Which of the following electromagnetic wave has the longest wavelength?

Step 1: Analyze the question You are asked to pick the electromagnetic wave with the longest wavelength from the four options.

Step 2: The concept All electromagnetic waves travel at the speed of light, c = fλ. So wavelength and frequency are inversely related: the higher the frequency, the shorter the wavelength.

The electromagnetic spectrum, from longest wavelength to shortest, is:

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

Step 3: Compare the options

• Radio waves: wavelengths from about 1 mm up to several kilometres (the longest)
• Infrared waves: about 700 nm to 1 mm
• Ultraviolet rays: about 10 nm to 400 nm
• Gamma rays: less than about 0.01 nm (the shortest)

Order of the options by wavelength: radio waves > infrared > ultraviolet > gamma rays.

Step 4: Answer Radio waves have the longest wavelength, so the correct option is A.

Tip: Remember the spectrum with the phrase "Raging Martians Invaded Venus Using X-ray Guns" (Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma). It runs from longest wavelength (lowest frequency and energy) to shortest wavelength (highest frequency and energy). If a question asks for the longest wavelength, pick the first one on the list that appears in the options. If it asks for the highest frequency or energy, pick the last one.