. Place a glass block on an A4 paper and
draw around it.
2. Place 2 pins on one side of the glass
block and label them p1 and p2.
3. Place 2 more pins on the other side of
the block such that they make a straight
line with the images of p1 and p2 in the
block. Label them p3 and p4.
4. Remove the glass block and the pins.
5. Draw a line passing through p1 and p2
up to the upper bounder of the block.
6. Draw another line passing through p4
and p3 up to the lower boundary of the
block.
7. Join the upper line to the lower line.
8. From the upper line, draw a normal and
measure the angles of incidence and
refraction.
9. Calculate the refractive index of the
glass block.
x p1
x p2
x p3
x p4

21. Refraction : Summary
 When light bends this is called refraction.
 Refraction happens because the light changes speed [or
velocity].
 When light enters a more dense medium [e.g. glass], it
bends towards the normal.
 When light enters a less dense medium [e.g. air], it
bends away from the normal.
 If the incident ray hits a surface at 0º (light passing through
the normal), no refraction occurs.
 Remember that the angle of reflection [r] and angle of
refraction [r] have the same symbol.
In reflection, <i = <r
In refraction, <i  <r

22. Consequences of Refraction of light
1. Apparent depth
R – Real Depth
A – Apparent Depth
If the chest is 20m below
the surface, calculate its
apparent depth.

23. If the chest is 20m below
the surface, calculate its
apparent depth.

24. Dispersion of light using a glass prism
• A prism is a triangular block of glass or plastic.
• The sides of a prism are not parallel.
• When light is refracted by a prism, it comes out in a
different direction. It is Deviated.
• If a narrow beam of white light is passed through a
prism, it splits into a range of colours called a
Spectrum. This effect is called Dispersion of light.

25. • The spectrum of white light is a continuous change of
colour from beginning to end.
• The spectrum has Red, Orange, Yellow, Green, Blue,
Indigo, Violet (ROY-G-BIV)
• In the white light spectrum,
– Red light is deviated (bent off-course) the least. It is
refracted the least.
– Violet is refracted the most.