Intensity Formulas - Definitions & Solved Example Questions
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Intensity is a measure of the energy transmitted by a wave. It is the quantity of energy the wave –conveys per unit time over unit area. It is generally measured with units of watts per square metre (i.e., Wm^{–2}). Intensity will depend on the strength and amplitude of a wave. Intensity is represented by \[I\]. The formula for intensity is given by, \[I = \frac{P}{A}\] Where\[I\]is the intensity, P is the power and A is the area of cross section.
Example: What is the intensity of light incident normal to a circular surface of radius 5 cm from a 100 W source of light? Solution: r = 5 cm = 5 × 10^{–2} m, P = 100 W, \[I\]=? \[I = \frac{P}{{\pi {r^2}}} = \frac{{100}}{{\pi \times {{(5 \times {{10}^{--2}})}^2}}} = 1.27 \times {10^4}W{m^{--2}}\]
Question: The intensity of a light source with power P on a circular surface of radius r placed at a certain distance is \[I\]. If at the same distance from another source, if the intensity on a circular surface of double the radius, is 4\[I\], then what is the power of the second source? Options: (a) 4 P (b) 16 P (c) 2 P (d) P / 4 Answer: (b)