Figure P-4 – Focus of a Biconvex Lens

Focus: When light enters a lens it is bent or refracted. Biconvex lenses were designed to bend parallel incoming rays (like the rays of the sun in this illustration) so that they all meet at a single point. That point is the focus. The focal length of the lens is the distance from the centre of the lens to the focal point.

 

Role in the Optical Machine: Focal length is important for understanding how the optical machine is supposed to work. For that you need to read the path of light in in this diagram in reverse. The image is located at the point of focus and the observer stands where the sun is. Rays from each point on the image surface fan out and then are bent parallel by the lens as they travel towards the viewer's eyes.

 

Here is how the lens of the optical machine affects the orientation of the two eyes.

 

Here is how the lens of the optical machine affects the focus of the single eye.

 

When a picture is placed at the focal distance from the lens, the things depicted in it should, according to 18^th-century theory, be perceived as though they were an infinite distance away.

 

For the purposes of these simple optics we can treat the sun as effectively at an infinite distance.