Because there are no angle of deviation in it due to the parallel sides of the mirror?

Introduction:
When light passes through a flat pane of glass, we do not see a spectrum of colors because of the phenomenon known as refraction. Refraction occurs when light waves change direction as they pass from one medium to another, such as from air to glass. This phenomenon is responsible for various optical effects, including the dispersion of light into its constituent colors. However, in the case of a flat pane of glass, the refraction is minimal, leading to the absence of a visible spectrum of colors.

Explanation:
Refraction is the bending of light as it passes from one medium to another due to a change in its speed. When light travels from air to glass, its speed decreases, causing it to change direction. The amount of bending depends on the angle at which the light ray strikes the surface of the glass and the refractive index of the glass.

Minimal refraction:
In the case of a flat pane of glass, the light rays pass through the glass parallel to each other and perpendicular to the glass surface. This means that the angle of incidence is zero, resulting in minimal refraction. Since the light rays do not change direction significantly, they remain relatively undisturbed, and the spectrum of colors is not separated.

Refraction and dispersion:
When light passes through a prism or a lens, which have curved surfaces, the refraction is more pronounced. The curved shape causes different parts of the light wave to bend at different angles, leading to the separation of colors. This separation is known as dispersion, and it occurs because different colors of light have different wavelengths and thus different refractive indices.

Conclusion:
In summary, when light passes through a flat pane of glass, the minimal refraction prevents the separation of colors into a spectrum. The absence of a visible spectrum is due to the parallel and perpendicular nature of the light rays passing through the glass. It is only when light encounters curved surfaces, such as in prisms or lenses, that the refraction becomes significant enough to disperse the light and create a visible spectrum of colors.