1. 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

2. Waves A Wave is a disturbance that travels through a medium.
A wave carries energy
A wave does not transport matter.

3. Transverse Waves The movement or vibration is “up and down”
A transverse wave’s motion is perpendicular to its travel
Travel
Vibration

4. 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

6. Properties of a Wave s c τ ν λ A Property Name Symbol Unit Frequency
Hertz (Hz)
Wavelength λ
Metres (m)
Amplitude A
Decibels (dB)*
Speed
(mechanical waves) s
Metres/sec (m/s)
Speed (of light)
(ElectroMagnetic Radiation) c
km/s
Period τ
Seconds (s)

7. Mechanical Waves
Mechanical waves need a substance or “medium” to travel through.
Examples:
Sound Waves
Seismic Waves
Ocean Waves

8. 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

9. 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

10. 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 km/s

11. 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

12. 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.

13. 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)

14. Infrasound and Ultrasound
Frequencies below 20Hz are called infrasound.
Human’s can’t hear them, but some animals, like elephants, can.
Frequencies above 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.

15. Different light frequencies are interpreted as colours
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

16. 400 THz
700 THz

17. Reflection
Normal
Incident Ray
Reflected Ray
Mirror

18. 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

19. 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

20. Converging, or convex, lenses focus light at a single focal point.
Optical Centre
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,
Focal point
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
Focal point
Optical Centre

21. How to Find the Image Size (Projected Image, Large)
2. Tip of object, parallel to base line
Lens Plane
Base line
3. Tip of object through F’. F’
2a. From lens plane, through Focal point (F)
Object
1. Tip of object through OC
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

22. How to Find the Image Size (Projected Image, small)
2. Tip of object, parallel to base line
1.Tip of object through OC
Lens Plane
2a. through Focal point (F)
3. Tip of object through F’
Base line F’
Upside down
Smaller
Real
Image
Object
3a. Parallel to base line
Converging
(convex)
Lens

23. How to Find the Image Size (Virtual Image, large)
2b. From focal point through lens
2. From lens, parallel to first line
Lens Plane
Base line . F’
Image
Right side up
Larger
Virtual
Object
1. Tip of object through O.C.
Converging
(convex)
Lens

24. 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.

25. 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.

26. Exercises
Workbook, pages 59, 60, 61