Waves Revision Workbook

Top Revision Tips

• Find a quiet place to work (no TV, Xbox, Netflix ect…)
• Use your class book, revision guides, textbooks, Doddle, BBC Bitesize, Youtube, to make revision notes, flash cards & concept maps
• Summarise information using bullet points & diagrams
• Try to revise with another student, and explain concepts to each other
• Put up posters with key points around your home
• Look at & work through past papers from www.AQA.org.uk
• Take regular breaks & get enough sleep

Contents

• Wave characteristics
• Transverse & longitudinal waves
• Frequency
• Waves required practical
• Reflection
• Refraction
• Refraction required practical
• Sound & hearing
• Ultrasound
• EM
• Lenses
• Infrared radiation

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Waves Revision

Waves are one of the ways in which energy may be transferred between stores. Waves can be described as oscillations, or vibrations about a rest position. For example:

• sound waves cause air particles to vibrate back and forth
• ripples cause water particles to vibrate up and down

The direction of these oscillations is the difference between longitudinal or transverse waves. In longitudinal waves, the vibrations are parallel to the direction of wave travel. In transverse waves, the vibrations are at right angles to the direction of wave travel.

Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a medium to travel through. Electromagnetic waves cause oscillations in electrical and magnetic fields.

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Draw a wave with a low amplitude & high frequency

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Draw a wave with a high amplitude & low frequency

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Waves Revision

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Longitudinal waves

Sound waves and waves in a stretched spring are longitudinal waves. P waves (relatively fast moving longitudinal seismic waves that travel through liquids and solids) are also longitudinal waves. In longitudinal waves, the vibrations are along the same direction as the direction of travel.

Compare transverse & longitudinal waves

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This is when:

• wave speed (v) is measured in metres per second (m/s)
• frequency (f) is measured in Hertz (Hz)
• wavelength (λ) is measured in metres (m)

Rearrange the equation above to give frequency

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Rearrange the equation above to give wave length

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Example

What is the wave speed when the frequency is 20Hz with a wave length of 3m?

Wave speed = Frequency X Wave length

Wave speed = 20Hz X 3m

= 60m/s

What is the wave speed when the frequency is 70Hz with a wave length of 10m?

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What is the wave speed when the frequency is 66Hz with a wave length of 11m?

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What is the wave speed when the frequency is 6Hz with a wave length of 3m?

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Give examples of both transverse and longitudinal waves.

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Waves Revision

A ripple tank can be used to measure and calculate frequency, wavelength and the speed of waves on the surface of the water. A ripple tank is a transparent shallow tray of water with a light shining down through it onto a white card below in order to clearly see the motion of the ripples created on the water’s surface. Ripples can be made by hand but to generate regular ripples it is better to use a motor.

Method

1. Set up the ripple tank as shown in the diagram with about 5 cm depth of water.
2. Adjust the height of the wooden rod so that it just touches the surface of the water.
3. Switch on the lamp and motor and adjust until low frequency waves can be clearly observed.
4. Measure the length of a number of waves then divide by the number of waves to record wavelength. It may be more practical to take a photograph of the card with the ruler and take measurements from the still picture.
5. Count the number of waves passing a point in ten seconds then divide by ten to record frequency.
6. Calculate the speed of the waves using: wave speed = frequency × wavelength.

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For the method to the left fill out the table above

1. Attach a string or cord to a vibration generator and use a 200 gram (g) hanging mass and pulley to pull the string taut as shown in the diagram. Place a wooden bridge under the string near the pulley.
2. Switch on the vibration generator and adjust the wooden bridge until stationary waves can be clearly observed.
3. Measure the length of as many half wavelengths (loops) as possible, divide by the number of half wavelengths (loops). This is half the wavelength, doubling this gives the wavelength.
4. The frequency is the frequency of the power supply.
5. Calculate the speed of the waves using: wave speed = frequency × wavelength.

Waves Revision

Waves - including sound and light - can be reflected at the boundary between two different materials. The reflection of sound causes echoes.

The law of reflection states that:

angle of incidence = angle of reflection

For example, if a light ray hits a surface at 32°, it will be reflected at 32°.

The angles of incidence and reflection are measured between the light ray and the normal - an imaginary line at 90° to the surface. The diagrams show a water wave being reflected at a barrier, and a light ray being reflected at a plane mirror.

Specular reflection

Reflection from a smooth, flat surface is called specular reflection. This is the type of reflection that happens with a flat mirror. The image in a mirror is:

• upright
• virtual

In a virtual image, the rays appear to diverge from behind the mirror, so the image appears to come from behind the mirror.

Carefully draw the path of the ray which is reflected from both mirrors. Draw an arrow on the ray to show the direction of the light.

Incident ray

Waves Revision

Refraction of waves

Different materials have different densities. Light waves may change direction at the boundary between two transparent materials. Refraction is the change in direction of a wave at such a boundary.

The density of a material affects the speed that a wave will be transmitted through it. In general, the denser the transparent material, the more slowly light travels through it.

Glass is denser than air, so a light ray passing from air into glass slows down. If the ray meets the boundary at an angle to the normal, it bends towards the normal.

The reverse is also true. A light ray speeds up as it passes from glass into air, and bends away from the normal by the same angle.

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Refraction Required Practicle

1. Set up a ray box, slit and lens so that a narrow ray of light is produced.
2. Place a 30 centimetre (cm) ruler near the middle of a piece of plain A3 paper. Draw a straight line parallel to its longer sides. Use a protractor to draw a second line at right angles to this line. Label this line with an ‘N’ for ‘normal’.
3. Place the longest side of a rectangular acrylic polymer block against the first line. With the normal near the middle of the block, carefully draw around the block without moving it.
4. Use the ray box to shine a ray of light at the point where the normal meets the block. This is the incident ray.
5. The angle between the normal and the incident ray is called the angle of incidence. Move the ray box or paper to change the angle of incidence. The aim is to see a clear ray reflected from the surface of the block and another clear ray leaving the opposite face of the block.
6. Using a pencil on the paper, mark the path of:
7. the incident ray with a cross
8. the reflected ray with a cross
9. the ray that leaves the block with two crosses - one near the block and the other further away
10. Remove the block. Join the crosses to show the paths of the light rays.
11. Repeat steps 2 to 7 for a rectangular glass block.
12. Measure the angle of incidence, angle of refraction and angle of reflection for each block.

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The data given in the table below was obtained from an investigation into the refraction of light at an air to glass boundary.

Describe an investigation a student could complete in order to obtain similar data to that given in the table above.

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Waves Revision

Sound waves are longitudinal waves. They cause particles to vibrate parallel to the direction of wave travel. The vibrations can travel through solids, liquids or gases. The speed of sound depends on the medium through which it is travelling. When travelling through air, the speed of sound is about 330 metres per second (m/s). Sound cannot travel through a vacuum because there are no particles to carry the vibrations.

Describe the range of human hearing

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Describe how sound is converted into a nerve impulse

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Waves Revision

Ultrasound

Ultrasound waves have a frequency higher than the upper limit for human hearing - above 20,000 Hertz (Hz). Different species of animal have different hearing ranges. This explains why a dog can hear the ultrasound produced by a dog whistle but humans cannot.

Uses of ultrasound

Uses of ultrasound include:

• breaking kidney stones
• cleaning jewellery

In both of these applications, the vibrations caused by the ultrasound shake apart the dirt or kidney stones, breaking them up. The principle is the same as the opera singer's trick, where a glass may shatter if the singer makes a high-pitched sound near to the glass.

How is an image of a foetus built up in an ultrasound scan?

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Echo sounding

High frequency sound waves can be used to detect objects in deep water and to measure water depth. The time between a pulse of sound being transmitted and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object. The process is very similar to ultrasound imaging. However, the sound waves used are within normal hearing range, and they are used to identify objects rather than internal structures.

This technique is applied in sonar systems used to find shipwrecks, submarines and shoals of fish. Bats and dolphins use a similar method called 'echolocation' to detect their surroundings and to find food.

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Reflections

When ultrasound waves reach a boundary between two substances with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time between an ultrasound wave after leaving the source to reach the detector. The further away the boundary, the longer the time between leaving the source and reaching the detector:

distance (metre, m) = speed (metre/second, m/s) × time (second, s)

For example, sound travels through water at about 1,400 m/s. If it takes 0.5 s for a sound to reach a boundary and reflect back to the detector, the total distance travelled is:

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distance = speed × time

= 1,400 × 0.50

= 700m

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The distance to the boundary is half this, which is 350m.

Waves Revision

The electromagnetic spectrum

Visible light is just one type of electromagnetic radiation: there are various types of electromagnetic radiation with longer wavelengths of light than red light and with shorter wavelengths than violet light. All the different types of electromagnetic waves travel at the same speed through space.

Waves Revision

Convex lenses

A convex lens is thicker in the middle than it is at the edges. Parallel light rays that enter the lens converge. They come together at a point called the principal focus.

In a ray diagram, a convex lens is drawn as a vertical line with outward facing arrows to indicate the shape of the lens. The distance from the lens to the principal focus is called the focal length.

Focal length

Concave lenses

A concave lens is thinner in the middle than it is at the edges. This causes parallel rays to diverge. They separate but appear to come from a principle focus on the other side of the lens.

In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.

To draw a ray diagram for a convex lens:

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Draw a ray from the object to the lens that is parallel to the principal axis. Once through the lens, the ray should pass through the focal point.

Draw a ray which passes from the object through the centre of the lens.

A magnifying glass is a convex lens used to make an object appear much larger than it actually is. This works when the object is placed at a distance less than the focal length. The image is:

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• upright
• magnified
• virtual

Waves Revision

For an object viewed through a concave lens, light rays from the top of the object will be refracted and will diverge on the other side of the lens. These rays will appear:

• from the same side of the principal axis meaning the image will be upright
• further from the principal axis, so the image will be larger than the object

Complete the diagram below for a convex lens

Complete the diagram below for a magnifying lens

Complete the diagram below for a concave lens

Example

An object that is 2 cm tall forms an image 250 cm tall. Calculate the magnification.

250 / 2 = 125

An object 3 mm tall forms an image 12 mm tall. Calculate the magnification.

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An object 6 mm tall forms an image 12 mm tall. Calculate the magnification.

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An object 9 mm tall forms an image 18 mm tall. Calculate the magnification.

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An object 45 mm tall forms an image 180 mm tall. Calculate the magnification.

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Waves Revision

Absorption of light

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Waves can be absorbed at the boundary between two different materials. When waves are absorbed by a surface, the energy of the wave is transferred to the particles in the surface. This will usually increase the internal energy of the particles.

When white light shines on an opaque object, some wavelengths or colours of light are absorbed. These wavelengths are not detected by our eyes. The other wavelengths are reflected, and these are detected by our eyes.

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For example, grass appears green in white light:

• red, orange, yellow, blue, indigo and violet are absorbed by the grass
• green light is reflected by the grass and detected by our eyes

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Transmission of light

Waves can also be transmitted at the boundary between two different materials. When waves are transmitted, the wave continues through the material. Air, glass and water are common materials that are very good at transmitting light. They are transparent because light is transmitted with very little absorption. Translucent materials transmit some light but are not completely clear. Lamp shades, shower curtains and window blinds are often translucent objects.

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Colour filters

When white light passes through a coloured filter, all colours are absorbed except for the colour of the filter. For example, an orange filter transmits orange light but absorbs all the other colours. If white light is shone on an orange filter, only the orange wavelengths will be observed by the human eye.

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Method

1. Place a Leslie cube on a heat-resistant mat. Fill it, almost to the top, with boiling water and replace the lid.
2. Leave for one minute. This is to enable the surfaces to heat up to the temperature of the water.
3. Use the infrared detector to measure the intensity of infrared radiation emitted from each surface, or the temperature of the surface. Make sure that the detector is the same distance from each surface for each reading.

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