# SI unit for refractive index

1. ### kindaichi

25
I am doing my homework and i come across a question.

What is the SI unit for refractive index?All my revision books did not have the answer.Or perhaps the book is wrong by any chance?

2. ### Integral

7,346
Staff Emeritus
Considering that the index of refraction is defined as a ratio of velocitys, What units MUST it have? What are the units of ANYthing defined as a ratio?

3. ### kindaichi

25

Hhmm..................no units?Sorry,i am a bit blur here.

4. ### zairiramli

1
hi kindaichi... for you info, refractive index have no si units, since from the definition refractive index is the ratio of velocity of light in a material divided with velocity of light in vacuum. this is what is meant by "integral". as it is a ratio between velocity of light in material vs velocity of light in vacuum, both unit for the velosity was cancelled out and the only thing left is the number which indicates the ratio. and that ratio is the refractive index. but there are a lot of expression can described the refractive index. for example in brix percent. the expression is depend on the application.

5. ### Hunterbender

54
Just adding on what's been said.

When I see these unit problem, I tend to look at the equations. For example, F = ma..even if I don't know the unit for force..I can derive it.

kg *m/s^2 is the equiv. of ma...so I know the unit for force must be kg m/s^2 (which we simplify and call it Newton, but both are correct).

Try that next time

6. ### lanca

1

For a RI detector, Refractive Index Range: 1 - 1.75 RIU.....what RIU mean?

7. ### dano603

1
RIU: Refractive Units Index. It is used in optical biosensing, for evanescent wave sensors (optical waveguides, ring resonators, interferometers, surface plasmon resonance). It is the minimun detectable change in the refractive index of the surface where the evanescent wave is travelling.

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