Consider the electromagnetic radiations having wavelengths 200 nm, 500 nm and 1000 nm. Which wavelength(s) of the following can make visual sensation to a human eye?

The correct answer is C, 500 nm only. The human eye responds only to visible light, which lies roughly between 400 nm (violet) and 700 nm (red), and 500 nm is the only one of the three values in that range. Light of about 500 nm looks blue-green to us, close to the part of the spectrum where the eye is most sensitive in daylight, around 555 nm. Radiation of 200 nm is ultraviolet, shorter than violet; it is absorbed largely by the ozone layer and cannot be seen. Radiation of 1000 nm is near-infrared, longer than red; it is felt as heat and is used in TV remote controls and night-vision devices, but it gives no visual sensation. Option A is wrong because it includes 200 nm, which is ultraviolet. Option B is wrong because it includes 1000 nm, which is infrared. Option D is wrong because both of its values lie outside the visible band. Exam tip: visible light = about 400–700 nm (VIBGYOR); below 400 nm = ultraviolet; above 700 nm = infrared.