Final answer:
To see the back of your head with mirrors, you need at least two placed at a right angle to each other, capitalizing on the properties of reflections from perpendicular surfaces.
Step-by-step explanation:
To devise an arrangement of mirrors that allows you to see the back of your head, the minimum number of mirrors you would need is two. By placing two mirrors at a right angle to each other, you can view the back of your head through the reflections. This setup utilizes the property that two mirrors at right angles can create a third image, as explained by part (a) of Figure 2.4 in your reference material, which results from rays reflecting from both mirrors.
Additionally, as stated by Figure 1.9, when a light ray strikes two mutually perpendicular reflecting surfaces, it is reflected back parallel to the direction it came from. This means that viewing at the right angle will allow you to see an image of the back of your head.