Inverse Square Law

Inverse square Law

BS-PIII

Institute of Physics

Inverse Square Law Objects of the experiment To understand and domanstrate the Inverse square law..

Introduction An inverse-square law is any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity. The intensity of light or other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source; so an object (of the same size) twice as far away, receives only ¼ the energy (in the same time period). For example, the intensity of radiation from the Sun is 9140 watts per square meter at the distance of Mercury; but only 1370 watts per square meter at the distance of Earth. Hence a threefold increase in distance results in a nine fold decrease in intensity of radiation. Some Applications: Photographers and theatrical lighting professionals use the inverse-square law to determine optimal location of the light source for proper illumination of the subject. The inverse square law is used in acoustics in measuring the sound intensity at a given distance from the source.

EQUIPMENT & COMPONENTS COMPONENTS: A micro-ammeter

EQUIPMENT: • Inverse square law Apparatus

A few connecting hard wires wires

Procedure: 1. Make all connections according to diagram. 2. Record the least count of Micro-Ammeter. 3. Place the Bulb in front of photocell such that both are at same height. 4. Fix a meter scale along the distance between the photocell and the bulb. 5. Switch on the electric bulb. 6. Adjust the distance "d" between the photocell untill micro-ammeter shows the full scale deflection. 7. Increase the distance of bulb from photocell such that the reading of ammeter decreases by its least count. Record this distance in "Distance increasing d1" colum of table.

1

Photocell

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mA

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Inverse square Law

BS-PIII

Institute of Physics

8. Take ten such readings. 9. Record the last reading of distance twice (for increasing and decreasing current). 10. Decrease the distance between bulb and photocell such that the ammeter readings are the same as in previous steps. Record this distance in "Distance decreasing d2" colum of table. 11. Take ten readings for distance decreasing. 12. Calculate mean distance "d" of two distances "d1" and "d2" for each reading. 13. Calculate the reciprocal of square of this mean distance (1/d2). 14. Plot a graph between "1/d2"and photo current "I". It will be a straight line showing proportionality relation. Observations: (i). Least count of the micro-ammeter = __________mA. (ii) Power of electric bulb = _________Watts S.no.

Photo current I(mA)

Distance Distance decreasing increasing d1 (cm) d2 (cm)

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

2

Mean distance d(cm)

d2 (cm2)

1/d2 (cm-2)