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Properties of Waves Including Light and Sound

Chapter: Properties of waves including light and sound

Transverse and Longitudinal waves

Transverse waves

The to and fro movements of any object are called oscillations. When the oscillations are up and down or from side to side, the waves are called transverse waves. In transverse waves, the oscillations are at right-angles to the direction of travel. Light waves are transverse waves, although it is electric and magnetic fields which oscillate rather than any material.

Longitudinal waves

When the oscillations are backwards and forwards, the waves are called longitudinal waves. The oscillations are in direction of travel in longitudinal waves. Sound waves are termed to be longitudinal waves.

Describing waves

The following terms are used to describe waves

Speed

The speed of the waves is measured in meters per second (m/s).

Frequency

This is the number of waves passing at any point per second. The SI unit of frequency is the hertz (Hz).

Wavelength

This is the distance between any point on a wave and the equivalent point on the next.

Amplitude

This is the maximum distance a point moves from its rest position when a wave passes.

Doubling the amplitude means that four times as much as energy is delivered per second.

The wave equation

The speed, frequency and wavelength of any set of waves are linked by the following equation:

Speed = frequency   x   wavelength

Wave effects

Reflection

Reflection is defined as the throwing back by a body or surface of light, heat, or sound without absorbing it.

Refraction

Refraction is defined as change in the direction of propagation of any wave when it travels at different speeds at different points along the wave front.

Diffraction

The waves bend round the sides of an obstacle, or extend out as they pass through a gap. The effect is called diffraction.

Sound, light, and radio signals all experience reflection, refraction and diffraction. This proves that they travel as waves.

Sound waves

When a loudspeaker cone vibrates, it moves forward and backward very fast. This compressions and rarefactions travel out through the air. These are sound waves. When they reach your ears, they make your eardrums vibrate and you hear a sound.

Nature of sound waves

  1. Sound waves are caused by vibrations
  2. Sound waves are longitudinal waves
  3. Sound waves need a material to travel through and this material is called a medium.
  4. Sound waves can travel through solids, liquids and gases.

The speed of sound

In air, the speed of sound is about 330 meters per second. The speed of sound depends on the temperature of the air. Sound waves travel faster through hot air than through cold air. The speed of sound does not depend on the pressure of the air. The speed of sound is different through different materials.

Speed of sound = distance travelled / time taken

Echoes

Hard surfaces such as walls reflect sound waves. When you hear an echo, you are hearing a reflected sound a short time after the original sound. From any surface, the sound has to travel to the wall and back again. The time it takes is the echo time. So:

Speed of sound = distance travelled / time taken

The principle of echo is used in several devices as following:

Echo-sounder

This measures the depth of water under a boat. It sends pulses of sound waves towards the sea-bed and measures the echo time. The longer the time, the deeper the water.

Electronic tape-measure

A surveyor can use this to measure the distance two walls. It works like an echo-sounder.

Radar

This uses the echo-sounding principle but with microwaves instead of sound waves. It detects the positions of aircraft by measuring the echo times of microwaves pulses reflected from them.

Ultrasound

The human ear can detect sounds up to a frequency of about 20000 Hz. Sounds above the range of human hearing are called ultrasonic sounds or ultrasound.

Uses of ultrasound:

Cleaning and breaking

Delicate machinery can be cleaned without dismantling it. The machine is immersed in a tank of liquid, and then the vibrations of high-power ultrasound are used to dislodge the bits of dirt and grease.

Echo-sounding

Ships use echo-sounders to measure the depth of water beneath them. An echo-sounder sends pulses of ultrasound downwards towards the sea-bed, and then measures the time taken for each echo to return. The deeper is the water for the longer time.

Metal testing

The echo-sounding principle can be used to detect flaws in metals. A pulse of ultrasound is sent through the metal as on the right. If there is a flaw in the metal, two reflected pulses are picked up by the detector. The pulse reflected from the flaw return first followed by the pulse reflected from the far end of the metal. The pulses can be displayed using an oscilloscope.

Scanning the womb

Echo sounding principle is used for scanning the womb of a pregnant mother. And the result is shown as image on the screen. Using ultrasound is much safer than using X-rays because X-rays can cause cell damage inside a growing baby. Also ultrasound can differentiate between different layers of soft issue which an ordinary X-ray machine cannot.

Light rays and waves

Features of light

Light is a form of radiation

This means that radiates from its source.

Light travels in straight lines

Light transfers energy

Energy is needed to produce light. Materials gain energy when they absorb light.

Light travels as waves

Light radiates from its source rather as ripples spread across the surface of a pond. The ripples are tiny, vibrating, electric and magnetic forces.

Light can travel through empty space

Electric and magnetic ripples do not need a material to travel through. That is why light can reach us from the Sun and stars.

Light is the fastest thing there is

In a vacuum, the speed of light is 300000 kilometers per second. Nothing can travel faster than this. The speed of light seems to be a universal speed limit.

The laws of reflection

When a ray of light strikes a mirror, it is reflected. The incoming ray is the incident ray, the outgoing ray is the reflected ray and the line at right angles to the mirror’s surface is called a normal.

There are two laws of reflection. They apply to all types of mirror:

  1. The angle of incidence is equal to the angle of reflection.
  2. The incident ray, the reflected ray, and the normal all lie in the same plane.

Images in a plane mirror

The image seen in the mirror looks exactly the same as the objects apart from one difference that the image is laterally inverted.

Real and virtual images

In a cinema, the image on the screen is called a real image because rays from the projector focus to form it. The image in a plane mirror is not like this. Although the rays appear to come from behind the mirror, no rays actually pass through the image and it cannot be formed on a screen. An image like this is called a virtual image.

Rules for image size and position

When a plane mirror forms an image:

  1. The object is the same size as the image.
  2. The image is far behind the mirror as the object and image passes through the mirror at right-angles.

Refraction of light

The broken object illusion occurs because light is bent by the glass block. The bending effect is called refraction. Refraction would also occur if the glass were replaced with another transparent material, such as water. But the angle of refraction would be slightly different. The material that light is travelling through is called a medium.

Why light is refracted

Light is made up of tiny waves. These travel more slowly in glass than in air. When a light beam passes from air into glass one side of the beam is slowed before the other. This makes the beam bend.

Refractive index

The refractive index of a medium is defined like this:

Refractive index = speed of light in vacuum / speed of light in medium

The medium with the highest refractive index has the greatest bending effect on light because it slows the light the most.

Refraction by a prism.

A prism is a triangular block of glass or plastic. The sides of prism are not parallel. So when a light is refracted by a prism, it comes out in a different direction. It is deviated. If a narrow beam of white light is passed through a prism, it gets split into a range of colours called a spectrum.  The effect is called dispersion. It occurs because white is not a single colour but a mixture of all the colours of the rainbow. The prism refracts each colour by a different amount.

Total internal reflection

The inside surface of water, glass, or other transparent material can act like a perfect mirror depending on the angle at which the light strikes it. The diagram above shows what happens to three rays leaving an underwater lamp at different angles. Angle c is called critical angle. For angles of incidence greater than this, there is no refracted ray. All the light is reflected. The effect is called total internal reflection.

The value of critical angle depends on the material.

Optical fibres

Optical fibres are very thin, flexible rods made of special glass or transparent plastic. They can carry telephone calls.

Snell’s law

When light is refracted, an increase in the angle of incidence i produces an increase in the angle of refraction r.

In 1620, the Dutch scientist Willebrord Snell discovered the link between the two angles: their sines are always in proportion.

When light passes from one medium into another,

Sin i / sin r is constant.

This is known as Snell’s law.

For a medium of refractive index n: sin c= 1/n.

Lenses

There are two main types of lens. They are:

Convex lens:

A lens may have two spherical surfaces which are bulging outwards. Such a lens is called convex lens. It is thicker at the center than at the edges and has a real focus. It converges a parallel beam of light on refraction through it.

The point where they converge is called the principal focus. Its distance from the center of the lens is called the focal length. A convex lens is known as converging lens.

Concave lenses

A concave lens is bounded by two spherical surfaces, curved inwards. It is thinner at the center than at the edges and has a virtual focus. It diverges a parallel beam of light on refraction through it. A concave lens is a diverging lens.

Ray diagrams

The rules followed by light rays while passing

 

Rule 1.

An incident ray which parallel to the principal axis of a lens, passes (in case of convex lens) or appears to be coming from the focus( in case of concave lens) after refraction.

 

Rule 2.

An incident ray passing through the optical center of a lens (concave or convex) goes straight after refraction.

 

Rule 3.

An incident ray passing through the focus of a convex lens becomes parallel to the principal axes after refraction.

Lens Formula and Magnification

The lens formula gives the relationship between object distance (u), image-distance (v) and the focal length (f).The lens formula is expressed as

Magnification

The magnification produced by a lens is defined as the ratio of the height of the image and the height of the object. It is represented by the letter m.

 

m = h' / h

Where h′ is the height of the image given by a lens

h is the height of the object

The camera

A camera uses a convex lens to form a small, inverted, real image at the back.

The lens

It is moved in or out to make focusing adjustments.

The shutter

It opens and shuts quickly to let a small amount of light into the camera. In some cameras, the speed of the shutter can be adjusted.

The image sensor

It is a light-sensitive microchip which captures the image electronically when the shutter opens.

The diaphragm

It is a set of sliding plates between the lens and the film.

The projector

A projector uses a convex lens to form a large, inverted, real image on a screen. The object is brightly lit piece of film or LCD with a picture on it.

The projection lens

It forms the image on the screen. To get a large image, the lens has to be a long way from the screen. The lens is moved backwards or forwards in its holder to make focusing adjustments.

The film or LCD

It must be upside down to get an upright picture on the screen. As the image is large and distant, the film or LCD must be positioned just outside the principal focus F of the projection lens.

The condenser lens

It concentrates light on the film or LCD so that it is very bright and evenly lit.

The enlarger

An enlarger is used when prints are being made from photographic film. Its job is to produce a printed image that is much larger than the one on the film. It magnifies in a similar way to a projector.

The human eye

The human eye uses a convex lens to form a small, inverted, real image of an object in front of it. It works just like a camera.

The cornea

It is a curved window over the front of the eye. The cornea and the watery liquid behind do most of the converging of the light.

The lens

It is used to make focusing adjustments: the process is called accommodation. The lens does not move backwards and forwards as in a camera. It is flexible and its shape is changed by the ring of ciliary muscles around it.

The iris

It is a bit that makes your eyes brown or blue. Its job is to control the amount of light entering the eye. The light passes through a gap in the middle called the pupil.

The retina

It is the screen on the back of the eye where the image is formed. It contains over 100 million light-sensitive cells. These react to light by sending nerve impulses along the optic nerve to the brain. The brain uses the nerve impulses to form a view of the outside world.

Correcting defects in vision

Short sight

In a short-sighted eye, the lens cannot be made thin enough for looking at distant objects. So the rays are bent inwards too much. They converge before they reach the retina. A concave lens is placed in front of the eye to correct the fault.

Long sight

In a long-sighted eye, the lens cannot be made thick enough for looking at close objects. So the rays are not bent inwards enough. When they reach the retina, they have still not met. A convex lens is placed in front of the eye to correct the fault.

From middle age onwards, the eye lens become less flexible and loses its ability to accommodate for objects at different distances. To overcome this problem, some people wear bifocals. They are the spectacles whose lenses have a top part for looking at distant objects and a bottom part for close ones.

Electromagnetic waves

Light waves belong to a whole family of electromagnetic waves. These have several features in common. They are:

  1. They can travel through a vacuum. They travel with a speed of 300000 kilometers per second.
  2. They are transverse waves. Their oscillations are at right-angles to the direction of travel.
  3. They transfer energy. A source loses energy when it radiates electromagnetic waves and it gains energy when it absorbs them.

The full range of electromagnetic waves is called the electromagnetic spectrum.

Where electromagnetic waves come from

Electromagnetic waves are emitted whenever charged particles oscillate or lose energy in some way. The higher the frequency of oscillation, or the greater the energy changes, the shorter the wavelength of the electromagnetic waves produced.

Infrared radiation and light

When a radiant heater or grill is switched on, you can detect the infrared radiation coming from it by the heating effect it produces in your skin. All objects emit some infrared because of the motion of their atoms or molecules. As an object heats up, it radiates more and more infrared and shorter wavelengths. At about 700 0C, the shortest wavelengths radiated can be detected by the eye. Short-wavelength infrared is often called infrared light, even though it is invisible.

Ultraviolet radiation

Hot objects such as the Sun emit some of their radiation beyond the violet end of the visible spectrum. This is ultraviolet radiation. It is also called ultraviolet light. It is harmful to living cells. It also causes skin cancer if it penetrates into the skin. Ultraviolet is used in some types of sterilizing equipment to kill bacteria.

X-rays

X-rays are given off when fast-moving electrons lose energy very quickly. Short wave-length X-rays are extremely penetrating. X-rays can be used to take photographs that reveal flaws inside metals. Airport security systems also use them to detect any weapons hidden in luggage. All X-rays are dangerous because they damage living cells deep in the body and can cause cancer or genetic changes. However concentrated beams of X-rays can be used to treat cancer by destroying abnormal cells.

Gamma rays

Gamma rays come from radioactive materials. They are produced when the nuclei of unstable atoms break up or lose energy. They tend to have shorter wavelengths than X-rays because the energy changes that produce them are greater.  Gamma rays can be used in the treatment of cancer. They are also used for sterilizing food and medical equipment.

Communications

Telephone, radio and TV are different forms of telecommunications. They are the ways of transmitting information over long distances. The information may be sounds, pictures or computer data. An encoder turns the incoming information into a form which can be transmitted. The signals pass along the transmission path to a decoder. This turns the signals back into useful information.

Analogue and digital transmission

The sound waves entering a microphone make the voltage across it vary. A continuous variation like this is called an analogue signal. Digital signals show signals represented by numbers.

Advantages of digital transmission

Signals lose power as they travel along. This is called attenuation. They are also spoilt by noise. To restore their power and quality, digital pulses can be cleaned up and amplified at different stages by regenerators. Analogue signals can also be amplified.

Storing and retrieving information

When you listen to a recording, the music is being recreated electronically from stored information. Vinyl disc, compact disc and MP3 player are the devices used to store information.