The RI of pure PMMA is ~ 1.4893 - 1.4899.

Lasers are often used to measure it.

Perhaps this reference will help you.

http://refractiveindex.info/?shelf=organic&book=poly%28methyl_methacrylate%29&page=Szczurowski

RI is a function of wavelength. HOw to determine this value for any particular wavelength?

Use spectroscopic ellipsometer. Many ellipsometer can measure DUV - NIR. Some goes down 190nm. Very few can go down 150nm.

I have a laser of wavelength 1 micron. Can I use this particular laser to determine its RI?

@Neeraj, Can you give me some details about the method or any link?

You could use your laser, irradiate the PMMA surface at a certain angle and measure the reflection. Then, the RI can be calculated on the basis of the Fresnel equations. But you have to consider back side refelctions, these can be avoided by spatial separation of the signals reflected from the back and front side (using a diaphragm) or by applying black lacquer to the back side. I used this method and compared the results to ellipsometric data, it works fine.

Thanks Mr. Christoph. Can you give me some more detail of your work or any link?

Unfortunately, I cannot provide any link since the work is not yet published. However, we worked at an AOI of 8°, so the simplified Fresnel equation for normal incidence can be used. This one is quite easy to solve for the RI and you just have to insert the measured reflectance...