Download Chapter 4 The Perception of Light & Sound

Chapter 4
The Perception of Light & Sound
Text Book pp. 91 to 116
Related concepts: Waves, Vision, Hearing, Telescopes
Radio
IR
UV
X Rays
Gamma
Waves
• A Wave is a disturbance that travels
through a medium.
– A wave carries energy
– A wave does not transport matter.
Transverse Waves
• The movement or vibration is “up and
down”
– A transverse wave’s motion is perpendicular
to its travel
Travel
Vibration
Longitudinal or Compression Wave
• The vibration is “forward and backward”
– The direction of vibration is the same as the
direction of travel
Vibration
Direction of Travel
Properties of a Wave
Property Name
Frequency
Wavelength
Amplitude
Speed
(mechanical waves)
Speed (of light)
(ElectroMagnetic Radiation)
Period
Symbol
Unit
ν
λ
Hertz (Hz)
A
Decibels (dB)*
s
c
Metres/sec (m/s)
τ
Seconds (s)
Metres (m)
300 000 km/s
Mechanical Waves
• Mechanical waves need a substance or
“medium” to travel through.
• Examples:
– Sound Waves
– Seismic Waves
– Ocean Waves
Electromagnetic Waves
• Electromagnetic waves do not need a substance
or medium in order to travel. They may travel
through empty space.
• Examples:
–
–
–
–
–
–
Radio waves
Infrared waves
Visible light
Ultraviolet light
X rays
Gamma rays
EMR Spectrum
• How EMR is organized
– WAVELENGTH
• Longest to shortest across the spectrum
– X-ray, Yellow visible light, purple vis. Light, Radio
waves, UV, Gamma Ray, Microwaves, Infrared
Sound
• Sound is carried by longitudinal,
mechanical waves
• Sound needs air to travel through
– In the vacuum of space, sound will not travel.
• The speed of sound through air is about
340 m/s or over 1000 km/h
– That seems fast, but it is way slower than
light, which travels 299000 km/s
The Decibel Scale
• The decibel scale measures the intensity, or
loudness of sound as perceived by the human
ear.
Sound
Decibels
Breathing, 3 m away
10
Murmur or whisper, 2 m away
20
Calm classroom
40
Intense road traffic, 3 m away
70
Motorcycle, without muffler, 2m away
100
Rock Concert / Jet engine (14m away)
120
Space Shuttle launch (50 m away)
200
Frequency & Wavelength
• Sound has the related properties of
frequency (how fast the vibration that
created the wave was) and wavelength
(how long the waves are)
– Frequency is measured in Hertz (Hz), also
called cycles per second. Its how many
vibrations there were per second.
– Wavelength is measured in metres or
centimetres
– The higher the frequency of a sound, the
shorter its wavelength is.
Frequency and Pitch of Sound
• High frequency sound wave (>1000Hz) have a very
high pitch sound (high treble).
• Low frequency sound waves (<100 Hz) have a low
pitch (bass)
• Mid-range sounds (100Hz to 1000Hz) are the most
comfortable to the ear.
– The mid-point of most musical compositions is C4 or
“middle-C” (262 Hz).
– The octave that contains middle C runs from 220 Hz
(A4) to 440Hz (A5)
• The range of human hearing is said to be:
– 20Hz to 20000Hz (on the average)
Infrasound and Ultrasound
• Frequencies below 20Hz are called
infrasound.
– Human’s can’t hear them, but some animals,
like elephants, can.
• Frequencies above 20000 are called
ultrasound.
– Human’s can’t hear them, but some animals,
like dogs and bats, can
– Bats use ultrasound to locate objects in the
dark (echolocation)
– Hospitals use ultrasound to “see” the
development of a fetus.
Visible Light
• Light is a form of electromagnetic radiation
(EMR) that humans can see with their
eyes.
– The different frequencies of light are
interpreted by our brains as different colours
Different light
frequencies are
interpreted as
colours
400 THz
700 THz
Normal
Reflection
Mirror
Refraction
• Refraction is the deviation (bending) of
light as it passes from one transparent
medium to another.
• For example, light refracts when it passes from air
into glass or water.
The spoon looks bent
Due to refraction
Air
Water
Lenses
• Lenses use refraction to collect, focus or
project light.
• Lenses are made of transparent material (like
glass) and have at least one curved surface.
• The two main types of lens are converging and
diverging.
• All of these devices use lenses:
Contact lens, eye
eyeglasses
telescope
projector
camera
photographic lens
Optical Centre
Focal point
Converging, or convex,
lenses focus light at a
single focal point.
Converging lenses can be used to
create “real” images… Images that
can be projected on a screen.
Converging lenses are often used
to enlarge or magnify images,
Diverging, or concave,
lenses do not focus light at
a single point.
Focal point
Optical Centre
The so-called focal point of a
diverging lens is a virtual point where
the image would appear to be when
viewed through the lens.
Diverging lenses cannot project
images onto a screen. Diverging
lenses usually make things look
small
How to Find the Image Size
(Projected Image, Large)
Lens Plane
2. Tip of object, parallel to base line
Object
Base line
F’
Image
Converging
(convex)
Lens
In this case, when the object is close to the
lens, the image is larger than the object, and
upside down, and real
How to Find the Image Size
(Projected Image, small)
Lens Plane
2. Tip of object, parallel to base line
Base line
F’
Image
Object
3a. Parallel to base line
Converging
(convex)
Lens
Upside down
Smaller
Real
How to Find the Image Size
(Virtual Image, large)
Lens Plane
2. From lens, parallel to first line
.
F’
Image
Object
Right side up
Larger
Virtual
Converging
(convex)
Lens
Base line
Object
Image
Small,
Virtual
Right side up
Diverging Lens (concave)
seen through a diverging lens, the image is smaller than
the object, but right-side up.
Glasses
• Myopia: Near-sighted
people need diverging
lenses in their glasses.
• Near sighted people can see
well close up, but not far away
• Hyperopia: far-sighted
need converging lenses in
their glasses
• Far-sighted people see things
far away well, but cannot see
close things well.
Exercises
• Workbook, pages 59, 60, 61