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Ch1. Reflection & Refraction
Physics & Chemistry Now
3TTO Physics
1.1 What is this chapter about?
• Reflection in mirrors:
– Mirror image point – geometric construction
– Rain droplets
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
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,…
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.
StarsLight bulb…
PlanetsClothes…
• Which are natural and which are artificial?
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?
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.
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.
1.2 What do you still know about light?
• We see things because they reflectlight into our eyes:
Homework
1.2 What do you still know about light?
• ShadowsShadows are places where light is “blocked”:
Rays of light
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.
The colours of the rainbow:
• Red
• Orange
• Yellow
• Green
• Cyan
• Blue
• Violet
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
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
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
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
• In different colours of light this kit would look different:
Red
lightShirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue
Some further examples:
Object Colour of lightColour 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
Using filters• Filters can be used to “block” out different colours of light:
Red Filter
Magenta
(violet) Filter
Investigating filters
Colour of filter Colours that could be “seen”
Red
Green
Blue
Cyan
Magenta (violet)
Yellow
Red
Magenta
White
Yellow
Blue Green
Cyan
1.3 Point and mirror image pointMirror image point A'
mirrorn
orm
al
Point A
L
L’
L
L’
M
M’
O
Field of view
O'
L
mirrorn
orm
al
i
i angle of incidence
t
t angle of reflection
1.4 Two angles
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
1.5 The angle of refractionLight 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.
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.
Three rules for calculating reflection and refraction.
2. The reflected angle is equal to the incident angle.
ir
3. Snell’s Law for refraction
ffii nn sinsin
Three rules for calculating reflection and refraction.
• 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?
• 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
• 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
Let’s review the definition of sine for a minute . . .
21.1sin
30sin42.2sin1
sinsin
f
f
idiamondfair nn
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.
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
Critical angle
• The critical angleof incidence results in a transmitted ray that is parallel to the boundary surface.
Total internal reflection
If the angle of incidence is greater than the critical angle, all the light is reflected and none is transmitted.
1
21sinn
ncritical
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.
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.
Plane parallel plate (flat sheet)
Plane parallel plate (flat sheet)
Prism
Converging (convex) lens
F
Focus F
1.6 Applications of lenses: burning glass
SUN rays
Ray beams are always reversible
F
Focus F
1.6 Applications of lenses: magnifying glass
A ray beam through the optical centre of the lens doesn’tchange direction.
+
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
+
v b
v (voorwerp): object distance
b (beeld): image distance
Magnification (N)
N = Image length/ object lengthN = b/v = Image distance / object distance
+
+
+
+
+
+
+
+
+
+
2f 2f
If v = 2f, then b = v and Magnification (N) = 1
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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!
+
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
+
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
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
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.
Example of a virtual image
Ray tracing
Converging(convex)
lens
Diverging(concave)
lens