Grade 10 Physical Sciences
Term 1 · Weeks 6–7

Electromagnetic Radiation

Paper 1 — Physics · Grade 10

Electromagnetic (EM) radiation includes light, radio waves, X-rays, and more. Unlike sound, EM radiation does not need a medium — it can travel through a vacuum. All EM waves travel at the same speed in a vacuum: c = 3 × 10⁸ m·s⁻¹.

Week 4–6

5.1 The Wave Model of Electromagnetic Radiation

Explain that some aspects of the behaviour of electromagnetic radiation can be modelled using a wave model of light.List properties of electromagnetic waves.Arrange different types of electromagnetic radiation in the electromagnetic spectrum.Calculate frequency or wavelength using c = fλ.Describe the dangers of gamma rays, X-rays and the damaging effect of ultraviolet radiation.

Electromagnetic radiation (EM radiation) is energy that travels through space as oscillating electric and magnetic fields. Unlike sound or water waves, EM radiation does NOT need a medium — it can travel through the vacuum of space. This is why we receive light and heat from the Sun even though space between us and the Sun is empty.

Properties of all electromagnetic waves

  • Transverse waves — the electric and magnetic field oscillations are perpendicular to the direction of travel
  • All travel at the same speed in a vacuum: c = 3 × 10⁸ m·s⁻¹ (the speed of light)
  • Can travel through a vacuum — no medium is required
  • Can be reflected, refracted, diffracted, and undergo interference
  • They carry energy — the higher the frequency, the more energy per photon

Formula

Speed of electromagnetic radiation (in vacuum)

c=fλc = f\lambda

c = speed of light in vacuum = 3 × 10⁸ m·s⁻¹, f = frequency (Hz), λ = wavelength (m)

SI unit: m·s⁻¹

Electromagnetic Spectrum← increasing frequency (Hz)increasing wavelength (m) →γ-rays10⁻¹²mX-rays10⁻¹⁰mUV10⁻⁸mVisible400-700nmIR10⁻⁵mMicro- wave10⁻²mRadio1–100mAll travel at c = 3×10⁸ m·s⁻¹ in a vacuum
Figure 5.1 — The electromagnetic spectrum. As frequency increases (left to right: radio → gamma), wavelength decreases and photon energy increases. Visible light occupies only a tiny portion of the full spectrum.

Worked Example

Green light has a wavelength of 550 nm (550 × 10⁻⁹ m). Calculate its frequency.

Given

  • λ = 550 × 10⁻⁹ m
  • c = 3 × 10⁸ m·s⁻¹

Find

f = ?

Solution

  1. 1c = fλ → f = c/λ
  2. 2f = (3 × 10⁸) / (550 × 10⁻⁹)
  3. 3f = 5,45 × 10¹⁴ Hz
Answer: f ≈ 5,45 × 10¹⁴ Hz

Worked Example

A radio station broadcasts at a frequency of 100 MHz (100 × 10⁶ Hz). Calculate the wavelength of the radio waves.

Given

  • f = 100 × 10⁶ Hz
  • c = 3 × 10⁸ m·s⁻¹

Find

λ = ?

Solution

  1. 1c = fλ → λ = c/f
  2. 2λ = (3 × 10⁸) / (100 × 10⁶)
  3. 3λ = 3 m
Answer: λ = 3 m
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Watch Out

DANGERS OF HIGH-FREQUENCY EM RADIATION: • Gamma rays (γ): Emitted by radioactive nuclei. Highly penetrating. Can damage and kill cells, cause cancer and genetic mutations. Used in controlled doses to destroy cancer tumours. • X-rays: Can penetrate soft tissue. Excessive exposure increases cancer risk. Radiographers wear lead aprons for protection. • Ultraviolet (UV): Damages DNA in skin cells, causing sunburn, premature ageing, and skin cancer. Also damages the retina of the eye. Use sunscreen and UV-protective sunglasses.

Uses of different EM radiation types

  • Radio waves: Broadcasting (TV, radio), mobile phone communication, Wi-Fi
  • Microwaves: Microwave ovens, radar, satellite communication
  • Infrared (IR): Remote controls, night-vision cameras, thermal imaging, physiotherapy heat lamps
  • Visible light: Vision, photography, fibre-optic communication
  • Ultraviolet (UV): Sterilising water and medical equipment, detecting forged banknotes
  • X-rays: Medical and dental imaging, security scanners at airports
  • Gamma rays: Cancer treatment (radiotherapy), food irradiation, sterilisation
Week 6

5.2 Photons and the Quantum Nature of Light

Define a photon.Relate the energy of a photon to its frequency and wavelength using E = hf.

While EM radiation behaves like a wave for phenomena such as interference and diffraction, it can also behave as a stream of tiny 'packets' of energy called PHOTONS. This dual nature (wave-particle duality) is one of the fascinating aspects of modern physics.

Definition

Photon

A photon is a packet (quantum) of electromagnetic energy. A photon has zero rest mass and always travels at the speed of light c = 3 × 10⁸ m·s⁻¹.

Formula

Photon energy

E=hfE = hf

E = energy of one photon (J), h = Planck's constant = 6,63 × 10⁻³⁴ J·s, f = frequency (Hz)

SI unit: J

Since c = fλ, we can also write f = c/λ and substitute into E = hf to get E = hc/λ. This shows: HIGHER FREQUENCY (shorter wavelength) → MORE energy per photon. This is why gamma rays (very high f, tiny λ) carry so much energy and are so dangerous, while radio waves (very low f, huge λ) carry very little energy per photon.

Worked Example

Calculate the energy of a photon of green light with frequency 5,45 × 10¹⁴ Hz. (h = 6,63 × 10⁻³⁴ J·s)

Given

  • f = 5,45 × 10¹⁴ Hz
  • h = 6,63 × 10⁻³⁴ J·s

Find

E = ?

Solution

  1. 1E = hf
  2. 2E = (6,63 × 10⁻³⁴)(5,45 × 10¹⁴)
  3. 3E = 3,61 × 10⁻¹⁹ J
Answer: E = 3,61 × 10⁻¹⁹ J
?

Practice Question

A UV photon has a wavelength of 300 nm (300 × 10⁻⁹ m). (a) Calculate the frequency of this photon. (b) Calculate the energy carried by this photon. (h = 6,63 × 10⁻³⁴ J·s, c = 3 × 10⁸ m·s⁻¹)

(6 marks)

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