In the whole human body the most beautiful and excellent part is the face. All the powers of hearing, seeing, speaking and all other powers, which create turmoil in the emotions and may lead men to sin are connected to the face. That's the reason the Shariah has ordered women to cover the face with a nose-piece and should not allow any non-Mahram men to have a glimpse of her face in particular.
The Hadith describe severe warnings for a man who sees any non-Mahram woman with lust and desire. The Holy Prophet (Sallallahu alaihi wa sallam) said:
Translation of Hadith: Whoever looks at a non-Mahram woman with lust, on the Day of Judgment, molten lead would be poured in his eyes. (Hidayah, Kitab Ul Karahiyya, Vol. 4, Pg. No. 458)
If there is any possibility of desire, then everyone agrees that it is prohibited to reveal the face before non-Mahram people. If there is no possibility of desire, then as per the Hanafi school of Fiqh, there is an allowance to keep the face uncovered. Hence, in light of the clear corruption in this age, the later Fuqaha (Islamic Law experts) have stressed that the face should be covered. Hence, while wearing the Burqa, it is more advisable and better to cover the hands up till the wrists, the ankles and including the face, the whole body should be covered.
As given in Radd Ul Muhtaar, Vol. 5, Pg. No. 261.