For the image of the overhead projector to be in focus, the distance from the projector lens to the image,
.z-dn.net/?f=d_%7Bi%7D" id="TexFormula1" title="d_{i}" alt="d_{i}" align="absmiddle" class="latex-formula"> , the projector lens focal length, f, and the distance from the transparency to the projector lens, , must satisfy the thin lens equation + . Which is the focal length of the projector lens if the transparency placed 4 inches from the projector lens is in focus on the screen, located 8 feet from the projector lens? I'm especially unsure of what the last sentence means...
A vertical polarizing filter is used on the lens of a camera, they block out the light that is horizontally polarized, so they allow all of the vertically polarized<span> light to pass through.</span>