The ceiling of a single-family dwelling in a cold climate should have an r-value of 46 ft2 ·f°·h/btu. to give such insulation, how thick would a layer of (a) polyurethane foam and (b) silver have to be? the thermal conductivity of polyurethane foam is 0.024 w/m·k, and the thermal conductivity of silver is 428 w/m·k.

With the simplified model of the eye, what corrective lens (specified by focal length as measured in air) would be needed to enable a person underwater to focus an infinitely distant object? (be careful-the focal length of a lens underwater is not the same as in air! assume that the corrective lens has a refractive index of 1.62 and that the lens is used in eyeglasses, not goggles, so there is water on both sides of the lens. assume that the eyeglasses are 1.90