Q.Why do submerged plants receive weak illumination than exposed floating plants in a lake?

Hydrophytes are plants living in water or permanently wet places, sub-divided by how much of the plant contacts water, mud and air: free-floating (touch water/air only — Eichhornia, Pistia, Wolffia), rooted-floating (roots in mud, leaves/flowers floating — Nelumbo, Nymphaea, Potamogeton, Marsilea), submerged-floating (entirely underwater, no soil/air contact — Ceratophyllum, Utricularia), rooted-submerged (rooted, fully underwater — Hydrilla, Vallisneria, Isoetes), and amphibious/rooted-emergent (adapted to both aquatic and terrestrial life in shallow water — Ranunculus, Typha, Sagittaria); a related smaller group, hygrophytes, grow in moist shady places rather than standing water (Habenaria, mosses). Morphologically: roots may be absent, poor or well-developed depending on type, with root pockets replacing root caps in some (Eichhornia); stems are long/slender/spongy when submerged, short/thick/stoloniferous when free-floating, or rhizomatous when rooted-floating, propagating readily via runners/stolons/tubers/offsets; leaves vary from ribbon-shaped (Vallisneria) to large …