Chapter 15: Sound and Light

Section 1: Sound

Section 2: The Nature of Light

Section 3: Reflection and Color

Section 4: Refraction, Lenses, and Prisms

Sound

Key Terms:

Sound Wave

Pitch

Infrasound

Ultrasound

Resonance

Sonar

Sound

Properties of Sound

Sound wave are longitudinal waves, in which particles of air vibrate in the same direction the wave travels. Sound waves have compressions and rarefractions.

Sound waves may be produced differently, but in all cases a vibrating object sets the medium around it in motion.

Sound

The Speed of sound depends on the medium (pg 491 Table 1)

The speed of sound in a particular medium depends on how well the particles can transmit the compressions and rarefractions.

Sound waves travel faster in solids and liquids than through gases.

However, some solids, such as rubber; dampen vibrations so that sound does not travel well. (Soundproofing)

Sound

Loudness is determined by intensity

The loudness of a sound depends partly on the energy contained in the sound wave.

Intensity describes the rate at which a sound wave transmits energy though a given area of the medium. It depends on the amplitude of the sound wave as well as the distance from the source.

Sound

A sound with twice the intensity of another sound does not seem twice as loud. For a sound to seem twice as loud the intensity would have to be 10 times the intensity of another sound.

Relative intensity is found by comparing the intensity of a sound with the intensity of the quietest sound a human can hear; the threshold of hearing. It is measured in decibels (dB)

0 dB Threshold of Hearing 120 dB Threshold of Pain

Sound

Pitch is determined by frequency

The pitch of a sound is related to the frequency of sound waves.

Higher-pitch = higher frequency (Shorter Wavelengths)

Lower-pitch = lower frequency (Longer Wavelenghts)

Sound

Humans hear sound waves in a limited frequency range.

Humans hear sound waves with a frequency between 20 Hz and 20,000 Hz

Below 20 Hz is call infrasound

Above 20,000 Hz is called ultrasound

Sound

Musical Instruments

Musical instruments rely on standing waves

By changing the length of the standing wave the frequency will change.

The primary standing wave has a wavelength that is twice the length of the string or column. This is the fundamental frequency

Sound

Harmonic give every instrument a unique sound

Harmonic is the fundamental frequency and a certain whole-number multiples of that frequency.

Every musical instrument has a characteristic sound quality resulting from the mixture of harmonics.

Sound

Resonance is a phenomenon that occurs when two objects naturally vibrate at the same frequency

These two vibrations are the natural frequency an instrument has plus a forced vibration.

Natural frequency depends on the shape, size, mass, and material an object is make of.

Sound

Hearing and the Ear

The human ear is a very sensitive organ that senses vibrations in the air, amplifies them, and then transmits signals to the brain.

Vibrations pass through three regions of the ear

Outer, Middle, and Inner (pg 496 Figure 7)

Sound

Sound waves pass though the ear canal and strike the eardrum, they cause the eardrum to vibrate. These vibrations pass from the eardrum through the three small bones of the middle ear (hammer, anvil, and stirrup). When the vibrations reach the stirrup, the stirrup strikes a membrane at the opening of the inner ear, sending waves through the cochlea.

Resonance occurs in the inner ear

Sound

The cochlea contains a long, flexible membrane called the basilar membrane. Different parts of the basilar membrane vibrate at different natural frequencies.

A wave of a particular frequency cause only a small portion of the basilar membrane to vibrate.

Sound

anvil - (also called the incus) a tiny bone that passes vibrations from the hammer to the stirrup.

cochlea - a spiral-shaped, fluid-filled inner ear structure; it is lined with cilia (tiny hairs) that move when vibrated and cause a nerve impulse to form.

eardrum - (also called the tympanic membrane) a thin membrane that vibrates when sound waves reach it.

hammer - (also called the malleus) a tiny bone

Sound

nerves - these carry electro-chemical signals from the inner ear (the cochlea) to the brain.

outer ear canal - the tube through which sound travels to the eardrum.

stirrup - (also called the stapes) a tiny, U-shaped bone that passes vibrations from the stirrup to the cochlea. This is the smallest bone in the human body (it is 0.25 to 0.33 cm long).

Sound

Sound

Ultrasound and Sonar

Sonar is used for underwater location

Sonar – Sound Navigation and Ranging – is a system that uses acoustic signals and echo returns to determine the location of objects or to communicate.

Ultrasound is above 20,000 Hz

Sound

Ultrasound imaging is used in medicine

Echoes of very high frequency ultrasound waves, between 1 million and 15 billion Hz are used to produce computerized images called sonograms

Some ultrasound waves are reflected at boundaries

The Nature of Light

Key Terms

Photon

Intensity

Radar

The Nature of Light

Waves and Particles

Two models of Light

1. Waves

2. Steam of Particles

Light produces interference patterns like water waves

The Nature of Light

1801 Thomas Young devised an experiment to test the nature of light. He was able show that light produces a striped pattern similar the pattern caused by water waves.

Light can be modeled as a wave

Young concluded that light must consist of waves.

The Nature of Light

Light waves consist of electric and magnetic fields. Because of this they are called electromagnetic waves.

We can describe transverse waves by amplitude, wavelength, and frequency.

The wave model also explains why light may reflect, refract, or diffract when it meets and object. Light wave can also interfere with one

The Nature of Light

The wave model of light cannot explain some observations

One example is when light strikes a piece of metal electrons may fly off the metal’s surface.

According to the wave model, very bright red light should have more energy then dim blue light because in bright light waves should have

The Nature of Light

Light can be modeled as a stream of particles

Energy from light is contained in small packets. A packet of blue light carries more energy than a packet of red light.

In the particle model of light, these packets are called photons, and a beam of light is considered t be a stream of photons.

The Nature of Light

Photons do not have mass; they are more like little bundles of energy.

The model of light used depends on the situation

The energy of light is proportional to frequency

Higher the frequency more energy

Lower the frequency less energy

The Nature of Light

The speed of light depends on the medium (pg 501 Table 2)

In a vacuum, all light travels at the same speed, called the speed of light 3 x 108 m/s (186,000 mi/s). Light is the fastest signal in the universe.

When light travels through a medium its speed slows down.

The Nature of Light

The brightness of light depends on intensity

Intensity is the rate at which energy flows through a given area of space.

Like the intensity of sound, the intensity of light source decreases as the light spreads out in spherical wave fronts.

The Nature of Light

The Electromagnetic Spectrum

We can detect light from 400nm (violet) to

700nm (red)

This is the visible spectrum and only makes up a

small part of the electromagnetic spectrum

The spectrum consists of light at al possible

energies, frequencies, and wavelengths.

The Nature of Light

Many modern tools take advantage of the different properties of electromagnetic waves. (Radar guns to cancer treatment)

Sunlight contains ultraviolet light (UV)

UV light has higher energy and shorter wavelengths than visible light.

The Nature of Light

X rays and gamma rays are used in medicine

X rays have wavelengths less than 10-8 m and

gamma rays have wavelengths as short as 10-14

m.

Because both x rays and gamma rays have very

high energies, they may kill living cells or turn

them into cancer cells.

The Nature of Light

Infrared light can be felt as warmth (IR)

Microwaves are used in cooking and

communication

Microwave ovens use waves with a frequency of

2450 MHz (12.2 cm wavelenght).

The Nature of Light

Radio waves are used in communications and radar

Radar – Radio Detection and Ranging – a system that uses reflected radio waves to determine the velocity and location of objects.

Reflection and Color

Reflection of Light

A light ray is an imaginary line running in the direction that the light travels.

Rough surfaces reflect light in many directions

The reflection of light off a rough surface is called diffused reflection

Reflection and Color

Smooth surfaces reflect light rays in one direction

The reflected light is reflected off a surface at the same angle that the incoming light struck the surface.

Law of Reflection

The angle of incidence equals the angle of reflection.

Reflection and Color

Mirrors

Flat mirrors create virtual images.

an image that forms at a location from which light rays appear to come but do not actually come. Behind the mirror.

Curved mirrors can distort images

Reflection and Color

Because the surface of a curved mirror is not flat, the line perpendicular to the mirror (the normal) points in many directions.

Mirrors that bulge out are convex mirrors

Mirrors that are indented are concave mirrors.

Concave mirrors can create real images

an image of an object formed by light rays that actually come together at a specific location

Reflection and Color

With a real image, light rays really exist a the point where the image appears; a virtual image appears to exist in a certain place, put there are no light rays there.

Telescopes use curved surfaces to focus light

Reflection and Color

Seeing Color

Objects have color because they reflect certain wavelengths

The color that we see is the color that the object reflects.

If an object appears green it reflects green wavelengths and absorbs all other

Reflection and Color

If an object is place under light without the color it reflects, it will appear black.

Colors may add or subtract to produce other colors

Additive primary colors red, green and blue can produce the secondary colors of yellow, cyan, and magenta. All three mixed together you get white

Subtractive primary colors yellow, cyan, and magenta can produce red, green, and blue. All

Reflection and Color

Black is the absence of color

Refraction, Lenses, and Prisms

Refraction of Light

Light waves bend as they pass from one medium to another. It bends because the speed of light is different in each medium.

Higher Speed to Lower Speed rays bend toward the normal

Lower Speed to Higher Speed rays bend away from the normal

Refraction, Lenses, and Prisms

Refraction makes objects appear to be in different positions.

The misplaced image of the object are virtual images.

Refraction in the atmosphere creates mirages

Because light travels at slightly different speeds in air of different temperatures, light will refract

Refraction, Lenses, and Prisms

Light can be reflected at the boundary between two transparent mediums

In order for this to occur, the angle at which light rays meet the boundary have to be at the correct angle.

This angle is the called the critical angle. This type of reflection is called total internal reflection.

Refraction, Lenses, and Prisms

Fiber optics use total internal reflection

Because fiber-optic cables can carry many different frequencies at once, they transmit computer data or signals for telephone calls more efficiently than standard metal wires.

Refraction, Lenses, and Prisms

Lenses

A lens is a transparent object that refracts light waves such that they converge or diverge to create an image.

Lenses rely on refractions

Light traveling through a flat piece of glass is refracted twice. When it enters then again when

Refraction, Lenses, and Prisms

When light passes through a curved lens, the direction of the light changes.

Converging lens bends light inward (Convex)

create either a real or virtual image depending

on the distance from the lens to the object

Diverging lens bends light outward (Concave)

Refraction, Lenses, and Prisms

Lenses can magnify images

Magnifying glasses are an example of a converging lens

Microscopes and refracting telescopes use multiple lenses

Microscopes use an objective lens first to form a larger real images and the eyepiece lens acts as a magnifying glass and creates an even larger

Refraction, Lenses, and Prisms

The eye depends on refraction and lenses

The cornea and lens refract light onto the retina at the back of the eye.

The retina contains rods and cones.

Rods are more sensitive to dim light, but cannot resolve details very well.

Cones are responsible for color vision, but they

Refraction, Lenses, and Prisms

Dispersion and Prisms

Prism a system that contains two or more plane surfaces of a transparent solid at an angle with each other.

Different colors of light are refracted differently

In the visible spectrum, violet light travels the slowest and red light travels the fastest.

Refraction, Lenses, and Prisms

Because violet light travels slower than red light, violet light refracts more than red light when it passes from one medium to another.

When white light passes through a prism, violet light bends the most and red light the least with the remaining visible colors in between.

Dispersion is the process of separating a wave of different frequencies into its individual

Refraction, Lenses, and Prisms

Rainbow are caused by dispersion and internal reflection

Sunlight is dispersed and internally reflected by water droplets to form a rainbow.

Red light comes from droplets higher in the air and violet light comes from lower droplets.