Strain sensor and method of measuring strain amount

A strain sensor comprising:

• a light source which emits light;
• a marker which is disposed on a surface of a measuring object in such a way that the light emitted from the light source is reflected by or transmitted through the marker;
• a detector which detects a light intensity of the light reflected by or transmitted through the marker;
• a signal processor which calculates a strain amount based on the light intensity detected by the detector, wherein
• the marker is a flat film comprising a first medium and a second medium having different refractive indexes;
• the second medium is periodically arrayed in the first medium and exists simultaneously with the first medium on a plane parallel to a mounting surface on which the marker is disposed;
• a maximum length of the second medium in a direction parallel to the mounting surface is shorter than a wavelength of the light emitted from the light source;
• and
• the first medium and the second medium deform in response to a load in a direction parallel to the mounting surface.