Question

• When you shine a light on an object, the momentum from the photons that make up that light impacts the object. For macroscopic objects, this will have no measurable effect. Describe why this is different for atomic-sized objects.
• Suppose you shine a very long wavelength light on one electron and a very short wavelength light on another electron. What differences will you observe?
• Why does shining very short wavelength photons on an electron not tell you exactly where the electron is?
• Describe two other examples of situations in which measuring something about an object somehow changes it.

Answer 1

The momentum of photons is very less to have measurable impact on mactroscopic objects. However for atomic sized object the photon momentum is large enough to impact the atomic sized object. For example the photon hitting an electron can change the speed as well as direction of the electron. This is known as Compton scattering, after hitting the electron, the incident photon loses some energy and its frequency changes due to the collision. The change in frequency or energy of the wavelength depends upon the mass of the scattering particle, in this case, electron.

Shining a long wavelength light on an electron does not change the position or momentum of the electron as the long wavelength photon will have less amount of energy to change the momentum of the electron. However if we shine a short wavelength light, for example an X ray, then the X, ray photon will scatter the electron and the electron initially at rest will be scattered away and gain momentum and therefore we will not be able to measure its position because of scattering by the X-ray photon.

A large wavelength photon does not interact with the electron while a short wavelength photon interacts with the electron because the wavelength of the short wavelength photon is of the order of the size of the electron. Hence we can say that the scattering effect is observed when the size of the object is of the order of the wavelength of the light.

This is a common phenomenon in quantum mechanics, that when we try to measure the position of a small object, then we need to shine light on the object so as to see it, but when we shine light on the object the light disturbs the postion of the object and hence we are not able to see the object at its original position. It is also known as the uncertainity principle, it states that one cannot measure the postion and momentum of an atomic particle simultaneously. Hence if we want to measure the momentum of the quantum particle we will lose information on its postion and if we want to measure the position of a quantum particle we will lose the information about its momentum or velocity.