How do infra-red waves differ from ultraviolet waves?
A. Infra-red waves are longitudinal
B. Infra-red waves have a lower speed in vacuo (in a vacuum)
C. Infra-red waves have lower frequencies
D. Infra-red waves have smaller wavelengths

Hint:Ultraviolet and infra-red waves are the part of the electromagnetic spectrum pattern when white light is allowed to pass through a diffraction medium in such a way that white light will split into seven visible colors and two invisible waves for naked eyes.

Complete step by step answer:
Sunlight, which we see from our naked eyes, is said to be as white light which has seven colors in it, and these colors are arranged in a pattern of seven colors starting from violet and ending at red color. We can say that there are two extremes of the spectrum. The upper extreme is violet colour and the lower one is red color. But this spectrum also has invisible parts of light categorized into two groups; ultra and infra.
The light of wavelength lower than the wavelength of a violet colour is called ultraviolet rays. The light of wavelength larger than the wavelength of red color is termed as infra-red. The wavelength of ultraviolet rays is smaller as compared to the wavelength of infra-red rays. Due to shorter wavelength, ultraviolet rays can penetrate our body through the skin resulting in severe life-threatening diseases. But we know that the frequency is inversely proportional to wavelength, so we can say that the frequency of infra-red waves is smaller than ultraviolet rays.Therefore, based on the above explanation, we can conclude that the infra-red waves have a lower frequency, and option (C) is correct.

Note:While establishing the relationship between infra-red and ultraviolet rays' wavelength and using that relationship to know which wave has a lower frequency, do not get confused. If wavelength is larger for a wave, its frequency will become smaller and vice-versa.