A.
Teeth whitening or dental bleaching, is the most commonly requested cosmetic dental treatment, as it is simple, safe and effective, when carried out by a dental professional.Teeth often become darker due to changes in the mineral structure of the tooth, as the enamel becomes less porous as we get older. Other reaons that teeth become darker are due to the effects of bacterial pigments, foodstuffs and tobacco. Certain antibiotic medications (like tetracycline) can also cause the teeth to look darker.

The way that the whitening procedures work is that oxidizing agents such as carbamide peroxide or hydrogen peroxide are used to lighten the shade of the tooth. The oxidizing agent penetrates the porous enamel and oxidizes the internal stain deposits. Over a period of time, the inner dentine layer, lying underneath the enamel, is also bleached.
In-surgery/Power bleaching uses light energy to speed up the bleaching process, so this procedure is complete within an hour. In addition to this procedure,there are many methods to whiten teeth: bleaching strips,natural bleaching, bleaching pen, bleaching gel and laser bleaching. Home whitening treatments involve the patient applying bleaching gel to the teeth using thin guard trays or by applying small strips that go over the front teeth. Teeth whitening procedures carried out by dentists are completely safe and effective. The only side effect you may experience is temporary sensitivity on your teeth, which disappears within 24 hours. Other risks that effect the longevity of your whitening treatment are lifestyle based, so smoking and the ingestion of dark colored liquids like coffee, tea and red wine reduce the effects of teeth whitening and repeat applications are necessary, more often.

If you have a crown on your teeth, it will not whiten during the whitening procedure so you may have to factor in extra treatment to replace the crown once the teeth are whitened, so you can match the new crown to the rest of the teeth.
I hope this helps,
Sunita