2. 1.1 What is this chapter about?
• Reflection in mirrors:
– Mirror image point – geometric construction
– Rain droplets
3. 1.1 What is this chapter about?
• Refraction at a boundary between two
materials (mostly with lenses)
– Refraction angles
– How does it work a magnifying glass and a camera
– Law of image formation in lenses
4. 1.2 What do you still know about
light?
• Natural vs. Artificial sources
• Convergent, divergent and parallel beams
• Light speed vs light year
• Primary colours / spectral colours
• Colour: spectral reflection
• Shadows
• Solar and moon eclipses
• Reflection vs. refraction
• Mirror reflection and glass refraction
• Field of view
• Focal length and lens power
• Lens (converging, diverging) and its images
• Eye accommodation
• Eye illnesses: long-sightedness,…
5. 1.2 What do you still know about
light?
• Luminous and non-luminous objects
A luminous object is one that produces light.
A non-luminous object is one that reflects light.
Stars
Light bulb
…
Planets
Clothes
…
• Which are natural and which are artificial?
6. 1.2 What do you still know about
light?
• Light beams:
– Divergent
– Convergent
– Parallel
• Light year:
– Is the distance (d) the light travels in 1 year:
– d = speed (m/s) x time (s)
– Can you calculate it?
7. 1.2 What do you still know about
light?
• Light travels in straight lines:
Laser
At this speed it can go around the
world 8 times in one second.
• Light travels very fast–
around 300,000 kilometres
per second.
8. 1.2 What do you still know about
light?
• Light travels much faster than sound. For
example:
1) Thunder and lightning
start at the same time,
but we will see the
lightning first.
2) When a starting pistol
is fired we see the
smoke first and then
hear the bang.
9. 1.2 What do you still know about
light?
• We see things because they reflect
light into our eyes:
Homework
10. 1.2 What do you still know about
light?
• Shadows
Shadows are places where light is “blocked”:
Rays of light
11. Colour
• White light is not a single colour; it is made
up of a mixture of the seven colours of the
rainbow.
We can demonstrate this by
splitting (dispersion of) white
light with a prism:
This is how rainbows are
formed: sunlight is “split up”
(dispersed) by raindrops.
12. The colours of the rainbow:
• Red
• Orange
• Yellow
• Green
• Cyan
• Blue
• Violet
13. Adding colours
• White light can be split up to make separate colours.
These colours can be added together again.
• The primary colours of light are red, blue and green:
Adding blue and red
makes magenta
(purple)
Adding blue and green
makes cyan (light blue)
Adding all three
makes white
again
Adding red and
green makes
yellow
14. Seeing colour
• The colour an object appears depends on the colours of
light it reflects.
For example, a red book only reflects red light:
White
light
Only red light is
reflected
15. A white hat would reflect all seven colours:
A pair of purple trousers would reflect purple light (and red and
blue, as purple is made up of red and blue):
Purple light
White
light
16. Using coloured light
• If we look at a coloured object in coloured
light we see something different. For
example, consider a football kit:
White
light
Shorts look blue
Shirt looks red
17. • In different colours of light this kit would look different:
Red
light
Shirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue
18. Some further examples:
Object Colour of light
Colour object
seems to be
Red socks
Red Red
Blue Black
Green Black
Blue teddy
Red Black
Blue
Green
Green camel
Red
Blue
Green
Magenta book
Red
Blue
Green
19. Using filters
• Filters can be used to “block” out different colours of light:
Red Filter
Magenta
(violet)
Filter
27. Clear (regular) vs. Diffuse Reflection
Smooth, shiny surfaces
have a clear (regular)
reflection.
Rough, dull surfaces have a diffuse
reflection.
Diffuse reflection is when light is
scattered in different directions
28. 1.5 The angle of refraction
Light refracts, which means that it bends when passing from one
medium to another. When light enters a more dense medium from one
that is less dense, it bends towards a line normal to the boundary
between the two media.
29. Three rules for calculating reflection
and refraction.
1. All angles are
measured from
the normal. The
normal is the
line
perpendicular to
the surface at
the point of
reflection.
30. Three rules for calculating reflection
and refraction.
2. The reflected angle
is equal to the
incident angle.
ir
31. 3. Snell’s Law for
refraction
ffii nn sinsin
Three rules for calculating reflection
and refraction.
32. • A ray of light strikes the
surface of a beaker of
hydrogen peroxide
(n = 1.414) making a
30o angle with the
surface normal.
• What angle does the
reflected ray make with
the normal?
• What angle does the
transmitted ray make
with the normal?
33. • a) The angle of the reflected ray is the same as the
incident ray, 30o
• b)
7.20
354.0sin
sin414.130sin1
sinsin
f
f
f
fperoxideiair nn
34. • A ray of light inside a
diamond encounters
a boundary between
the stone and air.
• The ray makes a 30o
angle with the
normal. What is the
angles of the
refracted beam?
• Index of refraction of
diamond = 2.42
35. Let’s review the definition of sine for a
minute . . .
21.1sin
30sin42.2sin1
sinsin
f
f
idiamondfair nn
36. Sine function
• The sine of an angle is the ratio between the side of
the triangle opposite the angle and the hypotenuse.
• The hypotenuse is the longest side of the triangle.
• So, the sine of an angle must always be less than 1.
37. Back to the diamond . . .
• Consider a ray of light inside the diamond with
an angle of incidence of 24.4o
• What is the angle of the transmitted ray?
90
1sin
2.24sin42.2sin1
sinsin
f
f
f
idiamondfair nn
38. Critical angle
• The critical angle
of incidence
results in a
transmitted ray
that is parallel to
the boundary
surface.
39. Total internal reflection
If the angle of incidence is greater than the
critical angle, all the light is reflected and
none is transmitted.
1
21
sin
n
n
critical
40.
41. Rainbows
Rainbows are phenomena that involve refraction, dispersion (split up),
and internal reflection. In order to see a rainbow, it is necessary to look
at a portion of the sky containing raindrops with the Sun directly behind
you. White light from the Sun enters the raindrops, and gets refracted
and dispersed inside the raindrop.
42. Maybe Too Much Information
When the dispersed light hits the back of the raindrop it gets internally
reflected, and when it emerges it gets dispersed even more. Because it refracts
more, blue light always emerges from the raindrop above the red light.
Consequently, only one color reaches your eye from any given raindrop. What
color you see depends on the angle at which you look.
In general you must look slightly higher up in the sky to see red light and lower
to see blue light. So you what you see is a band of color in the sky, with red on
top and blue on the bottom, and all the colors of the rainbow in between.
The reason rainbows appear as an arc in the sky is that the colors you see are
determined by the angle that your line of sight makes relative to the position of
the Sun behind your head. As your look along the blue arc of a rainbow, for
example, this angle remains constant.
50. A ray beam through the optical centre of the lens doesn’t
change direction.
51. +
1. A ray beam parallel to the optical axis bends through the focus
2. A ray beam through the optical center of the lens keeps straight
3. A ray beam through the focus bends parallel to the optical axis
52. +
v b
v (voorwerp): object distance
b (beeld): image distance
Magnification (N)
N = Image length/ object length
N = b/v = Image distance / object distance
77. +
When an object is placed on the focus, the ray beams coming from it don’t cross each
other after the lens: they become parallel:
There isn’t an image!
78. +
When an object is placed between the focus and the lens, all the ray beams
look like to come from a point-source before the lens and before the object
This is valid for any ray coming from the object
80. 1.7 Calculations with lenses
• di = distance from lens to image (b,u)
• d0 =distance from lens to object (v)
• f = focal length of lens0
111
ddf i
81. Lens Equation
• f is positive for converging lens
• f is negative for diverging lens
• Negative di is an image on the same side
of the lens as the object
• Positve di is an image on the opposite
side of the lens as the object
0
111
ddf i
82. Real and Virtual Images
• If the light rays actually pass through the point
they appear to come from, the image is real.
• If the light rays are not actually coming from
this position, the image is virtual.
