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.
When interest rates fall, newly issued bonds will pay a lower yield than existing issues, which makes the older bonds more attractive. Investors who want the higher yield may be willing to pay a premium to get it. Likewise, if interest rates rise, newly issued bonds will pay a higher yield than existing issues.