What is Tennyson trying to achieve through the contrast of dazzling light compared to Lancelot in "The Lady of Shalott"?
To answer the first question, one needs only to closely read section three of “The Lady of Shalott” as the images themselves are quite obvious. In this section, Tennyson’s imagery directly compares Lancelot to various sources of light and heat such as the sun, a star, a flame, and a meteor.
The second question posed is more difficult to answer as it depends upon one’s perspective. For starters, I suggest reading the enotes “Lady of Shalott” Theme page. (I have posted a link to this page below.) This page offers wonderful insight into the major themes of the poem which could help you formulate your own answer to the second question.
I will, however, offer my own thoughts on Tennyson’s use of light imagery in section three. Whether intentional or not, Tennyson’s imagery of light/heat associated with Lancelot and the Lady’s contrasting associated images of shadow seem to mirror the differences between the social status of men and women in Victorian England. During this time men occupied the public social sphere while women were expected by society to occupy the more private sphere of the home. Following this line of thinking, the poem could be read as a moral warning for young women or a possible commentary on the injustice of Victorian social codes. In either interpretation, The Lady is attracted not just to Lancelot’s fiery appearance but the publicity that such appearance/social status holds. However, when she leaves her allotted place in society (the shadows/private sphere), she dies. Such a death could be literal in the view of the poem as a moral warning or a metaphor for society’s criticism for women who dared to venture into the male social realm in the view of the poem as a social criticism. As always, such interpretation and justification lies in the hands of the reader.