Can you magnify radiation with a convex lens and dilute it with a concave lens?

Answer

The amount of radiation that passes through the glass (and is therefore not absorbed by the glass) in principle scales linearly with the amount of radiation that is sent to the glass (for simplicity we ignore reflection). When we say ‘radiation passes through a certain thickness of glass’, we mean that only a certain part of your radiation remains after it has passed through the glass – say one hundredth, or one thousandth. Focusing or defocusing does have an impact on the intensity of the radiation (ie radiation per surface area), but not on the amount of radiation: after passing through the glass, this remains one hundredth or one thousandth. Normally (de)focusing will therefore not affect the amount of radiation that passes through your glass.

In special situations, such as in the case of very strong rays (for example in the case of powerful laser beams), so-called ‘non-linear’ effects can indeed occur. If you focus such a strong laser beam, the atoms in the glass can be so upset by the extremely high intensity of your infrared radiation that they are pushed out of their normal position, for example. In that case, the absorption capacity of your glass can also change, and so the amount of light that passes through your glass can be influenced.