Which of the following radiations have the longest wavelength?

Step 1: Analyze the question You are asked which of the listed radiations has the longest wavelength. This is a question on the electromagnetic spectrum.

Step 2: The concept All electromagnetic waves travel at the same speed in a vacuum (c = 3 × 10⁸ m/s). They are related by:

c = fλ, so λ = c/f

This means the lower the frequency, the longer the wavelength.

Step 3: Order of the spectrum From longest wavelength (lowest frequency) to shortest wavelength (highest frequency):

Radio waves → Microwaves → Infra-red → Visible light → Ultra-violet → X-rays → Gamma rays

Step 4: Compare the options

• Radio waves: wavelengths from about 1 mm to over 100 km
• Infra-red: about 700 nm to 1 mm
• Light waves: about 400-700 nm
• Ultra-violet: about 10-400 nm
• Gamma rays: less than 0.01 nm

Radio waves have the longest wavelength of all the options.

Answer: D. Radio waves

Exam tip: Use the phrase "R-M-I-V-U-X-G" (Radio, Microwave, Infra-red, Visible, Ultra-violet, X-ray, Gamma). A popular memory line is "Rabbits Mate In Very Unusual eXpensive Gardens." The order goes from longest wavelength to shortest. Gamma rays have the highest energy and frequency, but the shortest wavelength, so don't mix them up.