Final answer:
To see the true size of an oblique surface, one must view it perpendicularly. This principle is essential in optics, where the direction of viewing surfaces affects the interpretation of image sizes and locations due to light refraction.
Step-by-step explanation:
To see the true size of an oblique surface, you have to look at the surface from a direction that is perpendicular to the surface. If you view an oblique surface from any other angle, the perspective will distort the dimensions, making the surface appear smaller or larger than it actually is. When dealing with optics, such as lenses and mirrors, understanding the interaction of light with surfaces is crucial. The direction of ray tracing is important to determine the accurate geometry of images.
For example, rays parallel to the optical axis in a converging lens focus at the focal point, whereas rays not parallel to the optical axis will meet at a point in the focal plane, which is perpendicular to the optical axis. In scenario concerning objects in media like glass or water, the refraction of light changes the perception of size and location due to the different indices of refraction.
Introducing an area vector, as mentioned in Figure 6.5, requires a direction to be arbitrarily chosen for an open surface to properly define the concept of normal or perpendicular direction that is necessary for precise optical measurements.