Class notes

1. The Perception
of Light and
Sound

2. 1 – What is a wave?

A wave is a disturbance that travels through a medium.

A wave transports energy.

A wave does not transport matter.

What is a disturbance?

A localized and temporary change in the properties of a
particular environment.

3. 1.1 – Wave Characteristics

There are 3 characteristics of waves:
1.
How the waves propagate (travel)
2.
The amplitude of the wave
3.
The frequency of the wave

4. How waves propagate:
Transverse and Longitudinal

A transverse wave is a wave that propagates perpendicular to
the motion its medium.

There are crests and troughs

5. 
A longitudinal wave is a wave that propagates parallel to the
motion of its medium

The particles move closer together and then farther apart BUT
always parallel to the wave’s motion.
Close together = area of compression
 Farther apart = area of rarefaction


6. 
The amplitude (A) of a wave corresponds to the maximum
distance travelled by a particle in the medium compared to its
position at equilibrium.

Amplitude of a transverse wave:

Amplitude of a longitudinal wave:
Smaller amplitude because it
is not as compressed as in the
second spring

7. 
The amplitude depends on the energy transmitted by a wave.

More energy = greater amplitude

Less energy = lower amplitude

Shall we give it a try?

8. 
Wavelength is the length of a wave’s complete cycle.

Symbolized by the Greek letter lamda, λ

9. 
The frequency is the number of cycles per unit time.

How fast the waves go by!

Frequency is measured in Hertz, Hz.

A Hertz is the number of cycles per second.
1 Hz = 1 full cycle per second

10. 1.2 – Types of Waves

There are two types of waves:
1.
Mechanical Waves
2.
Electromagnetic Waves

11. 
A mechanical wave is a wave that requires a medium in order
to propagate.

The medium may be a solid, a liquid or a gas.

Mechanical waves cannot travel in a vacuum.

Examples:

Waves travel in water

Sound waves travel in air (gas)

Seismic waves travel in the earth (solid)

12. 
Mechanical waves are caused by localized disturbances.

The disturbance changes the physical properties of the
medium; such as amplitude and frequency.

The changes are transmitted to the neighboring particles.

13. 
An electromagnetic wave is a wave that can travel through
both vacuum and a medium.

Light from the Sun is electromagnetic.

Travels through space and the atmosphere

The speed of electromagnetic waves depends on the medium it
is travelling in.

The speed of light in a vacuum is
300 000 km/s OR 300 000 000 m/s

14. Types of Waves
Mechanical Waves
Electromagnetic
Waves
Medium
Can only move in a
medium
Can travel in a
medium or a vacuum.
Examples
Seismic waves, sound
waves, water waves
Radio waves, light
waves, ultraviolet
waves, x-rays,
infrared waves,
gamma rays

15. The Electromagnetic Spectrum

This organizes all electromagnetic waves according to their
wavelength and frequency

17. E
D
R
A
N
G
E
E
L
L
O
W
R
E
E
N
L
U
E
N
D
I
G
O
I
O
L
E
T

18. Radio waves

The smallest frequency and the longest wavelength

They are invisible and transport little energy.

Microwaves are radio waves with a higher frequency which
makes certain particles vibrate and raises their temperature.

Radio, television

Magnetic resonance imaging (MRI)

Microwave ovens, cell phones, radar

19. Infrared

They are invisible but we feel the warmth they
emit

Heat waves!!!!

Infrared thermography, night vision goggles,
space observation satellites, short distance
communication (remote controls, optical
scanners, optical computer mouse, wireless
keyboard)

20. Visible Light

Can be seen by humans

ROYGBIV

Each color has its own wavelength

All of these colors together make up white light

Lighting, laser technology, photography, cinema, computer screens,
microscopes, telescopes

21. Ultraviolet Rays

UV rays are invisible to humans but can be seen by some
animals.

The greater frequency means more energy is transported by UV
light.

Cause suntans and can cause cancer

Our body needs UV rays to produce vitamin D

Treatment of rickets and jaundice, sterilization of medical
equipment

22. X-Rays

High frequency

Transmit a large quantity of energy!!!

Allow us to see through many opaque objects

Used to examine bones!!

Prolonged exposure can cause cancer and burns.

Radiography, baggage inspection, study of crystalline
substances

23. Gamma Rays

Travel very easily through matter.

1 cm lead, 6 cm concrete or 9 cm of earth only block 50% of UV
rays.

Can cause burns, cancer and genetic mutations.

Cancer treatment

Food preservation

24. 2 – Sound Waves

Sound is a longitudinal mechanical wave produced by the
vibration of an object and transmitted to the objects
environment.

25. 
Anything that creates sound produces a longitudinal
mechanical wave!!!!

Our larynx has folds on tissues that act like guitar strings.

They vibrate with the passage of air, creating sounds.

We only here sounds when the sound waves contact the
eardrum.

The eardrum vibrates in rhythm with the sound waves.

26. 
The eardrum vibrates near the fluid filled cochlea.

The cochlea has many receptor cells that capture the impulses.

The auditory nerve transmits this information to the brain
where it is analyzed.

Sound travels through solids and liquids, as well.

The vibrations can be felt!!!

27. 2.1 – The Speed of Sound

The speed of sound varies depending on the medium it is
travelling in.
Medium
Air
Speed (m/s)
346
Speed (km/h)
1246
Water
Plastic
Wood
Steel
1490
1800
4000
5200
5364
6480
14400
18720

28. 2.2 – The Decibel Scale

The volume of a sound depends on the energy it is transmitting.

Greater energy = louder

Greater amplitude = louder

The decibel scale is by factors of 10.

29. 
Example:

When a sound increases by 10 dB, it is 10 times louder.

A sound measuring 20dB is 100 times louder than a 0 dB sound

(10 x 10)

A 40 dB sound is 100 times louder than a 20 dB sound

Prolonged exposure to 100 dB sounds can lead to permanent
hearing damage.

Sounds 120dB and above can cause pain and immediate
damage

30. Sound Source
Intensity (dB)
Human breathing ( at a distance of
3 m away)
Murmuring ( at a distance of 2 m
away)
Calm classroom
Soft Music
Busy department store
Intense road traffic (3 m away)
Motorcycle – no muffler (2 m
away)
Rock music concert
Jet engine (14 m away)
Spacecraft engine (50 m away)
10
20
35-40
50
60
70
100
110-120
120
200

31. 2.3 – Frequency and the Perception of
Sound

Sound can also have different tones – higher and lower pitch
(frequency).

The tone of a sound depends on the frequency

The average humans can hear sounds with frequencies ranging
from 20 to 20 000 Hz.

32. Read page 104. Thoughts?

33. Infra-sounds and Ultra-sounds

Sounds under 20 Hz infra-sounds

Sounds higher than 20 000 Hz are ultrasounds

34. 
Bats use echolocation

Ultrasonic waves are emitted and their echoes are used to find
objects

Sonars used in boats use the same principles

35. 3 – Light Waves

Light is an electromagnetic wave that is visible to the human
eye.

Light travels in a straight line – hence, light rays!!

Light rays are reflected, refracted or absorbed when they strike
an object.

36. 3.1 - Reflection

This occurs when a light ray travels through one medium and
rebounds when it strikes another medium.

Air  mirror (simple example)  air

All objects reflect light to varying degrees.

37. Something to think about:

Black objects absorb all colors of light

White objects reflect all colors of light.

The actual color we see is the colors that are reflected!!

Colors are not usually pure.

An object has the main color and a bit of the colors on each
side of the main color!!!

38. Some terms to know:

Incident ray – the ray that contacts the
surface of an object

Reflected ray – the ray that rebounds
from the object.

The Normal – a line perpendicular to the
surface at the point of reflection.

Angle of Incidence – the angle formed by
the incidence ray and the normal.

Angle of Reflection – the angle formed
by the reflected ray and the normal

39. Lab Time!!!

Lab 33 – Image in a Plane mirror
Some terms to know:

A virtual image is an image that cannot be formed on a screen

A real image can be formed on a screen.

Inverted - An image in which up and down, as well as left and right, are
interchanged.

When an object is laterally inverted, the left and right are interchanged.

If the object is “upside-down” it is dorsally inverted

40. Lab 33 – Answers and thoughts!!!

41. What we learned from our lab on plane
mirrors!!!
 The
angle of incidence is always equal to the angle of
reflection.
 The
incidence ray and the reflected ray are always on
the same plane.

42. 
There are two types of reflection:

Diffuse reflection which occurs when objects have a rough surface

What happens?

Parallel light rays hit a rough surface and the rays are reflected in all
directions.

43. 
Specular reflection occurs when light rays hit a smooth surface.

When parallel light rays contact the smooth surface, the
reflected rays are parallel.

Produces a mirror image

45. Things to remember about images in plane mirrors:

The image appears to be behind the mirror, at a distance equal
to the distance between the mirror and the object

46. 
The image is virtual, the image cannot be captured on a screen.

47. 
The image is the same size as the object.

The image is horizontally inverted (left is right and right is left)

48. Application of plane mirrors:

The uses of plane mirrors are based on these two principles:
1.
They change the trajectory of light rays by reflection
2.
They increase the observer’s field of vision

Uses:

Rear view mirrors, mirrors used by dentists, mirrors in cameras,
microscopes…

49. 3.2 - Refraction

The deviation of a light ray as it passes from one transparent
medium to another.

When light bends as it passes from one medium to another

50. 3.3 - Lenses

Lenses use the principle of refraction.

Lenses are made of transparent material and have at least one
curved surface.

There are two types of lenses

Converging lenses bring the light rays together as the light
passes through the lens.

Diverging lenses spread the light rays aart as the light passes
through the lens

51. 
The optical center (OC) is the center of the lens.

The principal axis is the straight line that runs perpendicular to
the surface of the lens and through the OC.

52. Focal Point of a Lens

Converging lenses

The focal length of a converging lens is the real point where the
refracted rays actually meet when the incident rays run parallel.

53. 
Diverging Lens

The virtual point from which the refracted light rays appear to
emanate when the incident rays run parallel

55. 
Images obtained with a diverging lens are always the same, no
matter where the object was placed in relation to the lens.

The image is always:
virtual
not inverted (right side up)
smaller than the object

56. Normal Vision
The cornea of an eye is a converging lens
Muscles adjust the shape to allow objects to be focused on the retina
The image formed on the retina is smaller and inverted.
The brain analyzes the image

57. Myopia
- Nearsightedness – cannot clearly see objects that are at a
distance
- Caused by the image forming in front of the retina
- Corrected with a diverging lens

58. Hyperopia
- Farsightedness – cannot clearly see objects that are up
close
- Caused by the image forming behind the retina
- Corrected with a converging lens

59. Presbyopia

Presbyopia is caused by the aging of the lens ‘ muscles

The lens can no longer adjust its shape to clearly see objects
close up – no longer as elastic

The image is formed behind the retina.

Corrected by a converging lens.

60. Laser surgery
 Laser
surgery helps reshape the cornea
 Laser
surgery can correct myopia and hyperopia
 It
cannot correct presbyopia