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VICPhysicsQuick questions
Unit 4: How have new ideas and ways of thinking developed our understanding of the physical world?
Quick questions on Electromagnetic waves and the electromagnetic spectrum: VCE Physics Unit 4
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is visible light sub-bands?Show answer
Within visible light, the standard rainbow order (long wavelength to short) is:
What is radio?Show answer
AM (530 kHz to 1700 kHz), FM (87.5 MHz to 108 MHz), TV broadcast (VHF / UHF up to about 800 MHz). Long-range communication uses long wavelengths because they diffract around obstacles and reflect off the ionosphere.
What is microwave?Show answer
Mobile phones (around 0.7 to 2.7 GHz), Wi-Fi (2.4 GHz and 5 GHz), Bluetooth (2.4 GHz), radar (microwave + sub-microwave), satellite (1 to 30 GHz). Microwave ovens (2.45 GHz) heat water through dielectric absorption.
What is infrared?Show answer
Thermal imaging cameras detect body heat. Remote controls use near-IR LEDs around 940 nm. Optical fibres operate at IR wavelengths (typically 1310 nm and 1550 nm) where silica is most transparent.
What is visible?Show answer
Direct human vision. Photography, microscopy, plant photosynthesis, photovoltaic cells.
What is ultraviolet?Show answer
Sterilisation (UV-C around 254 nm destroys bacterial DNA). Fluorescence (UV light absorbed and re-emitted at visible wavelengths). Sunburn (UV-B).
What is x-ray?Show answer
Medical radiography (X-ray photons penetrate soft tissue but are absorbed by bone). CT scans (computed tomography). Crystallography (X-ray wavelengths comparable to atomic spacings, so diffraction reveals crystal structures).
What is gamma?Show answer
Cancer radiotherapy (gamma photons damage cancer cell DNA). Sterilisation of medical equipment and some foods. Gamma-ray astronomy (gamma sources include pulsars, supernovae, active galactic nuclei).
What is fM radio?Show answer
Frequency 100 MHz. Wavelength $\lambda = c / f = 3 \times 10^8 / 10^8 = 3$ m. Radio region.
What is green light?Show answer
Wavelength 550 nm. Frequency $f = c / \lambda = 3 \times 10^8 / (550 \times 10^{-9}) \approx 5.45 \times 10^{14}$ Hz. Photon energy $E = h f \approx 2.26$ eV.
What is medical X-ray?Show answer
Photon energy 30 keV. Frequency $f = E / h = 30 \times 10^3 \times 1.6 \times 10^{-19} / 6.626 \times 10^{-34} \approx 7.25 \times 10^{18}$ Hz. Wavelength $\lambda = c / f \approx 4.1 \times 10^{-11}$ m $= 41$ pm.
What is using $v$ in a medium when $c$ is asked?Show answer
In vacuum the speed is $c$. In a medium with refractive index $n$, the speed is $c / n$, which is lower. Frequency stays the same; wavelength reduces.
What is treating visible light as a single wavelength?Show answer
Visible covers 400 to 700 nm, almost a factor of two. Different wavelengths in this range are different colours and have different photon energies.
What is confusing ionising and non-ionising radiation?Show answer
The threshold is around 124 eV (UV-C). Microwave, IR, visible, and most UV are non-ionising. X-ray and gamma are ionising.
What is wrong region from wavelength?Show answer
Memorise the boundary orders: radio (m), microwave (cm), IR (microns), visible (hundreds of nm), UV (tens of nm), X-ray (nm to pm), gamma (pm and below).