CIE IGCSE Physics (0625) Electromagnetic spectrum Study Notes - New Syllabus
CIE IGCSE Physics (0625) Electromagnetic Spectrum Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Electromagnetic Spectrum
Key Concepts:
- Electromagnetic Spectrum – Regions and Properties
- Uses of the Electromagnetic Spectrum
- Harmful Effects of Electromagnetic Radiation
Communication Systems Using Electromagnetic Radiation
- Digital and Analogue Signals
Electromagnetic Spectrum – Regions and Properties
Electromagnetic Spectrum – Regions and Properties
All electromagnetic (EM) waves are transverse waves that can travel through a vacuum.
- Speed of all EM waves in a vacuum: \( 3.0 \times 10^8 \, \text{m/s} \)
Comparison Table – EM Spectrum Regions
| Order by Frequency (Low → High) | Order by Wavelength (Short → Long) |
|---|---|
| Radio Waves | Gamma Rays |
| Microwaves | X-rays |
| Infrared (IR) | Ultraviolet (UV) |
| Visible Light | Visible Light |
| Ultraviolet (UV) | Infrared (IR) |
| X-rays | Microwaves |
| Gamma Rays | Radio Waves |
Key Notes:
- As frequency increases, wavelength decreases.
- Gamma rays have the highest frequency and shortest wavelength.
- Radio waves have the lowest frequency and longest wavelength.
- All EM waves travel at \( 3.0 \times 10^8 \, \text{m/s} \) in a vacuum.
Example:
Both X-rays and radio waves travel at the same speed in a vacuum. Why can X-rays cause damage to living cells but radio waves generally cannot?
▶️ Answer/Explanation
The damaging effect of electromagnetic radiation depends not on speed, but on frequency and energy.
X-rays have a very high frequency and short wavelength, meaning they carry a large amount of energy per photon. This high energy can penetrate tissues and cause ionisation, which may damage DNA and cells.
Radio waves, on the other hand, have a very low frequency and long wavelength, so their energy is much smaller. They do not have enough energy to break chemical bonds or ionise atoms, making them generally safe for biological tissue.
Uses of the Electromagnetic Spectrum
Uses of the Electromagnetic Spectrum
| Region | Typical Uses |
|---|---|
| Radio Waves | Radio and TV broadcasting Astronomy (radio telescopes) RFID for tagging/tracking |
| Microwaves | Satellite TV communication Mobile phone signals Microwave ovens (heating food) |
| Infrared (IR) | Remote controls Intruder alarms Electric grills and heaters Thermal imaging Optical fibre communication |
| Visible Light | Human and animal vision Photography General lighting and illumination |
| Ultraviolet (UV) | Security marking (UV pens) Detecting fake currency Sterilising drinking water |
| Xrays | Medical imaging (e.g. bones) Security scanners at airports |
| Gamma Rays | Sterilising food/medical tools Cancer treatment and detection |
Example:
Infrared radiation and ultraviolet light are both part of the electromagnetic spectrum. Yet infrared is commonly used in remote controls and heating, while UV is used for sterilisation. Explain why they are used for such different applications.
▶️ Answer/Explanation
The main difference lies in the energy of the waves, which depends on their frequency.
Infrared has a lower frequency and longer wavelength than UV. It transfers energy more gently, making it ideal for heating and communication without damaging tissue.
Ultraviolet light has a higher frequency and carries more energy per photon. This high energy allows it to kill bacteria and viruses by damaging their DNA, which is why it’s used for sterilisation.
Example:
The frequency of an FM radio wave is \( 1.00 \times 10^8 \, \text{Hz} \). Calculate its wavelength in air. (Speed of light = \( 3.00 \times 10^8 \, \text{m/s} \))
▶️ Answer/Explanation
Use the wave equation: \( v = f \lambda \)
Rearranged: \( \lambda = \dfrac{v}{f} \)
\( \lambda = \dfrac{3.00 \times 10^8}{1.00 \times 10^8} = \boxed{3.00 \, \text{m}} \)
The wavelength of the radio wave is \( \boxed{3.00 \, \text{m}} \).
Harmful Effects of Electromagnetic Radiation
Harmful Effects of Electromagnetic Radiation
Excessive exposure to certain types of electromagnetic (EM) radiation can be dangerous. The severity of harm generally increases with the frequency and energy of the radiation.
| EM Wave | Harmful Effects |
|---|---|
| Microwaves | Can cause internal heating of body tissues. Prolonged exposure may affect internal organs. |
| Infrared (IR) | Causes skin burns due to heating of the skin’s surface. Risk increases with close, intense exposure. |
| Ultraviolet (UV) | Can damage surface cells and eyes. Prolonged exposure can lead to sunburn, skin cancer, and eye conditions like cataracts. |
| Xrays & Gamma Rays | Can cause mutations or damage to DNA and body cells. Risk of cancer, genetic mutation, and tissue damage. Classified as ionising radiation, meaning they can remove electrons from atoms. |
Higher frequency EM waves (like UV, Xrays, and gamma rays) are more harmful because they carry more energy and can cause cell damage or mutations.
Example:
Two hospital technicians work with different equipment. Technician A works near an infrared lamp used for muscle therapy. Technician B regularly operates an X-ray scanner. Which technician is at greater health risk and why?
▶️ Answer/Explanation
Technician B is at greater health risk because X-rays are ionising radiation, while infrared is not.
- X-rays have very high frequency and energy, and can penetrate the body, causing damage to DNA and potentially leading to cancer or mutations.
- Infrared radiation mainly causes of tissues, which may lead to burns but does not cause ionisation or DNA damage.
Therefore, even though both technicians are exposed to EM radiation, X-ray exposure is far more harmful in the long term.
Communication Systems Using Electromagnetic Radiation
Communication with Artificial Satellites Using Microwaves
Microwaves are used for satellite communication because they:
- Can pass through the Earth’s atmosphere with minimal loss
- Carry large amounts of data quickly
- Have shorter wavelengths, allowing smaller antennas and dishes
Microwave signals are used for:
- Satellite phones
- Satellite television (e.g., direct-to-home broadcast)
- Internet and data transmission in remote areas
Types of Satellites Used in Microwave Communication
| Type of Satellite | Description | Example Use |
|---|---|---|
Low Earth Orbit (LEO) | Orbits the Earth quickly (90–120 minutes) Not fixed above one point Provides low latency communication | Some satellite phones Earth observation and tracking |
Geostationary Satellite | Orbits once per day to stay above the same point on Earth Allows continuous coverage of one area Ideal for fixed-location receivers | Direct broadcast satellite TV Some satellite phones and internet services |
Key Point: Both low orbit and geostationary satellites use microwaves to send and receive signals from Earth because they experience less atmospheric absorption compared to lower-frequency waves.
Communication Systems Using Electromagnetic Radiation
Many modern communication systems rely on electromagnetic (EM) waves because they can transmit information wirelessly or through cables at high speeds. Each system uses a specific part of the EM spectrum depending on its purpose and required properties.
| System | Type of EM Radiation | Reason for Use |
|---|---|---|
Mobile Phones and Wireless Internet | Microwaves | Microwaves can penetrate some walls Require only short aerials for transmission and reception Suitable for short- to medium-range communication |
Bluetooth | Radio Waves | Radio waves can pass through walls Signal is weakened as it passes through obstacles Useful for short-range wireless connections between devices (e.g., headphones, keyboards) |
Optical Fibre Communication | Visible Light or Infrared | Glass is transparent to visible light and some infrared Light and short-wavelength IR can carry high data rates Light is internally reflected in optical fibres with low signal loss |
Example:
Why are microwaves used for mobile phone and Wi-Fi communication rather than radio waves, even though radio waves also travel through the air?
▶️ Answer/Explanation
Microwaves are used because they have a shorter wavelength than radio waves. This means they can:
- Carry more data per second (higher bandwidth)
- Be transmitted and received using small antennas, which is important for mobile devices
- Penetrate walls reasonably well for indoor communication
Radio waves have longer wavelengths, so they are better suited to long-range broadcasting like AM/FM radio but are less efficient for compact, high-speed data systems like mobile phones or Wi-Fi.
Example:
An infrared signal used in an optical fibre has a frequency of \( 2.0 \times 10^{14} \, \text{Hz} \). Calculate the wavelength of the signal. (Take speed of light \( c = 3.0 \times 10^8 \, \text{m/s} \))
▶️ Answer/Explanation
\( v = f \lambda \Rightarrow \lambda = \dfrac{v}{f} \)
Substitute values:
\( \lambda = \dfrac{3.0 \times 10^8}{2.0 \times 10^{14}} \)
\( \lambda = 1.5 \times 10^{-6} \, \text{m} = \boxed{1.5 \, \mu\text{m}} \)
Digital and Analogue Signals
Digital and Analogue Signals
Analogue Signal
An analogue signal is a continuous signal that can take on any value within a range.The signal varies smoothly and continuously over time.
Examples: natural sound waves, voltage in a microphone, old audio tapes
Digital Signal
A digital signal is a discrete signal that only takes on two values: 0 or 1 (binary).The signal is transmitted in small packets or pulses of binary data.
Examples: CDs, DVDs, modern mobile phones, computers
Transmitting Sound: Analogue vs Digital
1. Analogue Sound Transmission
- Sound is a longitudinal wave made of compressions and rarefactions in the air.
- Microphones convert these pressure variations into a continuous electrical signal an analogue signal.
- This signal varies smoothly in voltage, matching the shape of the original sound wave.
- When transmitted (e.g., through wires or radio waves), it is susceptible to noise even small disturbances affect the signal quality.
- Speakers later convert this electrical signal back into sound by vibrating accordingly.
2. Digital Sound Transmission
- Sound is first captured by a microphone as an analogue signal.
- The signal is passed through an Analogue-to-Digital Converter (ADC), which does two things:
- Sampling: The signal is measured at regular time intervals (e.g. 44,100 times per second for CDs).
- Quantisation: Each sample is assigned a numerical value (converted to binary).
- This produces a stream of binary data (1s and 0s), representing the sound digitally.
- This digital signal can be compressed, stored, and transmitted efficiently with little to no quality loss.
- At the receiving end, a Digital-to-Analogue Converter (DAC) converts the binary data back into an analogue signal to drive a speaker.
Key Difference: Analogue transmission is continuous and smooth but prone to noise; digital transmission involves sampling and digitising sound, allowing accurate regeneration and better resistance to distortion.
Benefits of Digital Signals
| Benefit | Explanation |
|---|---|
| Accurate Regeneration | Digital signals are easy to regenerate without losing quality. Even if noise is added, the 0s and 1s can be clearly restored. |
| Increased Range | The signal can travel long distances because it can be cleaned and amplified without distortion. |
| Higher Data Transmission Rate | Digital systems can transmit more data per second due to compression and binary encoding. |
While analogue signals are more natural and continuous, digital signals offer better noise resistance, range, and speed, which is why modern communication and audio systems prefer digital formats.
Example:
A live concert is broadcast through an FM radio station using an analogue signal. As the sound travels from the microphone to the radio receiver, what changes might happen to the signal, and why?
▶️ Answer/Explanation
The microphone converts sound waves into an analogue electrical signal — a continuously varying voltage.
The analogue signal is transmitted over a carrier wave via FM radio.
As the signal travels through the air, it may be affected by electrical noise (e.g., from other devices or weather conditions).
Since analogue signals are continuous, even a small distortion can affect the signal’s shape and quality, resulting in poor audio on the receiving radio.
Thus, the main issue with analogue transmission is signal degradation due to noise.
Example:
A song is streamed over the internet as a digital audio file. Describe how the sound is transmitted and why the quality remains high even after long-distance transmission.
▶️ Answer/Explanation
The song is stored in digital format (e.g., MP3 or WAV), consisting of binary data (1s and 0s).
This binary data is transmitted over the internet using packet-based systems.
Even if some distortion or loss occurs, error-correction and regeneration techniques can reconstruct the original signal without affecting quality.
At the user’s end, a DAC (Digital-to-Analogue Converter) converts the binary back into an electrical signal that drives the speaker to produce sound.
Conclusion: The use of binary allows accurate regeneration, which is why digital systems preserve high audio quality even over long distances.