When a glass fragment in silicon oil is heated and cooled, which indicates that the refractive index of the glass has been found?

Explanation:

The Beck Line test is a technnique used in optics. This test heps to determine the refractive index of a material in relation with other material. Basically, it consists in comparing a known refractive index with the unknown refractive index.

The beck line refes to a band formed by the refractive index of both materials, if the band doesn't disappear, it means those materials have differnece index. However, if the band disappears, it means both materials have the same refractive index.

Therefore, the right answer is "the disappearance of beck lines indicates that the refractive index has been found".