# Physics - Intensity

> Source: https://ollybritton.com/notes/a-level/physics/topics/intensity/ · Updated: 2021-03-18 · Tags: physics, waves, school, year-1

## See Also
- [Physics - Waves](https://ollybritton.com/notes/a-level/physics/topics/waves/)
- [Physics - Diffraction](https://ollybritton.com/notes/a-level/physics/topics/diffraction/)
- [Physics - Polarisation](https://ollybritton.com/notes/a-level/physics/topics/polarisation/)
- [Physics - Reflection and Refraction](https://ollybritton.com/notes/a-level/physics/topics/reflection-and-refraction/)

## Flashcards
##### What is the intensity of a wave??
The radiant power passing through a surface per unit area.

##### What is the SIMPLE formula for intensity??
$$
I = \frac{P}{A}
$$

##### What is the formula for intensity??
$$
I = \frac{P}{4\pi r^2}
$$

##### Why does the intensity of a wave decrease as you move further away??
Energy is conserved but spread over a larger area, so each piece of area gets less energy.

##### ![PHOTO SPHERE INTENSITY](sphere-intensity.png) What's the equation for the intensity of this wave as radiant energy spreads out from a point??
$$
I = \frac{P}{4\pi r^2}
$$

##### What's the formula for the volume of a sphere??
$$
\frac{4}{3} \pi r^3
$$

##### What's the formula for the surface area of a sphere??
$$
4\pi r^2
$$

##### What is intensity directly proportional to??
$$
\text{intensity} \propto \text{amplitude}^2
$$

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