Question

What do you mean by the invisible spectrum ? How will you investigate the existence of radiation beyond the red and violet ends of the spectrum ?

Answer 1

The part of spectrum beyond the red and the violet ends is called the $invisible$ spectrum as our eyes do not respond to the spectrum beyond the red and the violet extremes.
If the different radiations from the red part of the spectrum to the violet end and beyond it, are made incident on the silver-chloride solution, it is observed that from the red to the violet end, the solution remains unaffected. However just beyond the violet end, it turns violet and finally it becomes dark brown. Thus there exist certain radiations beyond the violet end of the spectrum, which are chemically more active than the visible light, called $ultravioletradiations$. Now, if a thermometer with a blackened bulb is moved from the violet end towards the red end, it is observed that there is a slow rise in temperature, but when it is moved beyond the red region, a rapid rise in temperature is noticed. It means that the portion of spectrum beyond the red end has certain radiations which produce a strong heating effect, but they are not visible. These radiations are called the $infrared$ radiations.