Chapter 4The Perception of Light & Sound

Text Book pp. 91 to 116

Related concepts: Waves, Vision, Hearing, Telescopes

UV X RaysIRRadio 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

Vibration

Travel

Longitudinal or Compression Wave

Direction of Travel

Vibration

• The vibration is “forward and backward”– The direction of vibration is the same as the

direction of travel

Properties of a Wave

Property Name Symbol UnitFrequency ν Hertz (Hz)

Wavelength λ Metres (m)

Amplitude A Decibels (dB)*

Speed (mechanical waves) s Metres/sec (m/s)

Speed (of light)(ElectroMagnetic Radiation)

c 300 000 km/s

Period τ Seconds (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

Reflection

Mirror

Nor

malIncident Ray

Reflec

ted R

ay

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.

Air

Water

The spoon looks bentDue to refraction

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

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. 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

Optical Centre

Optical Centre

Focal point

Focal point

How to Find the Image Size(Projected Image, Large)

Converging(convex)

Lens

Lens

Pla

ne

Object1. Tip of object through OC

3. Tip of object through F’.

2. Tip of object, parallel to base line

2a. From lens plane, through Focal point (F) Image

In this case, when the object is close to the lens, the image is larger than the object, and upside down, and real

F’

Base line

How to Find the Image Size(Projected Image, small)

Converging(convex)

Lens

Lens

Pla

ne

Object

1.Tip of object through OC

3. Tip of object through F’

2. Tip of object, parallel to base line

2a. through Focal point (F)Image

3a. Parallel to base line

Base line F’ Upside downSmallerReal

How to Find the Image Size(Virtual Image, large)

Converging(convex)

Lens

Lens

Pla

ne

Object

Base line

1. Tip of object through O.C.

2. From lens, parallel to first line

2b. From focal point through lens

ImageRight side up

LargerVirtual

.F’

Diverging Lens (concave)seen through a diverging lens, the image is smaller than

the object, but right-side up.

Object ImageSmall,Virtual

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