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.
Yield to maturity is a useful measure of the attractiveness of a seasoned bond that is held to maturity and redeemed at par value. For example, suppose you buy a $1000 par value ABC Company bond with a 5% coupon rate maturing in five years, and the market price for the bond is $900.