# Waves

Waves transfer energy from one point to another without transferring any matter.

In this chapter you will learn:

1. Types of Waves

2. Properties of waves

3. Reflection of waves

4. Refraction of waves

5. Sound waves

6. Seismic waves

7. Electromagnetic waves

9. Uses and dangers of EM waves

10. Types of lenses

11. Visible light

13. Perfect black bodies and radiation

## Types of Waves

These are two types of waves: transverse and longitudinal waves.

• In transverse waves, the vibrations are perpendicular to the direction of energy transfer.

• In longitudinal waves, the vibrations are parallel to the direction of energy transfer.

## Properties of waves

The amplitude of a wave is the maximum displacement of a point of a wave from its rest position.

Wavelength is the distance between the same point on two adjacent waves from peak to peak, or trough to trough.

Wavelength (λ) is measured in metres (m).

The frequency of a wave is the number of complete cycles passing a given point per second.

The period of a wave is the amount of time taken for one complete cycle of the wave.

The time period of a wave can be calculated using the following equation.

The units used in the equation above are as follows:

• Period (T) is measured in seconds, s

• Frequency (f) is measured in Hertz (Hz).

Question 1

Calculate the time period of a wave with a frequency of 40 Hz.

T = 1 ÷ 40

T = 0.025 s

The speed of a wave can be calculated using the following equation.

Wave speed = frequency × wavelength

The units used in the equation above are as follows:

• Wave speed is measured in metres per second, m/s

• Frequency is measured in Hertz, Hz

• wavelength is measured in metres, m

Question 2

What is the speed of a wave that has a frequency of 40 Hz and a wavelength of 5 m?

Wave speed = frequency × wavelength.

Wave speed = 40 × 6

Wave speed = 240 m/s

## Reflection of waves

Waves can be reflected at the boundary between two different materials.

Waves can be transmitted, reflected or absorbed.

Diffuse reflection is when light hits an object and reflects in lots of different directions.

When the surface is smooth, the light will reflects at the same angel as it hits the surface. This is called specular reflection.

Different materials reflect different amount of light.

## Refraction of waves

When light wave passes from a fast medium to slow medium, it bends. This is called refraction.

Both light waves and water waves are transverse waves.

The wavelength of a wave changes when it is refracted, but the frequency does not change. Frequency is the number of waves that pass a certain point per second.

## Sound waves

Sound waves are produced by vibrating objects.

Sound wave frequency is called pitch. Sound waves are longitudinal waves.

Sound wave needs a medium to travel from one place to another. Sound waves can't travel in space. Sound wave travels very fast in solids.

The auditory range of humans is generally 20-20,000 Hz. Frequency of sound wave is measured in hertz.

Some animals can hear higher frequencies than humans.

Sound wave with a frequency of more than 20,000 Hz, is called ultra sound. Ultrasound can be used in many different ways.

## Seismic waves

Seismic waves are produced by earthquakes in the Earth’s crust.

There are two types of seismic waves: P-waves and S-waves.

• P-waves are longitudinal. They can travel through the earth's core and they travel faster than S-waves.

• S-waves are transverse. They can't travel through the earth's core. They can't travel through liquids or gases. They are slower than P-waves.

## Electromagnetic waves

Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber.

All electromagnetic waves travel at the same velocity through a vacuum (space) or air.

Electromagnetic waves form a continuous spectrum over a range of frequencies.

The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency. In the descending order of their wavelength (or from low to high frequency) the groups are: radio, microwave, infrared, visible light (red to violet), ultraviolet, X rays and gamma rays.

EM radiation can have harmful effects on the human body.

Radio waves are EM (Electromagnetic)waves that have wavelengths longer than about 10 cm. Radio waves travel at the speed of light.

Radio waves can be produced by oscillations in electrical circuits.

The object called transmitter, in which charges oscillate to create the radio waves.

The frequencies of the waves produced will equal the frequencies of the alternating current.

When radio waves are absorbed by a conductor, they create an alternating current. This electrical current has the same frequency as the radio waves.

The combination of transmitter (e.g. radio mast) and receiver (e.g aerial) allows you to encode information onto a radio signal and transmit information from one place to another.

Radio waves are used mainly in communication. Radio waves are used to transmit information from one location to another through air easily.

Bluetooth devices use short wavelength radio waves to communicate and send data between devices.

You can now use high-frequency radio wave scanners to security checks for passengers at airports.

## Uses and dangers of EM waves

• Microwaves are used by satellites.

• Microwaves are used in cooking food.

• Infrared (IR) radiation is used by electrical heaters.

• Visible light is used in optical fibre to carry data over long distances.

• Gama rays and X-rays are used in medicine.

• Radiographers use X-rays and gama rays to treat people with cancer.

• X-ray imaging is also used for scanning the objects in airport security scanners.

• Ultraviolet light in sunlight can cause the skin to tan.

• x-rays and gamma rays can cause the mutation of genes, which can lead to cancer.

• Radiation dose is a measure of the risk of harm caused by exposing the body to ionising radiation.

## Lenses

A lens is generally made from glass. A lens forms an image by refracting light. Lenses can create virtual or real images. Lenses are used in microscopes, cameras, telescopes and binoculars.

There are two types of lens - convex and concave.

## Convex lenses

Convex lenses are thicker at the middle. Convex lenses converge light rays to form an image.

The image produced by a convex lens can be either real or virtual.

In a convex lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length.

## Concave lenses

Concave lenses are thinner in the middle than its edges. Concave lenses diverge light rays to form an image.

Concave lenses can form only virtual images.

## Visible light

Visible light is made up of a range of colour. Colour of visible light depends on its wavelength.

These wavelengths range from violet down at 400 nm up to red at 700 nm.

Different objects absorb, reflect and transmit different wavelength of light in different ways.

In an Opaque object the visible light cannot be transmitted through the material. If the object does not transmit light then some colour wavelengths are absorbed and other colours are reflected.

The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed.

For example a blue opaque object looks blue because all the other colours are absorbed and only blue wavelengths are reflected back into your eye.

The object appears white If all wavelengths are reflected equally . If all wavelengths are absorbed the objects appears black.

Objects that transmit light are either transparent or translucent.

The observed colour of a transparent material depends on which wavelengths are transmitted and reflected by it.

Colour filters work by transmitting only certain colours and absorbing the rest. They are used to filter out different wavelengths of light.

A colour filter absorbs some colours from white light but allows your desired colours to be transmitted.

All objects emit and absorb infrared radiation.

The distribution and intensity of the wavelengths emitted by the object only depends on the temperature of the object.

The hotter the object, the more infrared radiation it radiates in a given time.

Some colours and surfaces absorb and emit radiation better than others.

For example, dark and matt surface is better at absorbing and emitting radiation than a white and shiny surfaces. As they reflect all visible light wavelengths.

## Perfect black bodies and radiation

A perfect black body is an object that absorbs all of the radiation incident on it. It would not reflect or transmit any radiation

Object that are good at absorbing radiation are also a good emitter. So perfect black body would be the best possible emitter of radiation.

Radiation affects the earth temperature. The overall temperature of the Earth depends on many factors including: the rates of absorption and emission of radiation, reflection of radiation into space.

During in daytime a huge amount of EM radiation is transferred to the Earth's surface and atmosphere from the sun and absorbed. This warms up the surface and increases the temperature

At night, when more radiation is emitted than is absorbed, causing a decrease in the local temperature.