Because a reflection reverses the direction of the component of the wave vector of the light hitting the reflecting surface which is orthogonal to the surface. The component parallel to the surface will not change.
This means the light going towards a mirror, will go away from the mirror after reflection. But if it went from the left to the right, it will continue to go from the left to the right. Same with up and down.
In three dimensions this is the same as changing the handedness of the image.
It is a figure that is the same as another figure in the plane. A square is the same plane figure as another square, but a cube is same the same plane figure even tho it is made up of 6 squares.
An 8-sided plane figure is an octagon.
Reflection in the line.
A quadrilateral is a closed plane figure with four straight sides.
A three sided closed plane figure (shape) is a triangle.
This is a transformation which could be a rotation, translation or reflection.
A reflection in a line l is a correspondence that pairs each point in the plane and not on the linewith point P' such that l is the perpendicular bisector of segment PP'. IF P is on l then P is paired with itself ... Under a reflection the image is laterally inverted. Thus reflection does NOT preserve orientation...
Rotation, in the plane of the grid, through 180 degrees.
reflection
It is moved across the plane. Its size or orientation are not changed.
A transformation is moving or changing the shape of a figure on the Cartesian plane by a translation, by a reflection, by a rotation or by an enlargment
It could be a reflection with the mirror line outside the figure; it could be a rotation with the centre of rotation outside the figure; or it could be a translation.
plane figure
angle of incidence is equal to angle of reflection.
Area of plane figure
No it is not a plane but it is a solidfigure
rotation, reflection, translation, glide reflection