The grid below has 5 shaded squares, and a mirror stands along the dashed line MN on its right.

Explanation

1. 01

   A mirror standing along MN, to the right of the grid, swaps left and right but not top and bottom: a square in column c moves to column $6 - c$ and stays in its row.

2. 02

   So the shaded squares (1, 3), (1, 4), (1, 5), (2, 4), (2, 5) (row, column) move to (1, 1), (1, 2), (1, 3), (2, 1), (2, 2).

3. 03

   Figure 1 shows exactly these squares.