When we talk about internal tooth whitening we are generally referring to the deep whitening that is performed by professionals or dentists to get back a new whiter smile.
When we age the teeth change their colour and this makes the teeth look dirty and even dull in some cases and the light does not reflect so well off the teeth because of blocked enamel which is full of staining components we put in our mouths on a daily basis.
Some of these things that stain our teeth include - tea, coffee, blackcurrant juice, cola, fizzy drinks, sauces like pasta (tomato) soya sauces, curry and spices and even gravy will make our teeth change their colour.
Yes it does take a few years even with good dental care and is pretty much a given that the teeth will start to absorb some of this staining over a period of time even with regular brushing and flossing plus regular trips to the dentist.
The benefits of having a great smile can benefit just about every aspect of your life including work, social and even your love life. This is because a whiter smile is more attractive and makes you more aspiring to anyone you meet. The last time you met someone with very badly dis-colored teeth what did you think but dared to say, and this is why tooth whitening can be a low cost way of making your appearance look improved.
This desire for white teeth has been increased further by the amazing smile we see on TV, films and with just about all celebrities. Many of them have had veneers fitted, which gives them perfectly looking white teeth. Some people desire this look with tooth whitening but it is rare for our natural teeth to be pure white.
The methodology of teeth whitening is that is pushes the whitening gel inside the pores of the enamel which then releases the staining and makes the teeth whiter. This is why it is sometimes referred to as internal tooth whitening as the gel initially sits on top of the enamel and works it way in deep to target the more resilient staining which we receive when we get older as an accumulative process occurs.
Unless the colour of the teeth you see today before having the teeth whitened is not actually staining and is more like the natural colour of the dentin which beneath the enamel then it is likely to work for you in some way.
It is best to not set your expectations on the bright white smile we see on celebrities because like I have said previously pure white is not a natural base colour of our teeth and to really expect a brightening of the teeth and then they look more natural and not forced like pure white veneers.
Teeth whitening is an outside inside process and even though there are new technologies and system being developed it is likely stay that way for some time yet. Good luck!