The **Fourier transform**(FT) decomposes a function of
time (a signal) into the frequencies that make it up.

The Fourier transform of a funcition of time itself is a
complex-valued function of frequency, whose absolute value represents
the amount of that frequency present in the original function, and whose
complex argument is the phase offset of the basic sinusoid in that
frequency. The Fourier transform is called the *frequency domain
representation* of the original signal.

Because we are talking about Fourier transoform, should we take about light in wave or partical?

# Light in space-time

Light is part of electromagnetic wave, whose wavelength is about from 400nm-700nm and frequency range of roughly 430-750 THz. Electromagnetic wave refers to the waves(or their quanta, photons) of the electromagnetic field, propagation(radiating) through space-time, carring energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic waves are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum.

(Why energy can be transport by oscillations of electric and magnetic field?)

## Where light come from --- Light Source

- In energy
- In kinetics

Deceleration of a free charged particle, such as an electron, can produce visible radiation. (But Why electron's energy choose to pass the energy into electric and magnetic field?) (How the energy of electron change to electromagnetic field?) (How the behavir of electron determin the frequency of light?)

Electron's energy pass to electric and magnetic field.

## How light transport?

The wavefront of electromagnetic waves emitted from a point source is a sphere.

It is different after emitting from source: the Near and far fields.

After emssion, for point light source, light is in planar wave. For paraell light, how different light transport together?

How to explain light with Fouier tranform?

### The function of len and the mean of image

Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardised model of human brightness perception.

The len can do *Fouier transform* and *inverse Fouier
transform* while the light throught it. The first len will transform
space domain to frequency domain, then second len can transform
information of frequency domain to space domain, which means object
image will be recovered. If you want, you can modify the frequency
domain information at the focus distance after the first len.

image plane.

## How light is received?

Sense of light and Image of light. I wonder how to quantify the light of object.

# Sound in space and time dimension

How human and sensor sence the light and sound?

以声音和图形来理解傅里叶解析.我们听到的声音究竟是什么，而麦克风听到的又是 什么？声音也是穷级数的物理图像吗？

# 参考

- https://www.zhihu.com/question/19714540
- Wikipedia: Electromagnetic radiation