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Cambridge IGCSE0625

Electromagnetic spectrum

Physics 0625 Chapter Notes

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Electromagnetic spectrum
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Electromagnetic spectrum notes

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In this chapter

  1. 1The order of the spectrum
  2. 2One speed in a vacuum
  3. 3Uses of the regions
  4. 4Harm from excessive exposure
  5. 5Satellite communication
  6. 6Speed of electromagnetic waves and wavelength
  7. 7Communication systems
  8. 8Analogue and digital signals

1. The order of the spectrum

The main regions, from lowest frequency to highest, are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

Read the other way, the order is increasing wavelength: gamma rays have the shortest wavelength and radio waves the longest. Within visible light, blue has a shorter wavelength than red.

Examiner insight

“Although many candidates recognised that the wavelength of the radiations increases going from γ-radiation towards radio waves, a significant number thought it was the frequency that increased.” “Many candidates correctly identified the property as wavelength, but almost as many thought it was frequency.”

Worked example 11 mark

A planetary nebula emits blue light and red light at two specific wavelengths: 5.0 × 10⁻⁷ m and 6.6 × 10⁻⁷ m.

Explain why the light with a wavelength of 5.0 × 10⁻⁷ m is the blue light.

  1. 1

    blue light has a shorter wavelength (than red light) [1]

2. One speed in a vacuum

All electromagnetic waves travel at the same speed in a vacuum. That speed is high.

The regions differ in frequency and wavelength, not in speed. X-rays, gamma rays and radio waves all travel at the same speed in a vacuum.

Examiner insight

“Some candidates did not know that in a vacuum, radio waves and gamma rays travel at the same speed.”

Worked example 11 mark

Compare the speed of X-rays with the speed of radio waves in a vacuum.

Complete the sentence.

In a vacuum, the speed of X-rays is …

  1. 1

    the same (speed as radio waves) [1]

3. Uses of the regions

Radio waves: radio and television transmissions, astronomy, RFID. Microwaves: satellite television, mobile phones, microwave ovens. Infrared: electric grills, short range remote controllers, intruder alarms, thermal imaging, optical fibres.

Visible light: vision, photography, illumination. Ultraviolet: security marking, detecting fake bank notes, sterilising water.

X-rays: medical scanning, security scanners. Gamma rays: sterilising food and medical equipment, detection of cancer and its treatment.

Examiner insight

“Common misconceptions included suggesting that microwaves sterilise food, optical fibres use ultraviolet and security marking uses infrared.” “Some candidates stated that gamma rays were used in x-rays or were used to identify forged banknotes.”

Worked example 11 mark

Give one use of gamma rays.

  1. 1

    sterilising food / medical equipment, or detection of cancer, or treatment of cancer [1]

4. Harm from excessive exposure

Microwaves cause internal heating of body cells. Infrared causes skin burns.

Ultraviolet damages surface cells and eyes, leading to skin cancer and eye conditions.

X-rays and gamma rays cause mutation or damage to cells in the body.

Examiner insight

“A common error was to confuse the harmful effects of infrared and ultraviolet radiation.” “Some answers were too vague, such as unqualified ‘cancer’ or ‘damaging health’.”

Worked example 11 mark

Gamma radiation has many medical uses.

Describe the possible harmful effects of gamma radiation on people.

  1. 1

    mutation or damage to cells / DNA (in the body) [1]

5. Satellite communication

Communication with artificial satellites is mainly by microwaves.

Some satellite phones use low orbit satellites. Some satellite phones and direct broadcast satellite television use geostationary satellites.

Microwaves travel at the same high speed to both. The path to a geostationary satellite is much longer, so the signal takes longer.

Examiner insight

“Although most candidates recalled the correct electromagnetic wave used to receive and re-transmit television signals, a significant number of candidates thought that low-orbit satellites were used instead of geostationary satellites.” “The most common error was to give an incorrect electromagnetic wave used in signals for satellite television and mobile phones.”

Worked example 11 mark

Which region of the electromagnetic spectrum is used for direct broadcast satellite television?

A gamma rays B infrared C microwaves D ultraviolet

  1. 1

    microwaves: C [1]

6. Speed of electromagnetic waves and wavelength

The speed of electromagnetic waves in a vacuum is 3.0 × 10⁸ m/s. It is approximately the same in air.

Use wave speed = frequency × wavelength to find a wavelength or a frequency.

v = fλ

λ = v/f

f = v/λ

Examiner insight

“Candidates should recall and use this value to two significant figures, since this is how it is given in the syllabus.” “A commonly seen approach was dividing the frequency of the gamma ray by the speed of the gamma ray.”

Worked example 13 marks

The wave speed of a gamma ray is 3.0 × 10⁸ m / s. The frequency of the gamma ray is 5.0 × 10²⁰ Hz.

Calculate the wavelength of the gamma ray.

  1. 1

    v = f × λ [1]

  2. 2

    λ = 3.0 × 10⁸ ÷ 5.0 × 10²⁰ [1]

  3. 3

    λ = 6.0 × 10⁻¹³ m [1]

7. Communication systems

Mobile phones and wireless internet use microwaves. Microwaves can penetrate some walls and only need a short aerial.

Bluetooth uses radio waves. Radio waves pass through walls, but the signal is weakened.

Optical fibres carry cable television and high-speed broadband. Glass is transparent to visible light and some infrared, and these can carry a high rate of data.

Examiner insight

“Candidates were not credited for statements such as ‘electromagnetic waves travel quickly’ or ‘undergo total internal reflection’.” “Stronger candidates stated that radio waves lose energy or that the signal strength decreases when passing through walls.”

8. Analogue and digital signals

An analogue signal varies continuously over a range of values. A digital signal has one of only two possible states.

A sound can be transmitted as a digital signal or as an analogue signal.

Digital signalling gives an increased rate of transmission of data. It also gives an increased range, due to accurate signal regeneration: regeneration removes noise, so the signal can be transmitted over longer distances.

Examiner insight

“Stronger candidates explained that digital refers to the signal having one of only two possible states.” “Common mistakes were poorly drawn diagrams for digital signals that showed more than two states, or inadequate statements about analogue signals being continuous rather than continuously varying or varying over a range of values.”

Recap

  • Order of increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Wavelength is the reverse.
  • All electromagnetic waves travel at the same speed in a vacuum.
  • Uses: gamma sterilises and treats cancer; ultraviolet detects fake bank notes; infrared for remote controllers and thermal imaging.
  • Harm: microwaves heat cells, infrared burns skin, ultraviolet damages surface cells and eyes, X-rays and gamma rays mutate or damage cells.
  • Satellite communication is mainly by microwaves.
  • Extended: speed in a vacuum is 3.0 × 10⁸ m/s, and v = fλ.
  • Extended: a digital signal has only two states. Digital signalling gives a higher rate of data transmission, and a longer range due to accurate signal regeneration.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State which region of the electromagnetic spectrum has the longest wavelength.1 mark
  2. Put these in order of increasing frequency: ultraviolet, microwaves, X-rays, infrared.2 marks
  3. A student says that gamma rays travel faster than radio waves in a vacuum. Decide whether she is correct and explain.2 marks
  4. State the region of the electromagnetic spectrum used for (a) a television remote controller and (b) detecting fake bank notes.2 marks
  5. State the type of electromagnetic radiation used to communicate with satellites. Name one use of a geostationary satellite.2 marks
  6. A hospital sterilises medical equipment with one region of the electromagnetic spectrum. Its canteen cooks food under an electric grill, which uses another region. Name the region used for each. State one harmful effect of excessive exposure to each region.4 marks
  7. A Bluetooth speaker receives radio waves of frequency 2.5 GHz from a phone in the next room. (1 GHz = 1 × 10⁹ Hz.) (a) Calculate the wavelength of the radio waves. (b) State why the signal reaches the speaker, and why it is weaker than in the same room.5 marks
  8. Describe the difference between an analogue signal and a digital signal. State one benefit of transmitting sound as a digital signal.3 marks
  9. A satellite phone call is relayed by a geostationary satellite 36 000 km above the Earth’s surface. The microwaves travel from one phone up to the satellite and down to a second phone. Take both phones to be directly below the satellite. (a) Calculate the time taken for the signal to travel from one phone to the other. (b) The same call is relayed instead by a low orbit satellite 1200 km above the surface. Calculate how many times shorter the time is. (c) A student says the time is shorter because microwaves travel faster to a low orbit satellite. Explain why the student is wrong.7 marks
  10. A television remote controller emits infrared of wavelength 9.4 × 10⁻⁷ m. The television receives a digital signal from a geostationary satellite as microwaves of frequency 12 GHz. (1 GHz = 1 × 10⁹ Hz.) (a) Calculate the frequency of the infrared. (b) Calculate the wavelength of the microwaves. (c) Calculate how many times longer the wavelength of the microwaves is than the wavelength of the infrared. (d) State two benefits of the satellite signal being digital.8 marks

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