What the woman must do is cover her face because of a great deal of shari evidence which indicates that it is obligatory for the woman to cover her face in front of non-mahram men. And she has two ways in which she can cover:
Either she may lower down on her face something that will cover it completely and not let anything of it show, and she can see through that cover
or she may wear the niqab (face veil) which is that which covers the face but leaves the eyes uncovered.
In Saheeh al-Bukhaari (1838) it says: The woman in ihram should not wear niqab or gloves. The fact that the woman in ihram is forbidden to wear niqab indicates that it is permissible to wear it when not in ihram.
Hence the fuqaha' and scholars said that it is permissible for a woman to wear niqab, but that is on condition that she does not go to extremes in uncovering the eye socket or show too much of the cheeks.
Imam al-Ramli ash-Shaafai (may Allah have mercy on him) said:
It is haraam to look at a woman in niqab of whom nothing can be seen except her eyes, especially if she is beautiful. How much fitnah may be caused by looking at a woman's eyes. End quote from Nihaayat al-Muhtaaj, 6/188
It says in Fataawa al-Lajnah ad-Daa'imah: With regard to the niqaab: Abu Ubayd said, describing the niqab according to the Arabs: it is that from which the eye appears. It was known by other names, including burqa. As for the ruling on it, it is permissible.
The source for that is the hadeeth narrated from Ibn Umar (may Allah be pleased with him), according to which the Prophet (blessings and peace of Allah be upon him) said: The woman in ihram should not wear niqab or gloves. Narrated by al-Bukhaari. The fact that he (blessings and peace of Allah be upon him) forbade the woman in ihram to wear niqab indicates that it is permissible when not in ihram. End quote from Fataawa al-Lajnah ad-Daa'imah (vol. 1, 17/171).
Shaykh Abd al-Azeez ibn Baz, Shaykh Abd ar-Razaaq Afeefi, Shaykh Abdullah ibn Ghadyaan, Shaykh Abdullah ibn Qaood.
Once the above becomes clear, we will realise that there is nothing wrong with a woman wearing a niqaab from which only her eyes appear through a narrow opening which does not uncover anything more than what is needed in order to see. The ruling that this is permissible is not dependent upon there being any need; rather it is generally applicable and it is permissible for her to wear the niqab whether the woman needs to see where she is going or not, and whether she is sitting in a car or walking down the street. The evidence quoted above which indicates that it is permissible to wear the niqab was narrated in a hadeeth of the Prophet (blessings and peace of Allah be upon him), and the tafseer of Ibn Abbaas is general in application and is not restricted to cases of necessity.
And Allah knows best.
Reproduced from Islam QA