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### equations that apply to light only/everything else but light

Posted: **Sat Apr 21, 2018 4:06 pm**

by **Tina Wen 1G**

During Friday class the professor mentioned E=hv only applies to light and lambda=h/p applies to everything else with mass but light? I wonder why is that?

### Re: equations that apply to light only/everything else but light

Posted: **Sat Apr 21, 2018 4:32 pm**

by **Ellie Tsang 1I**

I'm confused about that too. I read that light doesn't have mass because it's made up of photons, which is massless and only energy and momentum. The momentum equation involves mass, so I don't get how a photon is massless.

### Re: equations that apply to light only/everything else but light

Posted: **Sat Apr 21, 2018 6:52 pm**

by **QuincyH1G**

Does light even have momentum?

### Re: equations that apply to light only/everything else but light

Posted: **Sat Apr 21, 2018 8:10 pm**

by **Fiona Grant 1I**

I don't think that light has momentum (p), because p = mv, and while light has velocity, it does not have mass. So the equation would not apply in this case. I am not sure though.

### Re: equations that apply to light only/everything else but light

Posted: **Sat Apr 21, 2018 11:46 pm**

by **Jack Martinyan 1L**

The photon is a massless particle. According to theory it has energy and momentum but no mass, and this is confirmed by experiments. Light carries momentum and will exert pressure on a surface. This is not evidence that it has mass since momentum can exist without mass. Sometimes people like to say that the photon does have mass because a photon has energy E = hv where h is Planck's constant and v is the frequency of the photon. Energy is equivalent to mass according to Einstein's formula E = mc2. They also say that a photon has momentum, and momentum p is related to mass m by p = mv. These formulas are referring to "relativistic mass", an old concept that can cause confusion. Relativistic mass is a measure of the energy E of a particle, which changes with velocity. Relativistic mass is not usually called the mass of a particle, so it is wrong to say the photon has mass in this way. In modern terminology, the mass of an object is its invariant mass, which is zero for a photon.

### Re: equations that apply to light only/everything else but light

Posted: **Sun Apr 22, 2018 9:52 pm**

by **Moris 1H**

How can something have momentum without mass? if mass is 0 wouldn't momentum also be 0?

### Re: equations that apply to light only/everything else but light

Posted: **Sun Apr 22, 2018 10:22 pm**

by **Kalsuda Lapborisuth 1B**

But if light has momentum as you said, wouldn't De Brogile's wave equation be applicable to photon also? I would like further clarification

on whether lambda=h/p applies to photon particles or not.

### Re: equations that apply to light only/everything else but light

Posted: **Tue Apr 24, 2018 5:44 pm**

by **Jack Martinyan 1L**

It does not apply to photons

### Re: equations that apply to light only/everything else but light

Posted: **Tue Apr 24, 2018 6:06 pm**

by **Chem_Mod**

Moris 1H wrote:How can something have momentum without mass? if mass is 0 wouldn't momentum also be 0?

Think of it as energy momentum.

In class I gave the example of light hitting the surface of a leaf. When sunlight hits a plant leaf energy is transferred (and there is a relationship between energy and momentum).