Electromagnetic Spectrum

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Electromagnetic Waves

The electromagnetic spectrum is a continuous range of waves arranged by wavelength or frequency.
Longer wavelength → lower frequency; shorter wavelength → higher frequency.
Order from longest to shortest wavelength: radio waves, microwaves, infrared, visible light (ROYGBIV), ultraviolet, X-rays, gamma rays.
All EM waves are transverse waves that transfer energy.
All EM waves travel at the same speed in a vacuum: 3.0 × 10⁸ m/s (also approximately the speed in air).
Visible light is only a small part of the EM spectrum.

The electromagnetic spectrum, showing the seven regions in order of decreasing wavelength (increasing frequency), from radio waves to gamma rays.

Uses of Electromagnetic Waves

Radio waves: radio/TV transmissions, astronomy, RFID.
Microwaves: satellite TV, mobile phones, microwave ovens, WiFi.
Infrared: remote controls, thermal imaging, optical fibres, intruder alarms.
Visible light: vision, photography, illumination.
Ultraviolet: security marking, detecting fake bank notes, sterilising water, fluorescence.
X-rays: medical scanning, security scanners (absorbed by bone, pass through tissue).
Gamma rays: sterilising food/equipment, cancer detection and treatment.

Dangers of Electromagnetic Waves

Higher frequency EM waves carry more energy and can ionise atoms (UV, X-rays, gamma rays).
Microwaves: internal heating of body cells (water absorption).
Infrared: skin burns.
Ultraviolet: skin cancer, eye damage; protection via sunscreen and sunglasses.
X-rays and gamma rays: cell mutation, DNA damage, cancer; radiographers use lead screens.
Ionising radiation risk increases with frequency and dose.

Communications

Microwaves are used for satellite communication (mobile phones, satellite TV) because they are not absorbed/reflected by the atmosphere.
Radio waves diffract around hills (similar wavelength to obstacles), used for local radio and Bluetooth.
Optical fibres use visible light or infrared for high-speed broadband; high frequency allows high data rates.
Geostationary satellites: orbit at 36 000 km, period 24 h, used for broadcasting; longer time delay.
Low‑orbit/polar satellites: orbit at ~200 km, shorter delay, used for weather and imaging.

A satellite in geostationary orbit above the Earth's equator, showing communication signals to other parts of the world.

Digital & Analogue Signals (Extended)

Analogue signals vary continuously and can take any value.
Digital signals have only two states (0/1, high/low).
Digital signals can be regenerated with minimal noise, giving larger range and higher data rate.
Extra data can be added to digital signals for error checking.
Digital sampling converts an analogue signal to digital by recording at set time intervals.

A digital signal, which can only take one of two states (0 or 1).

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练习题

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1.Which of the following lists the electromagnetic spectrum in order of decreasing wavelength?
Easy
- ARadio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays
- BGamma rays, X-rays, ultraviolet, visible, infrared, microwaves, radio waves
- CRadio waves, infrared, microwaves, visible, ultraviolet, gamma rays, X-rays
- DVisible, infrared, radio waves, microwaves, ultraviolet, X-rays, gamma rays
2.All electromagnetic waves travel at the same speed in a vacuum. What is this speed?
Easy
- A3.0 × 10⁸ m/s
- B3.0 × 10⁶ m/s
- C3.0 × 10¹⁰ m/s
- D3.0 × 10³ m/s
3.Which type of electromagnetic radiation is used for television remote controls?
Easy
- AInfrared
- BRadio waves
- CUltraviolet
- DMicrowaves
4.Which of the following is a harmful effect of excessive exposure to ultraviolet radiation?
Easy
- ASkin cancer
- BInternal heating of body cells
- CSkin burns
- DMutation of DNA
5.Which type of electromagnetic radiation has a wavelength of about 1 km?
Easy
- ARadio waves
- BMicrowaves
- CInfrared
- DVisible light
6.A geostationary satellite orbits at a height of 36 000 km above the Earth's surface. What is its orbital period?
Easy
- A24 hours
- B12 hours
- C48 hours
- D72 hours
7.Which property increases as you move from radio waves to gamma rays in the electromagnetic spectrum?
Easy
- AFrequency
- BWavelength
- CSpeed in a vacuum
- DAmplitude
8.Which type of signal can only take one of two discrete values?
Easy
- ADigital signal
- BAnalogue signal
- CRadio signal
- DMicrowave signal

更多题目

9.A student observes that radio signals can be received clearly in a valley surrounded by hills, but television signals cannot. Which property of waves explains this?
Medium
- ADiffraction
- BRefraction
- CReflection
- DAbsorption
10.Which of the following is a benefit of digital signalling over analogue signalling?
Easy
- AThe signal can be regenerated with minimal noise
- BDigital signals have a shorter range
- CDigital signals cannot be checked for errors
- DDigital signals have a lower data transmission rate
11.A radiographer stands behind a lead screen during X-ray procedures. Why is lead used?
Easy
- ALead absorbs a lot of X-ray radiation
- BLead reflects X-rays away from the radiographer
- CLead emits X-rays that cancel out harmful radiation
- DLead is a good conductor of electricity
12.Which type of electromagnetic radiation is used for sterilising medical equipment?
Easy
- AGamma rays
- BX-rays
- CUltraviolet
- DInfrared
13.A student is using a mobile phone to make a call. Which type of electromagnetic radiation is used to transmit the signal between the phone and the nearest mast?
Easy
- ARadio waves
- BMicrowaves
- CInfrared
- DVisible light
14.A geostationary satellite orbits at a height of 36 000 km above the Earth's surface. A signal is sent from Earth to the satellite and back. Given that all electromagnetic waves travel at 3.0 × 10⁸ m/s in a vacuum, what is the approximate time delay for the signal to make this round trip?
Medium
- A0.12 s
- B0.24 s
- C0.48 s
- D0.72 s
15.An X-ray machine uses a beam of X-rays to produce an image of a patient's bones. Which property of X-rays makes them suitable for this purpose?
Easy
- AThey have a long wavelength.
- BThey are absorbed by bone more than by soft tissue.
- CThey are reflected by bone.
- DThey travel at a lower speed in bone.
16.A technician uses an optical fibre of refractive index 1.3. What is the critical angle for the glass? (sin c = 1/n)
Medium
- A35.3°
- B40.5°
- C50.3°
- D55.6°
17.A student looks at an oscilloscope trace that shows a signal that varies continuously between a maximum and minimum value. What type of signal is this?
Easy
- ADigital signal
- BAnalogue signal
- CBinary signal
- DPulse signal
18.A polar satellite orbits at 200 km above sea level. A geostationary satellite orbits at 36 000 km. Which statement about the time delay for signals is correct?
Easy
- AThe polar satellite has a longer time delay because it is closer.
- BThe geostationary satellite has a longer time delay because it is farther.
- CBoth have the same time delay because they use the same type of wave.
- DThe time delay depends only on the frequency of the wave.
19.Which of the following lists the electromagnetic spectrum in order of increasing frequency?
Easy
- ARadio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays
- BGamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves
- CRadio waves, infrared, microwaves, visible light, ultraviolet, X-rays, gamma rays
- DVisible light, infrared, radio waves, microwaves, ultraviolet, X-rays, gamma rays
20.Which type of electromagnetic radiation has the highest frequency?
Easy
- ARadio waves
- BInfrared
- CUltraviolet
- DGamma rays
21.A technician uses an optical fibre made of glass with refractive index 1.5. What is the critical angle of the glass? (Use sin c = 1/n)
Medium
- A41.8°
- B48.6°
- C33.7°
- D56.4°
22.Which of the following is a correct use of ultraviolet radiation?
Easy
- Asterilising water
- Bheating food in a microwave oven
- Ctransmitting television signals from a satellite
- Dimaging bones in a hospital
23.A geostationary satellite orbits at a height of 36 000 km above the Earth's surface. A signal is sent from Earth to the satellite and back. Given that all electromagnetic waves travel at 3.0 × 10⁸ m/s in a vacuum, what is the approximate time delay for the signal to travel from Earth to the satellite?
Medium
- A0.12 s
- B0.24 s
- C0.48 s
- D1.2 s
24.Which type of electromagnetic radiation has the lowest frequency?
Easy
- Aradio waves
- Bmicrowaves
- Cinfrared
- Dvisible light
25.Which of the following statements about electromagnetic waves is correct?
Easy
- AAll electromagnetic waves travel at the same speed in a vacuum.
- BElectromagnetic waves are longitudinal waves.
- CElectromagnetic waves require a medium to travel through.
- DThe speed of electromagnetic waves in air is less than in a vacuum.
26.All electromagnetic waves travel at the same speed in a vacuum.
Easy
True or false?
27.Excessive exposure to which type of electromagnetic radiation is most likely to cause skin cancer?
Easy
- AUltraviolet
- BInfrared
- CMicrowaves
- DRadio waves
28.Gamma rays have a longer wavelength than radio waves.
Easy
True or false?
29.Which of the following statements about digital and analogue signals is correct?
Easy
- ADigital signals can take any value, while analogue signals have only two discrete values.
- BAnalogue signals vary continuously, while digital signals take only two discrete values.
- CDigital signals are used only for sound transmission, while analogue signals are used for video.
- DAnalogue signals have a higher rate of data transmission than digital signals.
30.Which of the following are uses of gamma rays? (Select all that apply.)
Easy
- ASterilising food
- BSterilising medical equipment
- CTreatment of cancer
- DSecurity marking
31.Match each type of electromagnetic wave to its primary use.
Easy
- Radio waves
- Microwaves
- Infrared
- Radio and television transmissions
- Satellite television
- Thermal imaging
32.Arrange the following regions of the electromagnetic spectrum in order of increasing frequency (lowest frequency first).
Easy
- Visible light
- X-rays
- Microwaves
- Gamma rays
33.Arrange the following types of electromagnetic radiation in order of increasing frequency (lowest frequency first).
Easy
- Ultraviolet
- Microwaves
- Gamma rays
- Visible light
34.A student uses a microwave oven to heat food. Which statement explains why microwaves can heat the food?
Medium
- AMicrowaves are absorbed by water molecules in the food, causing them to vibrate and produce heat.
- BMicrowaves are reflected by the metal walls of the oven, concentrating the energy on the food.
- CMicrowaves ionise the atoms in the food, releasing heat energy.
- DMicrowaves have a very high frequency, which directly heats the food.
35.Complete the sentence.
Easy
The electromagnetic spectrum is arranged in order of decreasing ____ and increasing ____.
36.What is the speed of all electromagnetic waves in a vacuum? Give your answer in m/s.
Easy
37.State one benefit of digital signalling over analogue signalling.
Easy
38.A geostationary satellite orbits at a height of 36 000 km above the Earth's surface. Calculate the time taken for a microwave signal to travel from Earth to the satellite. (Speed of electromagnetic waves in vacuum = 3.0 × 10⁸ m/s.)
Medium
39.Complete the sentence about the electromagnetic spectrum.
Easy
As the frequency of electromagnetic waves increases, their wavelength ____. (Fill in the blank with 'increases' or 'decreases')
40.What is the speed of all electromagnetic waves in a vacuum? Give your answer in m/s.
Easy
- A360000000 m/s
- B300000000 m/s
- C330000000 m/s
- D270000000 m/s
41.A geostationary satellite orbits at a height of 36 000 km above the Earth's surface. Calculate the time taken for a microwave signal to travel from Earth to the satellite. (Speed of electromagnetic waves in vacuum = 3.0 × 10⁸ m/s.)
Medium
- A-0.88 s
- B2.12 s
- C0.12 s
- D1.12 s

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