“Light is beautiful in itself, for its nature is simple and all things are like to it. Wherefore it is integrated in the highest degree and most harmoniously proportioned and equal to itself: for beauty is a harmony of proportions” -Franciscan Robert Grosseteste (a monk from the Gothic era)
Light has always played a role in art and architecture. The very light a work of art is placed in can effect it's meaning and purpose. And the purpose in the Gothic era was to make the work seem to be a reflection of heaven. So, new and innovative ways of architecture were worked out to make it possible for this "new light" or lux nova to filter its way into the Gothic cathedrals allowing this heavenly appearance to take place. Ribbed vaults resting on pointed arches allowed builders to eliminate the walls between chapels. It opened up the outer walls and they were able to fill them with stained glass windows. Stained glass played an important role in Gothic architecture and is where the inspiration for the phrase lux nova spoke by Abbot Sugar came from. He called the light "wonderful and uninterrupted." The stained glass was used to replace walls not cover them and provide a new and colorful way to tell Christian story. Stained glass occupied the punched holes in the stone work and later for the sake of filtering more light through the beautifully designed rose window bar tracery rather then plate tracery was developed allowing the window to be filled almost entirely of glass.
The rose window was introduced in the Gothic era and was a standard in French churches. The window would often times display the twelve signs of the zodiac and other calendar related icons. The tall narrow windows that ended in pointed arches were called lancets and they would line church walls where as in the Romanesque period nothing but wall would be placed there. Small rose windows would line there tops called oculi (singular oculus) and below the clerestory lancets as well.
It was important for the Gothic architectures to create a heaven on earth by use of light. Not only were they inspired by the Pseudo Dionysius, a mystic from the 5th and 6th centuries, who compared light to the goodness of God, but they were inspired by the turmoil of their time. It was during the Gothic era that the Hundred year war between the French and the English began and it was also during this time that the black plague killed off a great majority of the population in Europe. So, I gather it was their hope to release a little bit of God's light into the world during such dark times. And the musings of many saints and mystics also allowed for this thought process of light being good and beautiful and a quality of God's nature to be integrated into the building of churches. Over all it was important to replace the thick walls of the Romanesque church with light itself. Opening the church up to the public artistically and spiritually. Allowing light to shine down on the shrine in the church and on the people in the aisles.