Instructor Outline: Optics
Revised 04/09 A. Tomasch
Lab length: 70 minutes
Lab objective: To instruct the students about geometrical optics, reflection and
refraction, refractive index, dispersion and chromatic aberration.
Materials
1 ray box
1 box optical materials
-slit screen
-mirrors: flat, concave, and convex
-lenses: shallow and deep concave, convex, prism, trapezoid, hollow concave and
square
1 clear ruler
Exploration stage: 25 minutes - Group Lab Work
The students work in groups to observe reflection. They test mirrors of various
curvatures and observe converging and diverging rays. They deduce the “Law of
Reflection.”
Analysis stages: 15 minutes – Lecture
Concept development is done on reflection, refraction, refractive index, converging
and diverging optics, dispersion and chromatic aberration.
Application stage: 30 minutes – Group Lab Work
The students observe refraction through plastic. The observe dispersion and
chromatic aberration. They compare concave and convex lenses and mirrors. They
observe focusing optics and study the concepts of focal point and focal length.
Suggested Demos:
Diffraction of a laser pointer off of a CD and DVD to illustrate the wave nature of light
6D10.10 Two-Slit Laser Diffraction to illustrate the wave nature of light
6A61.10 Pin Hole Camera
Concepts developed:
1. The law of reflection states that the angle of reflection is equal to the angle of
incidence for a reflected ray of light. The angles are defined with respect to
the direction perpendicular to the reflecting surface called the normal.
2. Concave mirrors focus rays so that they converge at a common focal point in
front of the mirror. Convex mirrors produce diverging rays with a focal point
(the origin of all the diverging rays) behind the mirror.
3. The speed of light in a material such as glass, water or plastic is slower than
it is in vacuum. The ratio of (speed of light in vacuum)/(speed of light in
matter) is called the refractive index of the material.
4. The direction of a light ray changes as it enters a material with a higher
refractive index such that its angle with the normal decreases. This is how
lenses change the direction of light rays.
5. Convex lenses produce converging rays brought to a common focal point.
Concave lenses produce diverging rays.
6. The speed of light, and hence the index of refraction, is different in optical
materials such as glass or plastic for different wavelengths of light. This
effect is called dispersion.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
7. Blue light (short wavelength) is refracted (bent toward the normal) more than
red light (long wavelength). This is how a prism separates white light into its
component colors.
8. The dispersion of the glass or plastic material used to construct a converging
(convex) lens causes the different colors of light to have different focal
lengths (blue light has a shorter focal length than red light). This causes
lenses to exhibit chromatic aberration, where rings of color appear around the
image focused by the lens. For this reason, lenses are coated with additional
layers of different clear material with different refractive indices to correct for
this effect and bring all colors to a common focus.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109