A Level Physics 7.4 Electromagnetic Spectrum MCQs – 30 Practice Questions with Answers (Cambridge 9702)

Answer: C – Wavelength increases from gamma rays (shortest) through X-rays, ultraviolet, visible and infrared to microwaves and radio waves (longest). Frequency decreases along the same sequence because c = fλ.

Answer: A – Visible light spans roughly 400 nm (violet) to 700 nm (red), that is 4 × 10⁻⁷ m to 7 × 10⁻⁷ m. The shorter range in option D is ultraviolet, and option C starts in the infrared.

Answer: D – Microwaves cover roughly 10⁻¹ m down to 10⁻³ m. A wavelength of 5.0 × 10⁻³ m (5.0 mm) lies inside this range, just short of the infrared boundary near 10⁻³ m.

Answer: B – λ = c / f = (3.00 × 10⁸) / (1.5 × 10¹⁵) = 2.0 × 10⁻⁷ m. This is shorter than violet light (4 × 10⁻⁷ m) and lies in the ultraviolet range, about 4 × 10⁻⁷ m to 10⁻⁸ m.

Answer: A – Red light, at about 7 × 10⁻⁷ m, has the longest wavelength in the visible band. Since f = c / λ, the longest wavelength means the lowest frequency, about 4.3 × 10¹⁴ Hz.

Answer: A – Gamma rays have wavelengths shorter than about 10⁻¹¹ m. The X-ray range (about 10⁻⁸ m to 10⁻¹³ m) overlaps the gamma range, so at very short wavelengths the bands are often distinguished by how the radiation is produced: gamma rays come from the nucleus.

Answer: C – Infrared covers roughly 10⁻³ m to 7 × 10⁻⁷ m. A wavelength of 2.0 × 10⁻⁶ m (2.0 micrometres) is just longer than red light, so it is infrared, which is why hot objects are strong infrared emitters.

Answer: B – f = c / λ = (3.00 × 10⁸) / 200 = 1.5 × 10⁶ Hz = 1.5 MHz. Radio waves have wavelengths from about 0.1 m up to 10⁶ m, so 200 m is a typical long-wave radio wavelength.

Answer: A – Because every band travels at the same speed c in a vacuum, frequency rises as wavelength falls. Radio has the longest wavelength and lowest frequency; gamma rays have the shortest wavelength and highest frequency.

Answer: C – A wavelength of 3 × 10⁻² m (3 cm) is in the microwave range of about 10⁻¹ m to 10⁻³ m. Option A is ultraviolet, option B is an X-ray wavelength, and option D is a radio wavelength.

Answer: B – A high-frequency alternating current in a transmitting aerial makes charges oscillate, and the oscillating charges radiate radio waves of the same frequency. Option A describes X-ray production and option D describes gamma ray production.

Answer: A – In an X-ray tube, electrons are accelerated through a large potential difference and strike a metal target. Their rapid deceleration (and inner-shell electron transitions in the target atoms) produces X-rays with wavelengths of about 10⁻⁸ m to 10⁻¹³ m.

Answer: A – Gamma rays come from changes within the nucleus, for example when a nucleus formed in an excited state after alpha or beta decay drops to a lower energy state. Atomic electron transitions give visible or ultraviolet radiation, not gamma rays.

Answer: C – The discharge excites electrons in mercury atoms to higher energy levels. When they fall back, they emit photons; for mercury many of these transitions release enough energy per photon to be ultraviolet (wavelengths about 4 × 10⁻⁷ m to 10⁻⁸ m).

Answer: A – Infrared is detected by the heating effect it produces when absorbed. A thermopile (blackened junctions in series) or a thermometer with a blackened bulb absorbs the radiation and shows a temperature rise; photodiodes also respond to near-infrared. The eye cannot see it.

Answer: D – The aerial receives waves of many frequencies, which induce tiny alternating currents. The tuned circuit is adjusted to resonate at the wanted frequency, so only that station produces a large response and is selected.

Answer: B – Fluorescent materials absorb ultraviolet photons and re-emit the energy as visible light, which is also the basis of fluorescent security markings. Ultraviolet also affects photographic film. Like all electromagnetic waves it is uncharged, so option D is wrong.

Answer: C – A short aerial or dipole picks up the microwaves and the induced alternating signal is rectified by a diode so it can be measured or amplified. This is the detection method used in school microwave transmitter and receiver kits.

Answer: D – Atoms or semiconductor electrons are first raised to higher energy levels. As they return to lower levels they emit photons; for visible light the photon energies correspond to wavelengths of about 4 × 10⁻⁷ m to 7 × 10⁻⁷ m. Hot bodies also emit visible light across a broad range of wavelengths.

Answer: A – Both radiations are strongly ionising. In a Geiger-Muller tube the ions produced trigger a countable pulse; photographic film is also blackened by them, which is why film badges monitor workers exposed to X-rays and gamma rays.

Answer: D – Every electromagnetic wave is a transverse oscillation of electric and magnetic fields, needs no medium, and travels at c = 3.00 × 10⁸ m/s in a vacuum. The bands differ only in wavelength and frequency.

Answer: D – Only transverse waves can be polarised, because their oscillations are perpendicular to the direction of travel and can be restricted to one plane. Longitudinal waves, whose oscillations are along the direction of travel, cannot be polarised.

Answer: B – λ = c / f = (3.00 × 10⁸) / (100 × 10⁶) = (3.00 × 10⁸) / (1.00 × 10⁸) = 3.0 m.

Answer: C – λ = c / f = (3.00 × 10⁸) / (5.0 × 10¹⁴) = 6.0 × 10⁻⁷ m = 600 nm, which is orange-red visible light.

Answer: B – f = c / λ = (3.00 × 10⁸) / 0.12 = 2.5 × 10⁹ Hz = 2.5 GHz, the frequency used by microwave ovens and many radar systems.

Answer: D – The energy of a photon increases with frequency (the full treatment, E = hf, is in Topic 22). Gamma rays have the highest frequencies in the spectrum, so their photons carry the most energy, which is why gamma radiation is the most penetrating and the most damaging to living cells.

Answer: C – In a vacuum all electromagnetic waves travel at exactly the same speed c, whatever their wavelength or frequency. Their speeds differ only when they pass through a material medium, which is why refraction and dispersion can occur in glass.

Answer: B – At about 2.5 GHz the microwaves are strongly absorbed by water molecules, whose rotational energy levels respond at this frequency. The absorbed energy becomes internal (thermal) energy of the food. Microwaves are non-ionising, and all bands travel at the same speed, so options A and D are wrong.

Answer: C – The short wavelength of microwaves (centimetres) lets a dish produce a narrow, directional beam and gives sharp reflections from aircraft-sized objects. The time delay of the echo gives the range, since the waves travel at c. All electromagnetic waves travel at the same speed, so option A is wrong.

Answer: D – Diffraction is strongest when the wavelength is similar to the size of the gap or obstacle. Radio wavelengths of hundreds of metres spread around hills and buildings, while the much shorter television wavelengths diffract far less and need an almost line-of-sight path. All electromagnetic waves, including every band, show reflection, refraction and diffraction.