Plane Mirror Image - Light & Optics Step by Step

A plane mirror image is upright and the same distance behind the mirror as the object is in front.

Example

A plane mirror image is upright and the same distance behind the mirror as the object is in front. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Measure the object side

Place the object in front of the plane mirror. Its distance from the mirror is 4 metres, and its height is 2 metres.

d_{\text{object}} = 4\ \text{m}\qquad h_{\text{object}} = 2\ \text{m}

Back-projections meet behind the mirror

The reflected rays spread out in front of the mirror. If you trace them backward, the dashed lines meet behind the mirror. That meeting point is the virtual image.

\text{reflected rays spread; back-projections meet}

Distance behind matches distance in front

Keep the object height fixed and move the object. A plane mirror puts the virtual image the same distance behind the mirror as the object is in front.

\begin{array}{c|c}d_{\text{object}}&d_{\text{image}}\\2\ \text{m}&2\ \text{m}\\4\ \text{m}&4\ \text{m}\\6\ \text{m}&6\ \text{m}\\\end{array}

Image height matches object height

Now keep the object distance fixed and change the height. The plane mirror keeps the image upright and the same size in every row.

\begin{array}{c|c}h_{\text{object}}&h_{\text{image}}\\1\ \text{m}&1\ \text{m}\\2\ \text{m}&2\ \text{m}\\3\ \text{m}&3\ \text{m}\\\end{array}

The distance and height match

For a plane mirror, the image is 4 metres behind the mirror, matching the object distance in front. The image height is also 2 metres, so the image stays upright and the same size.

d_{\text{image}} = d_{\text{object}} = 4\ \text{m}\qquad h_{\text{image}} = h_{\text{object}} = 2\ \text{m}

optics The image is virtual: reflected rays do not pass through it, but their back-projections meet there.