To study the variation of photoelectric current with intensity of light
APPARATUS:
- Photo cell ,Sensitive galvanometer ,battery, rheostat, key electric bulb,
- Preferably point- type lamp, suitable case for the bulb and photo-cell and connecting wires.
THEORY
The intensity of light is equal to the light energy falling per second on a unit area held perpendicular to the path of light. It is measured in watts per square meter (W/m2).
The intensity pf light form a point source of light varies inversely as the square of the distance form the source. This is known as inverse square law. If /is the intensity of light and d is the distance from the source then
/∞
A photo-electric cell consists of a cathode and an anode enclosed in glass envelope which is completely evacuated. The cathode is made photo- sensitive form which electrons are emitted when light of attracted to thee anode (Fig.15a) and an electric current flows through the circuit. Such a current is known as photo electric current .The number of electrons emitted or the photo-electric current is proportional to the intensity of light falling on the cathode which in turn is inversely proportional to the square of the distance d between the source and the cathode.
PROCEDURE
- Draw the circuit diagram as shown in Fig15 (a).
- Clean the ends of connecting wires and arrange the apparatus as shown in Fig 15a where the cell is connected in series with a sensitive galvanometer, a rheostat, a battery \Band a key K. See that the positive pole of the battery is connected to the anode of the positive pole of the battery is connected to the photo cell.
- Move the lamp as far away from the photo cell as possible. Insert the plug in the key K and adjust the rheostat to get measurable deflection of the Galvanic on=meter. Note THE Deflection and the distance d of the photo cell form the lamp.
- Similar take at least six readings of the galvanometer by changing the distance d in regular steps. Calculate 1d2for each reading-draw a graph between θ and 1/d2 taking 1d2along x –axis .The graph will be a straight line. Record your observation as follow:
OBSERVATION & CALCULATION
| NO. OF Obs. |
Distance of lamp the photocell (d) (cm) |
Deflection of Galvanometer | Intensity of light1/d2(cm2) |
| 1 2 3 4 5 6 |
CONNCLUSION
The graph between deflection and -1d is straight line therefore defection is proportion 1 d. as deflection is proportion to the photo electric current is proportional to the Intensity of light.
PRECAUTIONS
All connections should be clean and tight.
Anode of the photo cell should be connected to the positive pole and the cathode to the negative pole of the battery
A point source of light should be used so that the intensity of light varies inversely as the square of the distance.
Experiment should be started with maximum distance of the lamp from the photo cell and this distance should be decreased in regular steps.
Viva voce
| Q.1: Define photo electric effects? Ans: The emission of electrons from a substance when illuminated by electromagnetic radiation of suitable frequency is called photo electric effect. Q.2: What is meant by threshold frequency? Ans: photo electric emission form a substance can only take place if the frequency of radiation falling on it, as above a critical frequency which is called the threshold frequency is less than the threshold frequency is less than the shoulder frequency cannot produce photo electric. |
Q.3: Define work function? The minimum energy needed to pull out an electron form a material is called the work function of the material. Q.4: what is photo electric call? Ans: photo electrical cell is device which can convert light energy into electrical energy under certain conditions. Q5: what is photon? A beam of light consists of stream of energy corpuscles. These corpuscles are called photon. Q.6: What is energy of photon? The energy of photon is E=hf |
