Name the physical quantity which remains the same for microwaves of wavelength $1mm$ and UV radiations of $1600 A^o $ in vacuum.

Hint: Both of them are electromagnetic waves. Thus they have different wavelengths. Recall what are the other properties which they have difference in and what properties are similar to all electromagnetic waves. Also note that the frequency of the waves are different as well.

Complete step by step solution:
All electromagnetic waves have some differences and similarities amongst them. The wavelengths and frequencies of these waves are different. However all of them do not require a medium for propagation and have the same speed. We know that speed is a physical quantity since it can be measured.
Thus, the physical quantity which remains the same for microwaves of wavelength $1mm$ and UV radiations of $1600 A^o $ in vacuum is velocity or speed of propagation.

Additional Information:
Electromagnetic waves or EM waves are waves that have the contribution of both electric and magnetic fields. These electric and magnetic fields are however not constant but vary with time. In other words, EM waves are composed of oscillating magnetic and electric fields. It includes radio waves, microwaves, infrared, even the visible light, the ultraviolet, X-rays, and gamma rays. The distance between adjacent crests or adjacent troughs in the adjacent cycles of a waveform signal propagated in space or along a wire is called the wavelength of the wave. The number of occurrences of a repeating event per unit of time is the frequency of the wave. In waves, it is no. of wavelets per second. It is measured in $Hz$ or hertz.

Note: Since speed is constant for an electromagnetic wave, the wavelength and frequency are inversely proportional to each other. Since UV light has smaller wavelengths than microwaves, UV light will have higher frequency. But will move with the same speed. All electromagnetic waves move with the same speed.