Skip to content

Physics · Ch 8 — Electromagnetic Waves

Electromagnetic Spectrum

8.4

Electromagnetic Spectrum

The Electromagnetic Spectrum: A Continuum of Waves

The electromagnetic spectrum is the complete classification of all electromagnetic (EM) waves arranged by their frequency (or wavelength). At the time of Maxwell, only visible light was well-known. Today, we know the spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Key idea: There are no sharp boundaries between different types of EM waves. The classification is based roughly on how they are produced and detected.

Ordering the Spectrum

The spectrum is typically presented in order of decreasing wavelength (or increasing frequency):

  1. Radio waves (longest wavelength)
  2. Microwaves
  3. Infrared (IR)
  4. Visible light (the tiny portion we can see)
  5. Ultraviolet (UV)
  6. X-rays
  7. Gamma rays (shortest wavelength)

All these waves travel at the same speed in vacuum: the speed of light, c=3×108 m/sc = 3 \times 10^8 \text{ m/s}.

The Fundamental Relation

For any electromagnetic wave, the relationship between its speed (cc), frequency (ν\nu), and wavelength (λ\lambda) is:

c=νλc = \nu \lambda

  • cc = speed of light in vacuum (3×108 m/s3 \times 10^8 \text{ m/s}) …
Figure 8.4The electromagnetic spectrum, with common names for various part of it. The various regions do not have sharply defined boundaries.
Fig. 8.4 — The electromagnetic spectrum, with common names for various part of it. The various regions do not have sharply defined boundaries.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your NCERT textbook's own diagram.

The figure is a two-scale vertical chart that maps the entire range of electromagnetic radiation by frequency and wavelength. The left vertical axis shows frequency in hertz (Hz), increasing from 1010 Hz at the bottom to 102310^{23} Hz at the top, with each tick mark representing a power of ten (decade). The right vertical axis shows wavelength in metres (m), decreasing from 10710^{7} m at the bottom to 10−1410^{-14} m at the top. The two axes are parallel and inversely related: as frequency increases upward, wavelength decreases upward.

Between the axes, brace-bracketed bands label the major regions of the electromagnetic spectrum from top to bottom: Gamma rays, X-rays, Ultraviolet, a highlighted Visible strip (around 101410^{14} Hz), Infrared, Microwaves, Short radio waves, Television and FM radio, AM radio, and Long radio waves. A dotted call-out fans from the Visible strip to a magnified vertical colour bar on the far right, marked 400 nm to 700 nm, with braces naming Violet, Blue, Green, Yellow, Orange, Red in order of decreasing wavelength (or increasing frequency).

Physical idea taught

The figure teaches that electromagnetic waves form a continuous spectrum with no sharp boundaries between regions. The classification is based on how waves are produced or detected, not on fundamental physical differences. All regions obey the same wave equation, but their vastly different frequencies and wavelengths lead to different practical uses and interactions with matter.

Key formula developed with this figure

The fundamental relation connecting frequency (ν\nu) and wavelength (λ\lambda) for all electromagnetic waves in vacuum is:

c=νλc = \nu \lambda

where:

  • cc = speed of light in vacuum (3.00×1083.00 \times 10^8 m/s)
  • ν\nu = frequency (Hz) …
Start your 14-day free trial to unlock the full solution →
← Previous · 8.3.2Example 8.2 — The magnetic field in a plane electromagnetic wave is given by By = (2…Next · 8.4.1 →Radio Waves