Section B -Activity 3
Aim: To study the effect of intensity of light (by varying distance of the source) on an LDR(Light Dependant Resistor)
Apparatus
LDR, Intense light source (lamp), Meter scale, Digital multimeter, DC power supply.
Procedure
- 1. Turn the selector switch and set it on R for the measurement of resistance, in Multimeter.
- 2. Plug the metallic ends of the black probe in the terminal marked common in Multimeter and that of red in the terminal marked as P (or +). Short the other metallic ends and adjust the ‘R adjusting’ to get a full-scale deflection reading at zero ohm in the meter.
- 3. Touch the metallic probes to the two metal ends of the L.D.R. [Fig. (a)] and read the value of resistance when (a) the source is kept at a distance of 2 cm, fixing the source of light in a stand and keeping the L.D.R. vertically below it.
- (i) Moving the source to 4 cm distance from the L.D.R. and
- (ii) Moving the source to 6,8 and 10 cm from L.D.R. and repeating observation three more times.
Observation
| Serial No. of Obs. | Distance between LDR and lamp ‘d’ cm | d2 (cm2) | 1/d2 (cm-2) | Resistance of LDR RL (kΩ) |
|---|---|---|---|---|
| 1 | 10 | 100 | 100*10-4 | 1.78 |
| 2 | 20 | 400 | 25*10-4 | 2.49 |
| 3 | 30 | 900 | 11.11*10-4 | 3.17 |
| 4 | 40 | 1600 | 6.25*10-4 | 4.25 |
| 5 | 50 | 2500 | 4*10-4 | 5.20 |
Graph
Conclusion
- As the distance between LDR and source of light (lamp) increases, the resistance of LDR increases.
- As the intensity of light varies inversely to the square of distance, the resistance of LDR varies inversely to the intensity of light. i.e. as the intensity of light decreases, the resistance of LDR increases.
- The graph of R against 1/d2 shows the exponential decrease.
Precautions
- No stray light should fall on the L.D.R. It is better to work in a dark room.
- Connect L.D.R. carefully to the voltage source.