Which of the following radiations has the longest wavelength?

Step 1: Analyze the question We need to find which radiation has the longest wavelength among gamma rays, radio waves, infrared rays and X-rays. All of these belong to the electromagnetic spectrum.

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

c = fλ, so λ = c / f

Higher frequency means shorter wavelength, and lower frequency means longer wavelength.

Step 3: Order of the electromagnetic spectrum

From longest wavelength (lowest frequency and energy) to shortest wavelength (highest frequency and energy):

Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays

Step 4: Compare the options

• Radio waves: wavelengths from about 1 mm to over 100 km (longest)
• Infrared: about 700 nm to 1 mm
• X-rays: about 0.01 nm to 10 nm
• Gamma rays: less than 0.01 nm (shortest)

Radio waves have the longest wavelength of the four.

Step 5: Answer The correct option is B. Radio wave.

Exam tip: Remember the spectrum with the phrase "Radio Mothers In Violet Underwear eXpose Girls", which gives Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma. Radio is at the long-wavelength end, and gamma is at the short-wavelength end. If a question asks for the highest frequency or energy, pick gamma. If it asks for the longest wavelength, pick radio.