A fan is configured to generate an air flow by rotating together with a rotation shaft. The fan includes multiple blades arranged radially about a central axis of the rotation shaft and a boss portion that connects the blades to the rotation shaft. The boss portion has an end face portion located on one end side of the rotation shaft. The end face portion has an annular region concentric with the central axis of the rotation shaft. One part of the annular region in a circumferential direction has an optical refractive index different from an optical refractive index of another part of the annular region in the circumferential direction.