What nm laser will burn?

Now, today lasers can go from less than 5mW all the way up to 5,000mW or 5W. So that means knowing where the burning threshold is and that threshold is at a minimum of 100mW as a general rule. That means that any laser below 100mW will be hard-pressed to burn anything and will flat out not be able to.

What wavelength is best for burning?

So, blue lasers at 445nm and typically over 1,000mW these days are the best suited for burning ability. Offering a high heat index as well as a great visibility to go with it.