No, electromagnetic waves, including infrared (IR) waves, are not necessarily emitted perpendicular to the emitting surface. The direction of propagation of an electromagnetic wave depends on various factors, such as the geometry and orientation of the emitting source.
In the case of a flat sheet of metal emitting infrared waves, the wavefronts would typically take the shape of plane waves. Plane waves have wavefronts that are flat and parallel to each other, extending infinitely in the direction of propagation. These waves propagate in a direction perpendicular to the wavefronts.
However, it's important to note that if the emitting surface is not completely flat or uniform, the wavefronts may not be perfectly planar. The shape of the wavefronts can be affected by irregularities or variations in the emitting surface.
Additionally, if the emitting source has a different geometry or structure, such as a curved surface or an irregularly shaped object, the wavefronts can take different shapes, such as spherical waves or waves with more complex wavefront contours.
In summary, while a flat sheet of metal emitting IR waves would generally produce plane wavefronts, the exact shape of the wavefronts can vary depending on the characteristics of the emitting surface and the specific circumstances of the emission.