Assalamualaikum ustaz. What is the status of a woman who wears thin socks that made her skin color visible?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
Covering Aurah in Prayer
Among the valid conditions of prayer is the aurah is covered. This means that if a person, whether it is a man or a woman, who prays while his/her aurah isn’t covered then the prayer is invalid. Aurah for a man is between his navel and knees, while for a woman it is her whole body except her face and hands up to her wrists according to madhhab Syafie. They based this ruling with the hadith of the Prophet PBUH narrated by Aisyah R.Anha:
لَا يَقْبَلُ اللَّهُ صَلَاةَ حَائِضٍ إِلَّا بِخِمَارٍ
“The Salat (prayer) of a woman, who has reached puberty, is not accepted unless she is wearing a Khimar.” 
Al-Tirmizi said: “This hadith from Aisyah is hasan and can be practised (be made as an argument) according to scholars.” Al-Nawawi said, “what is meant by حَائِضٍ is a woman who has reached puberty.” And this is also one of the valid conditions for men. 
The covering or wearing clothes that cover the aurah is as stated by Imam al-Syirazi: “And it is obligatory to cover the aurah with something that doesn’t let the skin color become visible.” 
Syeikh Zainuddin al-Malibari said: “(covering the aurah) with something that doesn’t let the skin color visible (that can be observed) in an al-takhatub event (a dialogue).” 
The extent of what can be seen in this speaking event is discussed by Syeikh Abu Bakar al-Dimyati as: “The situation means if a person looks carefully at the person praying, approaches and gets really close to see his skin color, then it doesn’t affect it (his prayer).” 
Syeikh al-Syirwani stated: “And a methodology that should be employed is that it doesn’t affect (the validity of prayer) when the skin is visible with a medium (light source shining) sun or fire of which if they are absent, then it is no longer visible.” 
Syeikh al-Nawai al-Bantani said: “[covering the aurah] with a pure matter that could hinder another from seeing the skin color, until that the fairness (of the skin) from its darkness in an al-takhatub event.” 
The term al-takhatub event is further detailed by Syeikh Hasan bin Ahmad al-Kaf in chapter regarding washing a thick beard: “approximately one and a half metres.” 
Among the muhaqqiq scholars who issued a fatwa with the analogical deduction of the al-takhatub event from the ablution chapter to the aurah chapter is Imam Ahmad bin Musa bin ‘Ajil, as stated by the author of Fath al-Muin (pg.86); Hasyiah Tuhfah al-Muhtaj (2/112). However, according to Syeikh Abu Makhramah for the final opinion, there is no difference between al-takhatub events or not, if a person has to get closer to the clothing to see then it isn’t considered if the aurah is just then visible.  Wallahu a’lam.
After analyzing the above explanations and facts, we are inclined to state the following:
- It is obligatory to cover the sole of a woman in prayer according to madhhab al-Syafie and it is one of the valid conditions of prayer. The weak opinion in madhhab Syafie and one of the opinions in madhhab Hanafi, the soles of a woman is not aurah in prayer. Madhhab Maliki categorized the soles as a light aurat and doesn’t affect the prayer if they are exposed. 
- Wearing thin clothing to cover the aurah is not sufficient to attain a valid prayer.
- There is khilaf for the measure of whether it is thin or thick among Syafieyyah scholars. We are inclined towards the explanation of Syeikh Ibn Ajil where he states it is enough if the clothing doesn’t show the skin color for a distance in an al-takhatub event, or approximately 1.5 metres between two people talking. However, the final opinion is as stated by Syeikh Abu Makhramah, where there is no dabit of al-takhatub, except if one has to really scrutinize the clothing is a careful opinion.
- Thus, for a woman’s socks seem thin but the skin color isn’t visible and the fairness or darkness of the skin can’t be differentiated, in a distance of 1.5 meters, it is permissible to be worn in prayer.
- It is sunnah to wear thicker socks or lengthen the praying clothes to avoid the khilaf and fitnah. Wallahu a’lam.
Hopefully, this explanation will give understanding to the questioner. We end with the following supplication:
اَللَّهُمَّ أَعِنِّي عَلَى ذِكْرِكَ وَشُكْرِكَ وَحُسْنِ عِبَادَتِكَ
“O Allah, help me to remember You, to give You thanks, and to perform Your worship in the best manner.”