Isn't Blu-ray a DVD

Answer 1

In so much as it carries Digital Video, yes - it's a Digital Video Disc. It's name - "Blu-Ray" - is a reference to the difference from 'regular' DVDs, though.

On a digital compact disc, information is stored as 'groves' (or 'gaps') and 'valleys' (or 'hills'), etched in a layer in the disc using a laser (a type of light). This information is read back from the disc by shining a laser at it and seeing how it reflects (the 'groves' reflect differently from the 'valleys').

Now, DVD (and HD-DVD) use a red laser, 650 nanometers in wavelength (as opposed to CDs, which use 780 nanometers). As Wikipedia puts it, "This permits a smaller pit to be etched on the media surface compared to CDs (0.74 micrometers for DVD versus 1.6 micrometers for CD), allowing for a DVD's increased storage capacity.".

Blu-Ray DVD uses a yet shorter wavelength of 405 nanometers, resulting in its laser's visible light crossing into the blue spectrum (as the name implies), rather than red. But, the technology, for the most part, remains the same.