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.
The current yield is the bond interest rate as a percentage of the current price of the bond. The yield to maturity is an estimate of what an investor will receive if the bond is held to its maturity date. Non-taxable municipal bonds will also have a tax-equivalent (TE) yield determined by the investor's tax bracket.