Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Effect of light on an LED


LED is a p-n junction diode having band gap in visible range. When current is given to the diode, electron-hole pairs are produced which recombine to give visible light. We have observed that if light falls on an LED, a voltage is created between the terminals. This experiment is to investigate this phenomena.


An LED enclosed in an opaque tube (we will call it sensor LED), another LED on a stand (we will call it source LED) multimeter. Two more LEDs of different colours.


  1. Look at the arrangement to enclose the LED in the opaque tube. This is made in a pen body. You can easily take out the LED and change it. You are given two kinds of LEDs red and green.
  2. Connect the terminals of the sensor LED to the multimeter in DC voltage mode. Set the range to 200 mV.
  3. Switch on the source LED. Push this LED towards the sensor LED tube. Check the voltage appearing on the multimeter. Adjust the position, orientation and distance from the sensor LED to maximize the voltage. You can push the source LED inside the sensor LED tube.
  4. Do step-3 for all combinations of colours of LEDs, red and green for source LED and also for sensor LED. Fill your observations in the table.
  5. Choose the combination of colours of LED which give maximum voltage. Now bring the source towards the sensor LED with their length perpendicular as shown in the figure. What do you observe?


Figure to be made