CK-12-Physics - Intermediate

20.4. Thin Lenses http://www.ck12.org

Drawing ray diagrams for converging lenses: Real images

Three principal rays can be drawn to construct ray diagrams for convex lenses, as shown inFigure20.18.

1. A ray parallel to the principal axis is refracted through the opposite focal point.


2. A ray passing through the optical center of the lens does not refract.


3. A ray passing through the focal point on the same side of the lens as the object is refracted parallel to the
   principal axis.

The intersection of the rays shows where the image is formed.

FIGURE 20.18

Note the similarities in ray construction between the concave mirror and the convex lens.

Again, with a bit of geometry, we can demonstrate that the object distancedo, the image distancedi, and the focal
lengthfare related by the Lens Equation, shown below:
1
do+

1

di=

1

f

This is the same equation as the Mirror Equation. All distances are measured with respect to the lens.

Drawing ray diagrams for converging lenses: Virtual images

Ray 1: A ray parallel to the principal axis is refracted through the opposite focal point.

Ray 2: A ray that passes through the focal point on the left, as shown inFigure20.19, refracts parallel to the
principal axis.

Ray 3: There is no refraction of a ray passing through the center of the lens. The ray passes straight through the lens.

The intersection of the rays shows where the image is formed.

Sign conventions for converging lenses (single lens system)

1. Distances are measured from the lens.


2. The focal length is positive.