(Projoy) Starshine? - No (or lengthening evenings)
(Raak) Phosphorescence - No, but *a slight murmur of appreciation*
(Chalky) The passage of time - No

(GL) Radiation - Yes. - Now, which sort?
(Projoy) Photosynthesis? - No
(Irouléguy) A mirage - No.

Come to think of it, we seldom refer to infra-red light, so perhaps ultra-violet instead?

Step forth, the immaculately-tanned Projoy! ULTRAVIOLET it is. Ironically there's an article about the current tanning fad in today's Grauniad.

---

This one might be very easy indeed or a bit of a bastard. It's NOT ANIMAL, VEGETABLE NOR MINERAL and although it is ABSTRACT in one way it's also a real thing.

[Raak] gravity, no
[Rosie] a tan, no

[all] Luurrve, nope
[GL] Fiction, nope
[Ig] the speed of light, nope

[GL] Atmos, no. [Ig] One thing, yes.

How well you know me, dear Raak, but no, it's not that.

[Kim, all] It is neither, nor is it, for the sake of brevity a human construct, or even an human construct.

[Raak] Can it be seen? No
[GL] Weather related... not really, only in the most unhelpful tangential way
[all] Electricity - no
[Raak] Form of sound - no
[Rosie] Natural disaster - debatable, but you could say yes

(Incidentally there have been a couple of answers so far where I perhaps should have used the studio audience, but thought it would make it much too easy. I'll bring them back if it seems like it's too hard)