good for IGCSE physics

1. LIGHT
by: Chris Lembalemba
(chrislembalemba@yahoo.com)

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
Reflection: the bouncing back of light as it strikes a barrier (mirror).
• Law 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.

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. Reflection: Image location using a plane mirror
1. Fix a plane mirror along the
centre of a piece of A4 paper
and draw around it.
Place a pin as the object in front
of the mirror.
2. Line up a ruler with the image of
the pin and draw along the edge of
the ruler on the paper. Repeat for 3
more positions of the ruler.
3. Remove the mirror and ruler.
Where the lines cross is the image
position.
I
Image
Object
What is the distance between the mirror and object?
What is the distance between the mirror and image?
What is your conclusion?

7. Reflection: Image location by construction
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

8. 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?

9. 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.

10. Diffuse vs Specular
Reflection
• Diffuse Reflection
– Light incident upon a
rough surface
– Law of reflection still
holds; Normals are not
parallel.
• Specular Reflection
– Mirror like reflection
– All Normals are parallel

11. REFRACTION

12. Refraction : Bending light
Refraction: the bending of light as it moves from one medium to another due to
change in wave speed.
 The speed of light waves depends on the material they are travelling
through.
Air = Fastest Glass = slower Diamond = slowest
 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].
Glass

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

14. Refraction : Investigating Refraction
Angle i
Angle r

15. 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.

16. 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.

17. 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

19. 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

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, 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

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

24. Exercise
1. Calculate the angle of refraction if light (in air)
strikes water at an angle of incidence of:
a. 24
b. 53
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.

25. CRITICAL ANGLE
• Critical angle is an angle of incidence that gives an
angle of refraction = 90o.
ic
ic = critical angle

26. To find the critical angle, we use:
n = sin 90
sin ic
= 1 .
sin ic
Or ic = sin-1 (1/n)
Exercise:
Calculate the critical angle of
a. Diamond
b. Crown glass
c. Perspex plastic
d. Water

27. INTERNAL REFLECTION
• Whenever light moves
from medium A (water)
to medium B (air), there
is a weak ray of light
that is reflected back
into medium A. This
phenomenon is called
Internal Reflection.

28. TOTAL INTERNAL REFLECTION
• When the angle of
incidence is greater
than the critical angler,
light undergoes Total
Internal Reflection.

29. APPLICATIONS OF TOTAL INTERNAL REFLECTION
1. Periscope
- light incident at 0o to
the normal is not
refracted.
- in a periscope, light is
incident at 0o to the
normal (i.e through the
normal) on the glass
blocks.
- the refracted ray
passes through the
normal of the other side
of the block.

32. Reflecting prisms
a
b
c
i. ii.
-Light entering the prism on side ‘a’ is at 0o
(i.e. through the normal) is not refracted.
-The ray of light reaches side ‘b’ at an
angle greater than the critical angle.
- The ray of light is totally internally
reflected.
-The ray of light leaves the prism through
the normal.

33. Optical fibres
• An optical fiber is a
flexible, transparent
fiber made of glass or
plastic, slightly thicker
than a human hair.
• It can function as a
waveguide, or “light
pipe” to transmit light
between the two ends
of the fiber.
1. Optical fibres
1. Engineering
2. Endoscope

34. THANK YOU