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.
Current yield, in contrast, is the interest or dividends divided by the current market price. In the case of bonds, the yield on your investment and the interest rate your investment pays are sometimes, but by no means always, the same. If the price you pay for a bond is higher or lower than par, the yield will be different from the interest rate.