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Chapter 25 | Geometric Optics 1153
Therefore, the increase in temperature in one minute is
(25.52)
(25.53)
Discussion for (c)
An array of such pipes in the California desert can provide a thermal output of 250 MW on a sunny day, with fluids reaching temperatures as high as . We are considering only one meter of pipe here, and ignoring heat losses along the pipe.
Figure 25.43 Parabolic trough collectors are used to generate electricity in southern California. (credit: kjkolb, Wikimedia Commons)
What happens if an object is closer to a concave mirror than its focal length? This is analogous to a case 2 image for lenses
( and positive), which is a magnifier. In fact, this is how makeup mirrors act as magnifiers. Figure 25.44(a) uses
ray tracing to locate the image of an object placed close to a concave mirror. Rays from a common point on the object are reflected in such a manner that they appear to be coming from behind the mirror, meaning that the image is virtual and cannot be projected. As with a magnifying glass, the image is upright and larger than the object. This is a case 2 image for mirrors and is exactly analogous to that for lenses.
