What is the effect of light on the weight of a glass prism?

• rohanlol7
In summary, the problem involves an equilateral glass prism illuminated by a 100 W laser beam of wavelength λ = 600 nm. The glass has a refractive index of 1.50 at that wavelength and the light travels parallel to the base of the prism. The objective is to calculate the change in weight of the prism when the beam is switched on. Using the equation P=nhf/t, and understanding that photons have no mass but do have momentum, and that changing momentum requires force, it can be calculated that the force exerted on the prism is vertical due to the change in momentum of the photons as they enter and exit the prism.
rohanlol7

Homework Statement

Figure 1.1 shows an equilateral glass prism illuminated by a 100 W laser beam of wavelength λ = 600 nm. The refractive index of the glass of the prism is 1.50 at λ = 600 nm. The path of the light in the prism is parallel to the base of the prism. The momentum of a photon is h/ λ. Calculate the change in weight of the prism when the beam is switched on.( basically an equilateral triangle and a laser parralell to its base)

P=nhf/t
v(1)/c=1/1.5

The Attempt at a Solution

I tried something which involved giving the photon a mass say M. and calculating the number of photons per unit time. I'm not sure how to continue.
I'm basically practicing for a contest but i don't have solutions available only questions

Hi Rohan,

Photons are known to have no mass. But they do have momentum. Changing momentum requires force. Note the exerise doesn't ask for the change in mass of the prism, only for the change in weight...

BvU said:
Hi Rohan,

Photons are known to have no mass. But they do have momentum. Changing momentum requires force. Note the exerise doesn't ask for the change in mass of the prism, only for the change in weight...

I tried again. This time I'm trying to use forces. As it enters the prism i can calulate the change in momentum of the photons. Then I can relate the change in momentum to the wavelngths. Then I can calculate a value for the Force by calculating the number of photons per unit time from the power. However i can't seem to see why the Force Has to be virtical or am i missing something? I am quite rusty on this topic

Light comes in moving slightly upwards and comes out moving slightly downwards. That's a change in vertical momentum.

CalcNerd
I have reached the abyss of stupidity! Thanks

What is the phenomenon of "Prism gains mass due to light"?

The phenomenon of "Prism gains mass due to light" refers to the observation that when a beam of light passes through a prism, the prism appears to have gained mass or weight.

Why does a prism gain mass when light passes through it?

This is due to the transfer of momentum from the photons of light to the atoms and molecules of the prism, causing them to vibrate and increase in kinetic energy. This increase in energy results in an increase in mass, according to Einstein's famous equation E=mc^2.

Does this mean that the prism actually gains physical weight?

No, the increase in mass is extremely small and not measurable by any physical means. It is a purely theoretical effect based on the principles of relativity and quantum mechanics.

How is this phenomenon relevant to science or everyday life?

While the increase in mass is negligible, the phenomenon itself is important in understanding the behavior of light and matter at the quantum level. It also has practical applications in areas such as spectroscopy and laser technology.

Is this phenomenon unique to prisms or does it occur with other transparent materials?

The phenomenon of mass gain due to light is not unique to prisms and can occur with any transparent material, although the effect may vary depending on the specific properties of the material.

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