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 coupon rate exceeds the current yield when a bond sells at a discount. The call price must equal the par value. An increase in market rates increases the market price of a bond. Decreasing the time to maturity increases the price of a discount bond, all else constant. Increasing the coupon rate decreases the current yield, all else constant.