What's the Order of the Electromagnetic Spectrum by Wavelength?

The electromagnetic spectrum, from longest to shortest wavelength, is: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. From shortest to longest wavelength, reverse this order.

This is part of C.2 Wave model and applies to both IB Physics SL and HL.

Why This Order Works

All electromagnetic waves travel through a vacuum at the same speed, $c = 3.00 \times 10^8\ \text{m s}^{-1}$. Their wave speed, frequency, and wavelength are related by

$$c=f\lambda$$

Because $c$ is constant in a vacuum, frequency $f$ and wavelength $\lambda$ are inversely proportional. Therefore, moving from radio waves toward gamma rays means wavelength decreases while frequency increases.

Photon energy is related to frequency by

$$E=hf$$

so energy also increases from radio waves to gamma rays.

For example, visible light with $\lambda = 600\ \text{nm} = 6.00 \times 10^{-7}\ \text{m}$ has $f=c/\lambda = 5.00 \times 10^{14}\ \text{Hz}$. Using , one photon has energy . This worked example shows quantitatively that decreasing wavelength increases frequency and photon energy, while the wave speed remains unchanged in a vacuum.

A useful mnemonic for longest to shortest wavelength is: Raging Martians Invaded Venus Using X-ray Guns.

The common misconception is that higher frequency means a higher wave speed. In a vacuum, every electromagnetic wave has the same speed; higher frequency instead corresponds to shorter wavelength. At boundaries, frequency remains constant while speed and wavelength may change.

IB Exam Technique

For a State question, give only the ordered list. For an Explain question, link the order to $c=f\lambda$, stating that constant $c$ makes wavelength inversely proportional to frequency. If energy is included, add $E=hf$. Remember that electromagnetic-spectrum boundaries are approximate rather than perfectly fixed.