Light Dependent Resistor (LDR) Connecting Wires Source of different power rating (bulbs) Bulb Holder Meter scale Multi Meter Battery 3 THEORY 1 . LDR and its characteristics When light is incident on it, a photon is absorbed and thereby it excites an electron from valence band into conduction band. Due to such device decreases. Thus the LDR or photo-conductive transducer has the resistance which is the inverse function of radiation intensity. NO = threshold wavelength, in meters e = charge on one electron, in Coulombs Ewe = work function of the metal used, in Eve Here we must note that any radiation with wavelength greater than the value obtained in above equation CANNOT PRODUCE any change in the resistance of this device.
The band gap energy of Cadmium Sulfide is 2. Eve and for Cadmium Slender it SSL . Eve. Due to such large energy gaps, both the materials have extremely high resistively at room temperature. 4 Characteristics of photosensitive cells Now when the device is kept in darkness, its assistance is called as dark resistance. This resistance is typically of the order of 1013 ohms. When light falls on it, its resistance decreases up to several kilo ohms or even hundreds of ohms, depending on the intensity of light, falling on it.
The spectral response characteristics of two commercial cells were compared in our laboratory. And we found that there is almost no response to the radiation of a wavelength which was shorter than Mann. It was very interesting to note that the Cadmium Sulfide cell has a peak response nearer or within the green color of the spectrum within a range of Mann. Thus it can be used nearer to the infra-red region up to Mann. It was found that the maximum response of Cadmium Shouldering is in the yellow-orange range at Mann and also it can be used in the infra-red region up to about Mann. Sensitivity The sensitivity of a photo detector is the relationship between the light falling on the device and the resulting output signal. In the case of a photocell, one is dealing with the relationship between the incident light and the corresponding resistance of the cell. 6 Spectral Response Like the human eye, the relative sensitivity of a photosensitive ell is dependent on the wavelength (color) of the incident light. Each photoreceptor material type has its own unique spectral response curve or plot of the relative response of the photocell versus wavelength of light. 7 2. Luminous flux variation: (or even a simple sphere), take a small element dad on the stentorian at a distance ‘r’ from the source. It comprises a small part of the energy radiated (deer). Now, go further to a distance ‘R’ (R>r) from the source, consider the same area element dad, it comprises a much smaller part of energy radiated (deer). [deer > deer]. It varies inversely as the square of the distance. PROCEDURE Choose a specific position for the source and mount it using a holder, make sure it is stable. Select the bulb with the lowest power rating and connect it to the holder as shown in the figure.
Connect the LDR, battery(IV) and the millimeter in series. Set the millimeter to ohm section and select suitable range and measure the resistance with a bulb on. Similarly switch to current section and move to micro ampere in the millimeter. This gives the value of the current. Repeat these steps with different power sources at different distances and note down observations. 9 OBSERVATIONS The experiment has been conducted by using various sources with different power ratings. Voltage of the battery = 6 V 1 . ) 15 watts (yellow)