Question

What is photo-cell? Write down its two applications.

Hint:

Think of a photo-cell as an "electric eye" or a light-activated switch. Any application where you need to detect the presence or absence of light is a potential use for a photo-cell.

Answer 1

Step 1: Understanding the Concept:
A photo-cell is a sensor or transducer that detects light. Its electrical properties (like current, voltage, or resistance) change when it is exposed to light. The fundamental principle behind its operation is the photoelectric effect, where electrons are emitted from a material (photosensitive surface) when light of a suitable frequency shines on it.
Step 2: Detailed Explanation:
Definition of a Photo-cell:
A photo-cell is an electronic device whose operation is based on the photoelectric effect. It consists of a photosensitive cathode that emits electrons when illuminated, and an anode to collect these electrons. When light falls on the cathode, the emitted photoelectrons are attracted to the anode, completing an electric circuit and allowing a small current (photocurrent) to flow. The magnitude of this current is proportional to the intensity of the incident light.
Two Applications:
1. Automatic Street Lighting:
Photo-cells are used as light sensors in automatic streetlights. During the day, the ambient light is strong, causing the photo-cell to conduct and keep the light circuit open (lights OFF). As evening approaches and the ambient light level drops, the photo-cell's conduction ceases, which triggers a relay to close the light circuit, turning the streetlights ON.
2. Burglar Alarms:
In security systems, a photo-cell is used with a light source (often invisible ultraviolet or infrared light). A beam of light is directed across a doorway or window onto a photo-cell. As long as the light beam is uninterrupted, a current flows, and the alarm is silent. If an intruder breaks the beam, the light no longer reaches the photo-cell, the current stops, and this interruption triggers the alarm.
Other applications include smoke detectors, automatic door openers, and sound reproduction in motion pictures (reading the optical soundtrack on film).
Step 3: Final Answer:
A photocell is a light-to-electricity converter based on the photoelectric effect, used in applications like burglar alarms and automatic streetlights.