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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 are visible light sub-bands?
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Within visible light, the standard rainbow order (long wavelength to short) is:
What is radio?
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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?
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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?
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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?
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Direct human vision. Photography, microscopy, plant photosynthesis, photovoltaic cells.
What is ultraviolet?
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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?
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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?
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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?
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Frequency 100 MHz. Wavelength λ=c/f=3×108/108=3\lambda = c / f = 3 \times 10^8 / 10^8 = 3 m. Radio region.
What is green light?
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Wavelength 550 nm. Frequency f=c/λ=3×108/(550×10−9)≈5.45×1014f = c / \lambda = 3 \times 10^8 / (550 \times 10^{-9}) \approx 5.45 \times 10^{14} Hz. Photon energy E=hf≈2.26E = h f \approx 2.26 eV.
What is medical X-ray?
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Photon energy 30 keV. Frequency f=E/h=30×103×1.6×10−19/6.626×10−34≈7.25×1018f = 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 λ=c/f≈4.1×10−11\lambda = c / f \approx 4.1 \times 10^{-11} m =41= 41 pm.
What is wrong region from wavelength?
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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).
What is q1?
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State three regions of the electromagnetic spectrum in order of decreasing wavelength, with one application each. [3 marks]
What is q2?
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A radar transmits at 1010 GHz. Calculate (a) the wavelength, and (b) the energy per photon in joules and in eV. [4 marks]
What is q3?
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Refer to Synchrotron X-rays at 1010 keV. (a) Calculate the wavelength. (b) Determine the frequency.

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