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.
A bond's coupon rate is the actual amount of interest income earned on the bond each year based on its face value. A bond's yield to maturity is the estimated rate of return based on the assumption it is held until maturity date and not called. Yield to maturity includes the coupon rate within its calculation. Investors are more likely to make investing decisions based on an instrument's yield to maturity than its coupon rate.