Image Formation In order to form a coherent image, the eye, or some optical device, must focus or converge at least two light rays reflecting off of the same point on that object. The image seen is then the summation of all of the focused rays reflect off of all of the parts of the image.

Virtual images vs. Real images Real Image: Light rays from the same point intersect in real space on a ray diagram. Real images created by single converging lenses are inverted in most cases. The image formed by a human eye is a great example of this… Virtual Image: No actual intersection. Image formed behind lens/mirror.

Eyeball Mirror

Converging Optics: focus parallel light rays to a single point Converging Mirrors Always concave in shape Real images form when o>f Virtual images form when o<f Converging Lenses Always convex in shape Always form real images

Converging lenses Convex surface

Diverging Optics: separate parallel light rays so that they NEVER intersect Diverging Mirrors Always convex in shape Images always virtual Diverging Lenses Always concave in shape Always form virtual images

Diverging Lenses Concave surface

Lens Vocabulary

Relationship between variables Distance between object and optic Focal length Distance between image formed and optic Magnification:

An important note: The image distance for virtual images is ALWAYS negative in the lens equation. Be very careful with this. Dropping this negative can mess up your whole calculation.

Drawing ray diagrams Draw a principal axis such that this line will pass through the center of the optical component, perpendicular to the optic.** Draw the object as an erect (upright) arrow on the left side of the optic. --The height of the arrow will be proportional to the object’s real size and its distance from the optical component proportional to the real distance, o.

Finding images on a ray diagram (Converging optics ONLY) Incident ray parallel to principal axis—comes out through the focal point. Incident ray into optics through focal point—comes out parallel to the principal axis. Draw image at point of intersection of rays. Other rays can be drawn. (e.g., center=straight reflection/refraction, etc..)

Finding images on a ray diagram (Diverging optics ONLY) Incident ray parallel to principal axis—comes out through the focal point. Extend the point of interface “behind” the optic, through the focal point. 2nd ray will be drawn to pass directly through the center of the lens/off the center of the mirror. Draw image at point of intersection of rays.

Note that both types of optical components follow the same rules. We will study these in more detail in the coming days.

Ray Diagrams: A real image… f1 = f2 o = i