## SI units

emmaferry2D
Posts: 129
Joined: Wed Sep 30, 2020 9:47 pm

### SI units

I'm confused on whether we use nm or m when we have wavelength in an equation. I assumed it was nm since Lavelle gave us the wavelengths of the visible light spectrum in nm, however when I was working on the homework this week, my answer was incorrect because I used nm instead of converting it to m. Can anyone help?

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### Re: SI units

You have to use meters because the other values are are in meters and ultimately, the units need to cancel. For example, the speed of light is 3.00E8 m/s. To make sure the units cancel in the equation $c=\lambda \nu$, you would need your wavelength in meters as well, since the frequency is in Hz, or s^-1.

Hope this helps!

Crystal Yu 1D
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### Re: SI units

Usually in the equations, you have to use whatever units the constants have in order to cancel them out. For example, Joules are kg·m^2·s^-2, so when there's Joules in the equation (Plank's constant), you need to use kg, meters, and seconds. However, sometimes the question will ask you to convert from m to nm after you have solved for an answer.

Emmeline Phu 1G
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### Re: SI units

Hi! Wavelength is usually measured in m, which is one of the basic SI units. Constants such as the speed of light (3.0 x 10^8 m.s-1) and Planck's constant (6.626x10^-34 kg.m^2.S^-2) are given in units of meters; therefore, in order for the units of the values to cancel out with each other during calculations, the unit m must be used. Hope this helps! :)

Kailani_Dial_2K
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### Re: SI units

Hello, so everyone who has answered meters above is absolutely correct. I would like to add a small explanation as to why the professor gives the answers in nm. So, he has said in lecture before that it is for convenience. Saying something like 178 nanometers is easier than having a mouthful of 1.78 *10^-10 m . The calculations that you go through have to be in meters that way your units cancel according to SI units, but for the purpose of convenient reporting sometimes it is easier to give the answer in nm rather than m.

Eunice_Castro_1G
Posts: 91
Joined: Wed Sep 30, 2020 9:46 pm

### Re: SI units

Hi! Wavelengths are supposed to be measured in meters (because of SI units), but Dr. Lavelle often writes it out in nm since it is just "easier" to say it than having to write x10-^9 m every time. Hope this helps!

gabbi_r2C
Posts: 91
Joined: Wed Sep 30, 2020 9:52 pm

### Re: SI units

Hi there! Seems like everyone is sort of in agreement about meters being used, and this is because you need to use meters to cancel or match with the units in the constant of the calculation, otherwise your numbers will be off.

Posts: 93
Joined: Wed Sep 30, 2020 9:49 pm

### Re: SI units

Hi! You should use meters because when you calculate the equations, the units will cancel out so you're left with meters. If the question asks you for the answer in nm, you would then convert m to nm.

MMorcus2E
Posts: 96
Joined: Wed Sep 30, 2020 9:47 pm

### Re: SI units

Hey! Meters are the correct units to use because of the SI system but nanometers are just easier to write on a more casual basis.

ShinwooKim_3E
Posts: 90
Joined: Wed Sep 30, 2020 10:00 pm

### Re: SI units

You have to use meters for this equation!