# Quantisation of light and Doppler effect

• B
• Vibin Narayanan
In summary, the doppler effect of light can be explained by the fact that light consists of quanta, or small packets of energy, and the frequency and energy of these photons are frame-dependent. This means that the energy of the light can appear to change depending on the motion of the source or observer. However, this does not violate conservation of energy as it is dependent on the frame of reference. This can be seen in the example of two cars traveling at different speeds, where a small change in velocity does not result in a significant change in energy.
Vibin Narayanan
Light consists of quanta(small packets of energy). Then how do we explain doppler effect of light the same way we do for sound? What is the valid explanation of light doppler effect which is consistent with the photon picture?

Vibin Narayanan said:
Light consists of quanta(small packets of energy). Then how do we explain doppler effect of light the same way we do for sound? What is the valid explanation of light doppler effect which is consistent with the photon picture?
The frequency of the photon (and hence its energy) is frame-dependent.

DrClaude said:
The frequency of the photon (and hence its energy) is frame-dependent.
That's it? I simply didn't get it. Energy changing with the motion of source or observer or both, doesn't violate conservation of energy?

Energy is conserved, but it is dependent on the frame. For example, I will measure a ball moving relative to me as having kinetic energy, but in a frame co-moving with the ball, it will have none.

Vibin Narayanan said:
That's it? I simply didn't get it. Energy changing with the motion of source or observer or both, doesn't violate conservation of energy?

Nope. Consider yourself in a car moving down the road. If another car comes up behind you 1 mph faster than you're going and taps your bumper, your car is not totaled, despite the fact that you both may be traveling 70+ mph.

## 1. What is quantisation of light and how does it relate to the Doppler effect?

Quantisation of light refers to the concept that light is composed of discrete packets of energy called photons. The Doppler effect, on the other hand, describes the change in frequency of a wave due to the motion of the source or observer. Together, they explain the phenomenon of redshift and blueshift, where the frequency of light is shifted towards the red or blue end of the spectrum, respectively, when the source or observer is in motion.

## 2. How does the Doppler effect affect our perception of light from distant objects?

The Doppler effect can cause the light from distant objects to appear shifted towards the red end of the spectrum, known as redshift. This is because the universe is expanding, causing distant objects to move away from us and thus their light to be stretched to longer wavelengths. This redshift is used as evidence for the expanding universe and the Big Bang theory.

## 3. Can the Doppler effect be applied to other types of waves besides light?

Yes, the Doppler effect can be applied to any type of wave, including sound waves. In fact, the Doppler effect was first observed in sound waves, such as the change in pitch of a siren as an ambulance passes by. The principle is the same, where the frequency of the wave is affected by the motion of the source or observer.

## 4. How does the quantisation of light explain the photoelectric effect?

The photoelectric effect is the phenomenon where electrons are emitted from a metal surface when light is shone on it. This can be explained by the quantisation of light, as the energy of the photons of light must be high enough to overcome the binding energy of the electrons in the metal. If the energy of the photons is too low, no electrons will be emitted.

## 5. What practical applications does the understanding of quantisation of light and the Doppler effect have?

The understanding of quantisation of light and the Doppler effect has many practical applications. For example, it has been used in astronomy to study the movement and composition of distant galaxies. In medicine, the Doppler effect is used in ultrasound technology to measure blood flow and diagnose medical conditions. Additionally, the quantisation of light has led to the development of technologies such as lasers, solar cells, and LED lights.

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