Edexcel GCSE Physics (1PH0) — Light and the electromagnetic spectrum
Every time you tune into a local FM station or listen to a sports match on long-wave, invisible waves are carrying that information miles across the country. Radio waves are the ideal choice for this because they can travel incredibly long distances without being absorbed.
Radio waves are produced by oscillations within an electrical circuit. When an alternating current flows through a transmitting aerial, it causes electrons to rapidly move back and forth, generating an electromagnetic wave.
The transmission step depends on the wavelength:
Finally, the detection step occurs at the receiving aerial. When the radio waves are absorbed by a metal conductor, they induce a new alternating current. Crucially, this induced current has the exact same frequency as the radio wave itself.
A local radio station broadcasts its signal at a frequency of (). Calculate the wavelength of this radio wave. (Speed of electromagnetic waves in a vacuum = )
Step 1: State the wave equation and rearrange for wavelength.
Step 2: Substitute the values into the equation.
Step 3: Calculate the final answer with units.
Have you ever wondered how a live television broadcast from the other side of the world reaches your screen almost instantly? It relies on a carefully orchestrated relay between the ground and space.
Unlike lower-frequency radio waves, high-frequency communication microwaves (1 GHz to 30 GHz) undergo atmospheric penetration. They have short enough wavelengths to pass straight through the Earth's ionosphere without being reflected or absorbed.
The communication process follows a strict sequence:
Because microwaves travel in perfectly straight lines, they require a clear "line-of-sight" between the transmitter and receiver.
Popping a cold meal into a metal box and taking it out piping hot two minutes later feels like magic, but it relies entirely on molecular vibrations. Microwave ovens operate at a specific frequency (usually 2.45 GHz) designed to heat food from the inside out.
Step-by-step, the cooking process works like this:
Microwaves are a type of non-ionising radiation, meaning they cannot mutate DNA, but they could still heat human tissue. To prevent this, the oven uses metal walls and a metal grid in the glass door to reflect the waves safely back inside.
Why does a toaster glow red when you press the lever down? It is emitting intense infrared radiation to crisp your bread. All objects above absolute zero naturally emit and absorb infrared, but hotter objects emit more of it at shorter wavelengths.
When cooking with infrared (like in a grill or toaster), the radiation does not penetrate deeply. The food absorbs the infrared radiation strictly at the surface. Once the surface heats up, the energy transfers towards the centre of the food through conduction.
Infrared is also used for thermal imaging. Specialized cameras detect emitted infrared and convert it into electrical signals. A computer then processes these signals to create a thermogram, a false-colour image mapping out temperature differences. Security systems often use a PIR (Passive Infrared) sensor to detect the body heat of intruders against a cooler background, making them highly effective at night.
You can easily control a television from the sofa, but try doing it from another room, and the signal fails. TV remotes use tiny LEDs to send pulses of infrared radiation encoded with digital signals. Because infrared requires a line-of-sight and cannot penetrate walls, it acts as an excellent, interference-free method for short-range communication.
For long-distance data transfer, infrared is sent through optical fibres. These thin glass or plastic strands transmit data pulses using total internal reflection.
The structure of an optical fibre relies on two layers:
Infrared is chosen for optical fibres instead of visible light because it suffers from lower attenuation (less absorption and scattering), allowing the signal to travel much further before needing a boost.
An infrared pulse travels through an optical fibre. The glass core has a refractive index () of 1.45. Calculate the critical angle for the core-cladding boundary.
Step 1: State the equation linking critical angle and refractive index.
Step 2: Substitute the refractive index into the equation.
Step 3: Use the inverse sine function to find the angle.
Students often confuse how microwaves and infrared heat food. Remember that microwaves penetrate a few centimetres and are absorbed by water molecules, whereas infrared is only absorbed at the food's surface.
Students sometimes say radio waves bounce off the 'atmosphere'. To secure the mark, use the precise Edexcel terminology and state they reflect off the ionosphere.
In a 3- or 4-mark question describing radio receivers, you must explicitly state that the radio wave induces an alternating current of the exact same frequency as the wave itself.
When explaining satellite communications, distinguish between wave types: explicitly state that microwaves are chosen for high-speed data because they pass through the ionosphere, while specific radio frequencies are also used for satellite broadcasts and GPS.
For optical fibre questions, always state clearly that the core has a higher refractive index than the cladding; examiners frequently award a specific mark for this structural detail.
Oscillations
The rapid back-and-forth movement of electrons in an electrical circuit that produces radio waves.
Alternating current
An electric current that periodically reverses direction and changes its magnitude continuously with time.
Ionosphere
A region of the Earth's upper atmosphere containing a high concentration of ions and free electrons that reflects certain radio frequencies back to Earth.
Satellite transmissions
The relaying of signals (like TV, navigation, or data) from a ground station to an orbiting satellite and back to Earth.
Atmospheric penetration
The ability of electromagnetic waves to pass through the atmosphere and ionosphere without significant reflection or absorption.
Internal heating
Heating that occurs within the body of a substance (the outer few centimetres), characteristic of microwave interaction with water molecules.
Thermal energy store
The total energy of all the vibrating or moving particles within a substance, which increases as the substance heats up.
Conduction
The transfer of thermal energy through a material without the material itself moving, typical in solids.
Convection
The transfer of thermal energy through the movement of fluids (liquids or gases).
Non-ionising radiation
Radiation that lacks sufficient energy to remove electrons from atoms, such as radio waves, microwaves, and infrared.
Thermogram
A visual map produced by a thermal imaging camera where different colours represent different temperatures.
PIR (Passive Infrared) sensor
A sensor that detects the infrared radiation naturally emitted by warm objects to trigger alarms or lights.
Total internal reflection
The complete reflection of a wave back into its original medium when hitting a boundary with a less dense medium at an angle greater than the critical angle.
Core
The central part of an optical fibre, designed to have a higher refractive index than the surrounding cladding.
Cladding
The outer layer of an optical fibre that has a lower refractive index to ensure total internal reflection occurs inside the core.
Put your knowledge into practice — try past paper questions for Physics
Oscillations
The rapid back-and-forth movement of electrons in an electrical circuit that produces radio waves.
Alternating current
An electric current that periodically reverses direction and changes its magnitude continuously with time.
Ionosphere
A region of the Earth's upper atmosphere containing a high concentration of ions and free electrons that reflects certain radio frequencies back to Earth.
Satellite transmissions
The relaying of signals (like TV, navigation, or data) from a ground station to an orbiting satellite and back to Earth.
Atmospheric penetration
The ability of electromagnetic waves to pass through the atmosphere and ionosphere without significant reflection or absorption.
Internal heating
Heating that occurs within the body of a substance (the outer few centimetres), characteristic of microwave interaction with water molecules.
Thermal energy store
The total energy of all the vibrating or moving particles within a substance, which increases as the substance heats up.
Conduction
The transfer of thermal energy through a material without the material itself moving, typical in solids.
Convection
The transfer of thermal energy through the movement of fluids (liquids or gases).
Non-ionising radiation
Radiation that lacks sufficient energy to remove electrons from atoms, such as radio waves, microwaves, and infrared.
Thermogram
A visual map produced by a thermal imaging camera where different colours represent different temperatures.
PIR (Passive Infrared) sensor
A sensor that detects the infrared radiation naturally emitted by warm objects to trigger alarms or lights.
Total internal reflection
The complete reflection of a wave back into its original medium when hitting a boundary with a less dense medium at an angle greater than the critical angle.
Core
The central part of an optical fibre, designed to have a higher refractive index than the surrounding cladding.
Cladding
The outer layer of an optical fibre that has a lower refractive index to ensure total internal reflection occurs inside the core.
