Aqa Science P2 7.3 Homework Sheet Answers To Guess

P1.5 The use of waves for communication and to provide evidence that the universe is expanding

Electromagnetic radiations travel as waves and move energy from one place to another. They can all travel through a vacuum and do so at the same speed. The waves cover a continuous range of wavelengths called the electromagnetic spectrum.

Sound waves and some mechanical waves are longitudinal, and cannot travel through a vacuum.

Current evidence suggests that the universe is expanding and that matter and space expanded violently and rapidly from a very small initial 'point', ie the universe began with a 'big bang'.

Candidates should use their skills, knowledge and understanding to:

  • compare the use of different types of waves for communication

Additional guidance:

Knowledge and understanding of waves used for communication is limited to sound, light, microwaves, radio waves and infrared waves.

  • evaluate the possible risks involving the use of mobile phones
  • consider the limitations of the model that scientists use to explain how the universe began and why the universe continues to expand.

P1.5.1 General properties of waves

a) Waves transfer energy.

b) Waves may be either transverse or longitudinal.

Additional guidance:

Candidates should understand that in a transverse wave the oscillations are perpendicular to the direction of energy transfer. In a longitudinal wave the oscillations are parallel to the direction of energy transfer.

c) Electromagnetic waves are transverse, sound waves are longitudinal and mechanical waves may be either transverse or longitudinal.

d) All types of electromagnetic waves travel at the same speed through a vacuum (space).

e) Electromagnetic waves form a continuous spectrum.

Additional guidance:

Candidates should know the order of electromagnetic waves within the spectrum, in terms of energy, frequency and wavelength.

Candidates should appreciate that the wavelengths vary from about 10–15 metres to more than 104 metres.

f) Longitudinal waves show areas of compression and rarefaction.

g) Waves can be reflected, refracted and diffracted.

Additional guidance:

Candidates should appreciate that significant diffraction only occurs when the wavelength of the wave is of the same order of magnitude as the size of the gap or obstacle.

h) Waves undergo a change of direction when they are refracted at an interface.

Additional guidance:

Waves are not refracted if travelling along the normal. Snell's law and the reason why waves are refracted are not required.

i) The terms frequency, wavelength and amplitude.

j) All waves obey the wave equation: v = f x λ

Additional guidance:

v is speed in metres per second, m/s

f is frequency in hertz, Hz 

λ is wavelength in metres, m

Candidates are not required to recall the value of the speed of electromagnetic waves through a vacuum.

k) Radio waves, microwaves, infrared and visible light can be used for communication.

Additional guidance:

Candidates will be expected to be familiar with situations in which such waves are typically used and any associated hazards, eg:

  • radio waves – television, and radio (including diffraction effects)
  • microwaves – mobile phones and satellite television
  • infrared – remote controls
  • visible light – photography.

P1.5.2 Reflection

a) The normal is a construction line perpendicular to the reflecting surface at the point of incidence.

b) The angle of incidence is equal to the angle of reflection.

c) The image produced in a plane mirror is virtual.

Additional guidance:

Candidates will be expected to be able to construct ray diagrams.

P1.5.3 Sound

a) Sound waves are longitudinal waves and cause vibrations in a medium, which are detected as sound.

Additional guidance:

Sound is limited to human hearing and no details of the structure of the ear are required.

b) The pitch of a sound is determined by its frequency and loudness by its amplitude.

c) Echoes are reflections of sounds.

P1.5.4 Red-shift

a) If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency. This is known as the Doppler effect.

Additional guidance:

The following should be included:

  • the wave source could be light, sound or microwaves
  • when the source moves away from the observer, the observed wavelength increases and the frequency decreases
  • when the source moves towards the observer, the observed wavelength decreases and the frequency increases.

b) There is an observed increase in the wavelength of light from most distant galaxies. The further away the galaxies are, the faster they are moving, and the bigger the observed increase in wavelength. This effect is called red-shift.

c) How the observed red-shift provides evidence that the universe is expanding and supports the 'Big Bang' theory (that the universe began from a very small initial point).

d) Cosmic microwave background radiation (CMBR) is a form of electromagnetic radiation filling the universe. It comes from radiation that was present shortly after the beginning of the universe.

e) The 'Big Bang' theory is currently the only theory that can explain the existence of CMBR.

Suggested ideas for practical work to develop skills and understanding include the following:

  • reflecting light off a plane mirror at different angles
  • using a class set of skipping ropes to investigate frequency and wavelength
  • demonstrating transverse and longitudinal waves with a slinky spring
  • carrying out refraction investigations using a glass block
  • carrying out investigations using ripple tanks, including the relationship between depth of water and speed of wave
  • investigating the range of Bluetooth or infrared communications between mobile phones and laptops
  • demonstrating the Doppler effect for sound.

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