When you stand beside the highway, pay attention to a fast moving car engine sounds, you will find it moving to your tone of voice becomes high (ie, the frequency becomes higher), it leave you when the pitch becomes somewhat lower (ie, low frequency changes). This phenomenon is called the Doppler effect. Also exists in the optical phenomenon in the Doppler effect, when the light source to you fast movement, will increase the frequency of light, reflected light blue color to the direction of bias (as in the visible, the Blu-ray high frequency), the spectral blue shift; and fast away from you when the light source, the light will reduce the frequency, showing the color of light will shift to the red direction (as in the visible light, the red light of low frequency), that spectrum red shift occurs.

Before the Doppler effect in the further study, let us look at the basics of the wave:

If we put a small stone into calm water, the water will produce ripples, and continue to spread forward. Office of the surface wave source then once every vibration, the water will generate a new wave train.

Wave source based vibration cycle T, the vibration of a wave source every time T, then the two adjacent surface wave train on the distance between the VT, where V is the wave propagation velocity in water. In physics, we have shown that the distance between adjacent wave is called wavelength λ, said symbols. This wave of wavelength, wave velocity and the relations between vibration period can be expressed as: λ = VT (1)

As a wave source vibration time required for the T, the wave source in the number of vibrations per unit time on the 1 / T. Physics, the wave source in the number of vibrations per unit of time known as the wave frequency, with f expressed. Thus, the relationship between it and the cycle can be expressed as f = 1 / T, or T = 1 / f (2)

Integrated (1) type and (2) Type available: λ = VT = V / f (3)

 This style is the wave we discuss issues related to the basic formula, although the spread is summed up by waves, but it is applicable to all of a wave.

Experimental results show that: for the determination of the media, wave propagation velocity V is a constant. So, when the wave propagation of a defined medium, its wavelength λ is proportional to its period (frequency is inversely proportional). The higher the wave frequency, the smaller the period, the shorter its wavelength; the other hand, lower frequency waves, the greater cycle, the longer the wavelength.

On the sound, the sound frequency determines the pitch of the sound. The higher the frequency of the sound wave, the higher the sound pitch, sound, and the more pointed, more detailed, and even more strident. Based on the above findings, resulting in slower pitch of the sound source vibration, vibration cycle length, corresponding to the wavelength of sound waves are longer. For example: 10000Hz sound waves sound waves the wavelength is the wavelength of 100Hz 1 / 100.

In visible light, the light waves determine the frequency of the color of colored light. Frequency from low to high order corresponding to red, orange, yellow, green, blue, indigo, violet. One red light the lowest frequency, longest wavelength; violet highest frequency, but the shortest wavelength.