Why CD devices can not read DVD?

I'm not an engineer, but while lasers are used to read both CD's and DVD's, the required laser is different. I suppose it is due to the frequency that the laser is reading. Anyway, DVD's are about 3 times tighter packed than CD's, so the CD's laser can't focus in on the data. This is why if you tried to store a movie on CD (using the CD-i format, for example), you would end up with 3 or 4 CD's compared to one DVD.