The ruling on looking at the physical charms of a disbelieving woman

The answer – and Allah is the One who grants success – is that Allah, Glorified and Exalted, has intended to shut the doors that lead to temptation and to close the inlets of Satan through which he comes in order to make those who are morally responsible fall into the unlawful. Allah, Exalted, has commanded the believing men and believing women to lower their gaze, and He has commanded the believing women to be chaste, and forbidden them from displaying their adornment before unrelated men, and forbidden them from being soft in speech, and from stamping their feet so that what they conceal of their adornment may be known. Allah, Exalted, has forbidden the Muslims from entering houses except after seeking permission. And in Sūrat al-Aḥzāb He has said to the Mothers of the Believers: “And stay in your homes and do not display yourselves with the display of the former times of ignorance.” [al-Aḥzāb:33]
And there has come in the ḥadīth, in this chapter, a great deal – among it the ḥadīth: “It is not lawful for a woman who believes in Allah and the Last Day to travel the distance of a barīd post-stage) except with a man who is a maḥram to her,” or as he (May Allah bless him and his family and grant them peace) said.
Among it also is the prohibition of a man being alone with a woman who is not a maḥram to him. And among it is the ḥadīth: “Women are incapacity and nakedness, so veil their incapacity with silence and their nakedness with (remaining in) their houses.” And the ḥadīth: “Do not follow one glance with another, for you have the first and not the second,” and many other narrations.
The purpose of all these teachings that have come in the Book and the Sunnah is to shut the doors of temptations and the causes of desire.
When you know this, then know that looking at a disbelieving woman, if it stirs desire and is a cause leading to falling into temptation, is forbidden. There is no doubt that, as has come in the report, the gaze is an arrow from the arrows of Iblīs. The poets and sages have spoken about this, and the poet said:
A look, then a smile, then a greeting, Then speech, then an appointment, then a meeting.
Yes, if the Muslim is safe from the stirring of desire and from falling into temptation, then it is permissible for him to look at the disbelieving woman and the openly immoral, reckless woman; and similar to that is looking at a female slave. To this is attached looking at images displayed on television screens.
If you say: Looking at television screens does not lead to falling into temptation, because the image displayed is far from the viewer and he cannot reach her.
We say: If looking at images of women and their charms causes the stirring of desire which leads to falling into the unlawful, then looking (in that case) is forbidden – and there is no difference between the unlawful act being with the woman whose image is seen or with some other woman.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2