The transverse wave in figure (6a-1) produced by
oscillating the free end of rope in the plane of diagram - perpendicular
to its length is not obstructed by the slit, but in figure (6a -
2) it is obstructed by the slit and therefore, the wave does not
pass through the slit. The part of rope beyond the slit remains
at rest along horizontal.

The longitudinal wave in figure 6(b), that is produced
by oscillating the rope along the horizontal direction, is not obstructed
by a slit kept in any position and therefore the wave always passes
through the slit.

Consider that a beam of light, say from sodium
vapor lamp, is incident on a tourmaline crystal. Let another identical
tourmaline crystal be kept behind the first. Observe the brightness
of light as it comes out of the second crystal, which is being rotated
about the path of light, as the axes of the rotation. It will be
observed that the brightness of light is maximum when the reference
axes (called optic axes) of both the crystals are parallel to each
other and the brightness is minimum (almost dark) when the optic
axes are perpendicular to each other. This observations implies
that light is a transverse wave and is polarized by the first crystal
such vibrations are restricted along the optic axes. Therefore,
when the second crystal has its optic axes parallel to the first
the vibrations pass through it, but when the second crystal's optic
axes is perpendicular to that of the first the vibrations cannot
pass through it. (see figure 7a & 7b.)