Final answer:
A concave lens refracts incident light rays in such a way that rays parallel to the principal axis diverge to appear to come from the focal point on the same side of the lens. To illustrate this, a ray diagram can be drawn showing the focal point, principal axis, incident ray, and the emergent ray parallel to the axis. The point on the principal axis where the extended incident ray meets it is called the focal point.
Step-by-step explanation:
A ray of light that passes through a concave lens and emerges parallel to the principal axis must have been incident to the lens in such a way that its extension backwards would pass through the focal point of the lens.
To draw the ray diagram for this scenario:
- Draw the principal axis, and the concave lens with its focal points marked on either side.
- Identify the focal point (F) on the same side of the lens as the incident ray.
- Extend a straight line from F to the point where the ray will enter the lens.
- Draw the incident ray so that it follows this line and refracts through the lens.
- Since the emergent ray is parallel to the principal axis, draw the refracted ray straight and parallel to the principal axis after it exits the lens.
The point where the backward extension of the incident ray intersects the principal axis is known as the focal point.