Download Conceptual Physics Chapter 29 Reflection and Refraction

Conceptual Physics Chapter 29
Reflection and Refraction
29.1 Reflection
Wave is totally reflected when it reaches a completely rigid boundary
Comparable boundary with light: a mirror
Wave is partially reflected and partially transmitted when it reaches a flexible boundary
Comparable boundaries with light: glass (4% reflection); water (2% reflection)
29.2 The Law of Reflection
The angle of incidence equals the angle of reflection.
This law holds for all cases of reflection.
Angles are always measured from ray to normal
29.3 Mirrors
The image is
 Located behind the mirror
 A virtual image – no real light rays start here
 As far behind the mirror as the object is in front
 The same size as the object
1
The Law of Reflection also applies to curved mirrors:
a. Convex Mirror
Outwardly curving
Virtual Image smaller &
closer than object
b. Concave Mirror
Inwardly curving
Virtual Image larger and
farther away than object
29.4 Diffuse Reflection
Reflection is from a “bumpy” surface
Incoming parallel Light Rays seem to reflect at a variety of angles
2
Radio waves are very long and “see” this dish as a polished surface and reflect off it:
Microscopic view of paper: light waves “see” the paper as rough and there is diffuse
reflection from paper:
Diffuse reflection from paper
Light reflects at a variety of angles
No image can be formed
Regular reflection from a mirror
Light reflects off in parallel rays
Image can be seen
3
29.5 Reflection and Sound
Echo is reflected sound
Reflection is
 better off a hard smooth surface
 less from soft and irregular surface
Acoustics
The study of sound and its interaction with surfaces
If walls ceiling and floor are
 too reflective, multiple reflections called reverberations make garbled sound
 If not reflective enough then dead and muffled sound
Shiny plates above the stage reflect sound:
4
29.6 Refraction
Lawnmower goes from
 pavement to grass: wheels bend toward the normal
 grass to pavement: wheels bend away from the normal
29.7 Refraction of Sound
Sound waves are refracted when parts of a wave front travel at different speeds.
This happens when
 Uneven winds
 Air layers are of uneven temperature
5
Warm Day:
 Air near ground is warmer than air above
 Sound travels faster in warmer air
 Sound travels faster near the ground, slower above
 Sound wave bends upward
 Sound bending upward from the ground gives the impression that sound does not
carry well
Cool Day or at Night:
 Air near ground cooler than air above
 Warm air above sound travels faster
 Sound wave bends toward ground
 Gives the impression that sound carries well
29.8 Refraction of Light
As light wave passes from air into water, its speed decreases.
The refracted ray is closer to the normal than the incident ray
6
The laser beam bends toward the normal when it enters the water, and away from the
normal when it leaves.
(left) Because of refraction, the apparent depth of the glass block is less than the real
depth
(middle) The fish appears to be nearer than it actually it.
(right) the full glass mug appears to hold more root beer than it actually does.
29.8 Atmospheric Refraction
The refraction of light in air causes a mirage
 Air near ground is warmer than air above
 Creates a refraction effect similar to sound refraction
 Light bends upward, away from the ground
 Image appears as if it were reflecting from water
7
Light refracts through the atmosphere at sunset
 Light bends through layers of atmosphere
 Sun still appears to be visible even after sun has set
Atmospheric refraction causes the pumpkin sun:
8
29.10 Dispersion in a Prism
Dispersion is the separation of light into its frequencies:
Different frequencies of light
 travel at different speeds in transparent materials
 refract differently
 bend at different angles
Isaac Newton used dispersion to produce the spectrum.
29.11 The Rainbow
A rainbow is produced by reflection and by dispersion (refraction) inside raindrops.
How rainbows work:
 The sun is behind you
 The rain is in front of you
 When you see a rainbow, you are seeing light which has refracted inside a
raindrop, reflected from the back of the drop, refracted again
 Violet is bent the most and is on the inside of the curve (bottom)
 Red is bent the least and is on the outside of the curve (top)
9
Rainbows actually extend 360º around
The ground gets in the way so we only see part of the arc
Millions of drops produce the whole spectrum
Only raindrops along the dashed arc disperse red light to the observer on a 42º angle
Double reflections in a drop produce a secondary rainbow
10
29.11 Total Internal Reflection
As light refracts from water to air
 At a certain angle the refracted light no longer reaches the air
 Light is all reflected back into the water
 That angle is called the critical angle
 The critical angle for light passing from water to air = 48º
 There is total internal reflection
The critical angle for glass is 43º
The critical angle for diamond is 24.6º
Smaller than any other known substance
Wide dispersion in a diamond produces flash and color
11
Optical cables (fiber optics) work by internal reflection
Used in communication
How the internal reflections work in optical cables:
Polar bears’ fur are optical cables that carry UV radiation to their skin which is black.
12