Chapter 18.1 Notes: LIGHT

Chapter 18.1b notes

Plane Mirrors:

Definitions and Concepts:

A plane mirror is a planar reflecting surface, on which specular (regular) reflection is observed.

A virtual image is formed by a plane mirror. The rays reflecting from the mirror appear to have originated from the location of the virtual image.

On a diagram, a virtual image is usually depicted by a broken (dotted) arrow.

A real image (formed by other kinds of optical devices) can be focused on a screen, whereas a virtual image can not. Real images are typically formed by curved mirrors..

On a diagram, a real image is usually depicted by a solid arrow.

In any optical system there are four important image characteristics which need to be considered:

  • magnification
  • attitude (erect or inverted)
  • type (real or virtual)
  • position

The image in a plane mirror is located as far behind the mirror as the object forming it is in front of the mirror. It is the same size as the object. (magnification = +1)

magnification (m) = height of image /height of object   =  Hi / Ho   = - di / do

where di is the distance from the image to the mirror, and do is the distance from the object to the mirror.

The image formed in a plane mirror is erect but laterally inverted. (Lateral inversion is also referred to as "left-right reversal, or "left-right inversion" )

Ray diagrams:

A solid line on a ray diagram is used to illustrate the path of a light ray.

Arrows are used on the solid lines to show the direction of propagation of light.

By convention, incident rays are usually depicted on a ray diagram as travelling from left to right.

A dotted line is used as a construction line. Light rays do not travel along those dotted lines.

Lenses and mirrors are shown in profile on ray diagrams.

Glossary of Terms

Go to part b of ch 18.1

Interactive simulation of ray diagrams

Ray tracing for mirrors:

1.An incident ray that is parallel to the principal axis is reflected such that it passes through the focus (or appears to have originated at the focus).
2.An incident ray passing through (or heading toward) the focus is reflected such that it travels parallel to the principal axis.
3.An incident ray passing through (or heading toward) the center of curvature reflects back along the same path.

Sign conventions:

1.The focal length (f) is positive for converging mirrors and lenses, and negative for diverging ones.
2.The object distance (do) is positive for real images. (The distance is negative for a virtual object.)
3.The image distance (di) is positive for all real images and negative for virtual images.
4.Heights (Ho and Hi) are positive if measured upward from the principal axis and negative if measured downward.
5.Magnification (m) is positive if the image is erect and negative when inverted.