---
title: 'new course  : overview'
---

### The mirror

#### What is a mirror ?

##### Objective

* initial : to **reflect** light, to **focus or disperse light**.
* Ultimate : to **realize images**, alone or as part of optical instruments.

##### Physical principle

* uses the **phenomenon of reflection**, described by the law of reflection.

##### Constitution

*  Usually plane or curved (spherical for the most simple to realize,
parabolic or elliptical) **surface, highly polished** so that its surface 
state deviates from its theoretical form of less than $`\lambda/10`$ at each point
of its surface ($`\lambda`$ being the wavelength in vacuum of the light to be reflected).
To increase the reflectivity of the mirror (percentage of reflected light 
intensity per the total of the incident light intensity), the surface is
**most often metallized**.

##### Interest in optics

* **One of the most importante simple optical component** that is used *alone or 
* combined in a series in most optical instruments* : some telephotos, 
* reflecting telescopes.

#### Why to study plane and spherical mirrors?

* **Plane and spherical mirrors** are the *most technically easy to realize*, 
so they are the *most common and cheap*.
* In paraxial optics, the optical properties of a **plane mirror** are those 
of a *spherical mirror of infinite radius of curvature*.
Plane mirror, concave and convex spherical mirror

![](plane-concave-convex-mirrors.png)
Fig. 1. a) plane    b) concave    c) convex   mirrors

#### Are plane and spherical mirrors stigmatic?

##### Perfect stigmatism of the plane mirror

* A plane mirror is **perfectly stigmatic**.
* Object and image are symmetrical on both side of the surface of the plane mirror.
* A real object gives a virtual image.<br> A virtual object gives a real image.

##### Non stigmatism of the spherical mirror

* In each point of the spherical mirror, the law of reflection applies.
* A spherical mirror is not stigmatic: The rays (or their extensions) 
* coming from an object point generally do not converge towards an image 
* point (see Fig. 2.)






