Looking at a work of art with raking light (light shining almost parallel to the paint surface) gives us a greater understanding of the relief of the painting (the shape of its surface) and how the artist has used the paint. It can also show any lifting cracks in the paint, any distortions caused by uneven stretching, and sometimes changes that an artist has made in paint.
In the photograph of Ophelia taken in raking light, we can see that Millais made relatively few changes to the composition. We can also see that the surface of the paint is very smooth. What we can see clearly are the cracks in the paint that have occurred over time. These cracks are caused by the ageing of the paint and the stretched canvas.