Give some important uses of photo-cells.

Explanation

Application of photo cells :
`implies` Photo cells have many applications, especially as switches and sensors .
`implies` Automatic lights that turn on when it gets dark use photocells , as well as street lights that switch on and off according to whether it is night or day . `implies ` Photo cells are used for reproduction of sound in motion pictures and are used as timers to measure the speeds of athletes during a race .