The electromagnetic spectrum and risk

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課程筆記

Big idea: the electromagnetic spectrum and risk

Key concept: Relationships. All electromagnetic waves travel at the same speed in a vacuum. Higher frequency means shorter wavelength and greater photon energy.
Related concepts: Models and evidence. Use a scientific explanation to make predictions, then test it against observations.
Global context: Scientific and technical innovation. Different spectrum regions support communication, imaging and treatment.

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.
Radiation risk depends on radiation type, absorbed dose, exposure and the tissue affected. Greater electromagnetic frequency increases photon energy but does not by itself determine risk.

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

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).

Think like a scientist

Analyse teacher-provided frequency and wavelength values for electromagnetic waves in a vacuum.
Comparison: the frequency of the radiation. Outcome: the corresponding wavelength.
Control: compare propagation in a vacuum. Explain why this makes the comparison fairer.
Evidence: Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause.
Safety: Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it.
Inquiry task: State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction.

Evaluate the science

Different spectrum regions support communication, imaging and treatment.
Risk depends on type, dose and exposure; non-ionising radiation can still cause harm through heating or other interactions.
Evaluation task: Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

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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

複習筆記

## Big idea: the electromagnetic spectrum and risk - **Key concept: Relationships.** All electromagnetic waves travel at the same speed in a vacuum. Higher frequency means shorter wavelength and greater photon energy. - **Related concepts: Models and evidence.** Use a scientific explanation to make predictions, then test it against observations. - **Global context: Scientific and technical innovation.** Different spectrum regions support communication, imaging and treatment. ## 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. ## 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. - Radiation risk depends on radiation type, absorbed dose, exposure and the tissue affected. Greater electromagnetic frequency increases photon energy but does not by itself determine risk. ## 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. ## Digital & Analogue Signals - **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. ## Think like a scientist - Analyse teacher-provided frequency and wavelength values for electromagnetic waves in a vacuum. - **Comparison:** the frequency of the radiation. **Outcome:** the corresponding wavelength. - **Control:** compare propagation in a vacuum. Explain why this makes the comparison fairer. - **Evidence:** Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause. - **Safety:** Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it. - **Inquiry task:** State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction. ## Evaluate the science - Different spectrum regions support communication, imaging and treatment. - Risk depends on type, dose and exposure; non-ionising radiation can still cause harm through heating or other interactions. - **Evaluation task:** Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Analogue & Digital Signals: Distinguish between analogue and digital signals and state advantages of digital signalling.
EM Spectrum Order: Recall the order of the electromagnetic spectrum by wavelength and frequency, from radio waves to gamma rays.
EM Wave Dangers: Explain the harmful effects of excessive exposure to electromagnetic waves, particularly ionising radiation.
Satellite Communications: Compare geostationary and low-orbit satellites and explain their use in communications with microwaves.
EM Wave Uses: Describe typical uses of each region of the electromagnetic spectrum, including communication, medicine, and security.
EM Wave Properties: State that all electromagnetic waves are transverse, travel at the same speed in a vacuum, and transfer energy.
Designing scientific inquiries: Plan a fair comparison: Analyse teacher-provided frequency and wavelength values for electromagnetic waves in a vacuum.
Evaluating evidence and impacts: Different spectrum regions support communication, imaging and treatment. Risk depends on type, dose and exposure; non-ionising radiation can still cause harm through heating or other interactions.
Electromagnetic Waves: The **electromagnetic spectrum** is a continuous range of waves arranged by **wavelength** or **frequency**.
Uses of Electromagnetic Waves: **Radio waves**: radio/TV transmissions, astronomy, RFID.
Dangers of Electromagnetic Waves: Higher frequency EM waves carry more energy and can **ionise** atoms (UV, X-rays, gamma rays).
Communications: **Microwaves** are used for satellite communication (mobile phones, satellite TV) because they are not absorbed/reflected by the atmosphere.
Digital & Analogue Signals: **Analogue signals** vary continuously and can take any value.
electromagnetic spectrum: The **electromagnetic spectrum** is a continuous range of waves arranged by **wavelength** or **frequency**.
wavelength: The **electromagnetic spectrum** is a continuous range of waves arranged by **wavelength** or **frequency**.
frequency: The **electromagnetic spectrum** is a continuous range of waves arranged by **wavelength** or **frequency**.

字卡

Which of the following lists the electromagnetic spectrum in order of decreasing wavelength?

Radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays The electromagnetic spectrum is arranged from longest to shortest wavelength: radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays. Option A is correct.

All electromagnetic waves travel at the same speed in a vacuum. What is this speed?

3.0 × 10⁸ m/s The speed of all electromagnetic waves in a vacuum is 3.0 × 10⁸ m/s. Option A is correct.

Which type of electromagnetic radiation is used for television remote controls?

Infrared Television remote controllers use infrared radiation. Option A is correct.

Which of the following is a harmful effect of excessive exposure to ultraviolet radiation?

Skin cancer Ultraviolet radiation can cause skin cancer and eye conditions. Internal heating is from microwaves, skin burns from infrared, and DNA mutation from X-rays/gamma rays. Option A is correct.

Which type of electromagnetic radiation has a wavelength of about 1 km?

Radio waves Radio waves have wavelengths of around a kilometre. Option A is correct.

A geostationary satellite orbits at a height of 36 000 km above the Earth's surface. What is its orbital period?

24 hours Geostationary satellites have an orbital period of 24 hours. Option A is correct.

Which property increases as you move from radio waves to gamma rays in the electromagnetic spectrum?

Frequency As wavelength decreases from radio to gamma, frequency increases. Speed in a vacuum is constant. Option A is correct.

Which type of signal can only take one of two discrete values?

Digital signal A digital signal can only take two values (e.g., 0 or 1). An analogue signal varies continuously. Option A is correct.

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?

Diffraction Radio waves have longer wavelengths (about 1 km) similar to the size of hills, so they diffract around obstacles. TV signals have shorter wavelengths and diffract less. Option A is correct.

Which of the following is a benefit of digital signalling over analogue signalling?

The signal can be regenerated with minimal noise Digital signals can be regenerated accurately, reducing noise and increasing range. They also allow error checking and higher data rates. Option A is correct.

A radiographer stands behind a lead screen during X-ray procedures. Why is lead used?

Lead absorbs a lot of X-ray radiation Lead absorbs a lot of radiation, protecting the radiographer from exposure. Option A is correct.

Which type of electromagnetic radiation is used for sterilising medical equipment?

Gamma rays Gamma rays are used to sterilise food and medical equipment by killing bacteria. Option A is correct.

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?

Microwaves Mobile phones use microwaves for communication.

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?

0.24 s Distance = 2 × 36 000 km = 72 000 km = 7.2 × 10⁷ m. Time = distance / speed = (7.2 × 10⁷) / (3.0 × 10⁸) = 0.24 s.

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?

They are absorbed by bone more than by soft tissue. X-rays are absorbed by denser materials like bone, leaving a shadow on the photographic film. Soft tissue absorbs less, so the bone appears clearly.

A technician uses an optical fibre of refractive index 1.3. What is the critical angle for the glass? (sin c = 1/n)

50.3° Critical angle c = sin⁻¹(1/n) = sin⁻¹(1/1.3) ≈ sin⁻¹(0.769) ≈ 50.3°.

How would you design the inquiry for the electromagnetic spectrum and risk?

Analyse teacher-provided frequency and wavelength values for electromagnetic waves in a vacuum. Compare the frequency of the radiation; record the corresponding wavelength; compare propagation in a vacuum.

What limitation matters when applying the electromagnetic spectrum and risk?

Risk depends on type, dose and exposure; non-ionising radiation can still cause harm through heating or other interactions.

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