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 following year, the yield on Bond A has moved to 3.5% to match the move in prevailing rates (as reflected in the 3.5% yield on Bond B). Since the coupon stays the same, the price must rise to $1142.75. Due to this increase in price, the yield declines (because the $40 coupon divided by $1142.75 equals 3.5%).