The Perception of Light and

Sound

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.

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

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

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

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

The amplitude depends on the energy transmitted by a wave.

More energy = greater amplitude

Less energy = lower amplitude

Shall we give it a try?

Wavelength is the length of a wave’s complete cycle.

Symbolized by the Greek letter lamda, λ

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

1.2 – Types of Waves

There are two types of waves:

1. Mechanical Waves

2. Electromagnetic Waves

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)

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.

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

Types of Waves Medium Examples

Mechanical Waves Can only move in a medium

Seismic waves, sound waves, water waves

Electromagnetic Waves

Can travel in a medium or a vacuum.

Radio waves, light waves, ultraviolet waves, x-rays, infrared waves, gamma rays

The Electromagnetic Spectrum

This organizes all electromagnetic waves according to their wavelength and frequency

ED

RANGE

ELLOW

REEN

LUE

NDIGO

IOLET

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

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)

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

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

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

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

2 – Sound Waves

Sound is a longitudinal mechanical wave produced by the vibration of an object and transmitted to the objects environment.

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.

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!!!

2.1 – The Speed of Sound

The speed of sound varies depending on the medium it is travelling in.

Medium Speed (m/s) Speed (km/h)

Air 346 1246

Water 1490 5364

Plastic 1800 6480

Wood 4000 14400

Steel 5200 18720

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.

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

Sound Source Intensity (dB)

Human breathing ( at a distance of 3 m away)

10

Murmuring ( at a distance of 2 m away)

20

Calm classroom 35-40

Soft Music 50

Busy department store 60

Intense road traffic (3 m away) 70

Motorcycle – no muffler (2 m away)

100

Rock music concert 110-120

Jet engine (14 m away) 120

Spacecraft engine (50 m away) 200

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.

Read page 104. Thoughts?

Infra-sounds and Ultra-sounds Sounds under 20 Hz infra-sounds

Sounds higher than 20 000 Hz are ultrasounds

Bats use echolocation

Ultrasonic waves are emitted and their echoes are used to find objects

Sonars used in boats use the same principles

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.

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.

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!!!

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

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

Lab 33 – Answers and thoughts!!!

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.

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.

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

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

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

The image is the same size as the object.

The image is horizontally inverted (left is right and right is left)

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…

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

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

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.

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.

Diverging Lens

The virtual point from which the refracted light rays appear to emanate when the incident rays run parallel

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

Normal VisionThe cornea of an eye is a converging lensMuscles adjust the shape to allow objects to be focused on the retinaThe image formed on the retina is smaller and inverted.The brain analyzes the image

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

Hyperopia- Farsightedness – cannot clearly see objects that are up

close- Caused by the image forming behind the retina- Corrected with a converging lens

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.

Presbyopia

Laser surgery

Laser surgery helps reshape the cornea

Laser surgery can correct myopia and hyperopia

It cannot correct presbyopia