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.
For example, if a $1,000 par value bond has a 5% coupon rate, each year the holder of that bond will earn 5% of $1,000, or $50 (0.05 x $1,000 = $50). Yield-to-maturity is the yield calculation used to compare the values of bonds with different issue and maturity dates, coupon rates, and par values.