Optical element and optical sensor

A method of measuring a refractive index,

• in which a liquid crystal layer is formed of a composition including a liquid crystal compound,
• the liquid crystal layer has a liquid crystal alignment pattern in which a direction of an optical axis derived from the liquid crystal compound changes while continuously rotating in at least one in-plane direction,
• the liquid crystal compound is twisted and aligned in a thickness direction,
• the liquid crystal layer further has a resonance structure,
• a relationship between a refractive index of a material disposed on the liquid crystal layer and a position of a peak wavelength of reflected light is grasped in advance as reference data,
• an object to be measured of which a refractive index is not known is disposed on the liquid crystal layer and a position of a peak wavelength of reflected light is measured to acquire the refractive index of the object to be measured, and
• wherein in the liquid crystal layer, a twisted angle of the liquid crystal compound in the thickness direction is 45° to 120°.