Light guide plate and backlight module

A light guide plate includes a light emitting surface, a bottom surface, a light incident surface and many optical units. Each of the optical units includes a first optical microstructure and at least one second optical microstructure. The second optical microstructure has a first surface and a second surface. A first cross-sectional line obtained by cutting the first surface along a direction perpendicular to the light incident surface and the light emitting surface is an inclined straight line. A first end of a second cross-sectional line obtained by cutting the second surface along the direction perpendicular to the light incident surface and the light emitting surface and a second end of the second cross-sectional line are connected to form a reference straight line. An acute angle included by the reference straight line and a normal line of the light emitting surface is [alpha]. The acute angle [alpha] satisfies an equation: [90 DEG + sin - 1 ( sin [psi] n ) ] 2 - 15 DEG