This phenomenon is called Bathochromic Shift – which is the change in the ability of a liquid to absorb light of longer wavelengths(red light) caused by the substitution or changes in the molecules of the liquid.
So something happens to the tea molecules when you add lemon, that changes its ability to absorb red light. (That’s my plain English definition).
Why does it happen, let me see if I can keep the explanation simple and interesting.
When tea leaves are picked they are green. Like all leaves, when you pluck them, they start to wilt because they lose water. This is the start of the natural oxidation process. To make our household regular tea, oxidation has to happen at a controlled and specific rate, plus its heated. Oxidation means that water is driven out of the leaf and the chlorophyll (the pigment that make it green) slowly breaks down by the action of the enzymes naturally present in the leaves. Oxidation of the leaves, creates a tannin called Thearubigins – that’s what gives the tea its brown colour and slightly bitter taste.
Now when you add lemon juice to your tea, the citric acid in the lemon reacts with the tannin (thearubigins). The acid doesn’t allow the tannin to get ionized (ionization= converting a molecule to ions). This changes the nature of the liquid to absorb low frequency light.
I hope I got it right. Enjoy the resulting brownish orange chemical solution.
Heres what Everything.com has to say:
Adding a squeeze of lemon juice to black tea has the effect of clearing the liquid; it transforms it from a dark, nearly-opaque brown to a transparent orangey yellow in a matter of seconds. This is because the hydogen ions produced by the acid in the lemon juice suppress the ionisation of thearubigins (tannins), the polyphenols that otherwise give tea its brown colour, an effect known as the bathochromic shift - it's a fun chemistry demonstration as well as a tasty hot drink!* The reaction makes the thearubigins lose not just their brown colour but also their astringency, so strong black tea can be made drinkable this way - especially with the addition of a little sugar to take the edge off the bitterness. Unlike milky tea, lemon tea made this way remains tasty at any temperature, and many recipes for iced tea call for the addition of lemon. Incidentally, orange juice is also acid enough to remove most of black tea's astringency, as well as sweetening it; it sounds weird, but it's actually pretty good.
Just to remind you of the chem lab days, I used to love titrations