✔ 最佳答案
The crucial point is the intensity of the infared.
Although there exists an infared background from other objects and from the environment, their intensity is rather low. The infared sensor on the appliance has been tuned not to respond to such low intensity infared radiation.
When you shoot a beam of infared from your remote control to the appliance, the sensor detects an infared level that is much higher than background and is sufficient to trigger the sensor operation, thus switching on the applicance.