Q.Mention two harmful effects of excess UV radiation on organisms.

Light is both an energy source for photosynthesis and a signal organisms use to time their lives. Several distinct light-related phenomena are recognised.

Phototaxis is directed whole-body movement towards (positive) or away from (negative) a light source — for example, many insects moving towards a lamp, or earthworms burrowing away from light. Photokinesis is different: it is a change in the speed of an organism's random movement in response to light intensity, without any directed orientation towards or away from the source — the organism simply moves faster or slower as light changes, not purposefully towards or away from it.

Biological rhythms are activities that repeat on a regular schedule tied to environmental light cycles. A circadian rhythm repeats roughly every 24 hours — sleep-wake cycles and hormonal fluctuations are classic examples — and persists via an internal "clock" even without external light cues, though it is normally reset daily (entrained) by the light-dark cycle. Photoperiodism is a response to the relative duration of light versus darkness across the 24-hour cycle (day length); many organisms use changing day length, rather than temperature, as the more reliable cue for timing seasonal reproduction, migration or flowering, since day length changes predictably with the calendar regardless of short-term weather. …