* **Emerging ray** (or its extension) = *straight line between impact point and B'*.
* **Emerging ray** (or its extension) = *straight line between impact point and B'*.

##### Determining the incident ray corresponding to any emergent ray
##### Determining the incident ray corresponding to any emergent ray
* **deflection of a ray by a thin optical element** : *local interaction the point of impact* (refraction or reflection) between emergent ray and thin optical element $\Longrightarrow$ *independent of the distance of the image point*.
* **deflection of a ray by a thin optical element** : *local interaction the point of impact* (refraction or reflection) between emergent ray and thin optical element $\Longrightarrow$ *independent of the distance of the image point*.