A photocell is any of a wide range of sensors that react based on the presence of light or electromagnetic energy. They can be found in many different forms, from solar panels that use light to generate electricity, to photoresistors, which shift in resistance based on how much light is present.
Bond prices and yields are like a seesaw: when bond yields go up, prices go down, and when bond yields go down, prices go up. In other words, a move in the 10-year Treasury yield from 2.2% to 2.6% indicates negative market conditions, while a move from 2.6% to 2.2% indicates positive market performance.
However, the absolute value of the benefit is not very high when interest rates are low. Only two years ago, at the beginning of 2011, one could buy a twenty year AA-rated municipal bond with a yield of 5.25 percent. Now, the yield on such a bond is only around 3.75 percent, down about a third.