Light guide, illuminating device for image reading, and image reading apparatus

A light guide for use in an image reading apparatus, the light guide comprising:

• an emitting surface, through which light rays entering the light guide are emitted;
• a reflecting surface configured to reflect the light rays to the emitting surface;
• a cross-sectional surface of the light guide, which is a sectional surface of the light guide along a plane orthogonal to a predetermined direction in which the light guide extends, the cross-sectional surface having a shape comprising a combination of an ellipse and a parabola that is convex outward with respect to the light guide;
• and
• a tapered end portion near an entrance surface through which the light rays enter the light guide
• wherein the parabola on the cross-sectional surface of the light guide defines the reflecting surface;
• a focal point of the parabola is located in the light guide, on a first focal point of the ellipse that is closer to the reflecting surface;
• and
• the tapered end portion is in such a shape that an angle of incidence of a light ray to the thin end portion is greater than a critical angle.