Instructor Outline: Optics-Image Formation
UM Physics Demo Lab 07/2013
Lab length: 70 minutes
Lab objective: To instruct the students about geometrical optics: ray tracing and the
formation of real and virtual images by a pinhole and a thin converging lens. Also to
demonstrate practical instruments based on a single thin lens: light house, slide
projector and camera.
Materials
40-Watt “candle” lamp on wooden base
Power strip
Mesh stand for candle lamp
Cookie sheet pinhole projector
White cardboard projection screen
Desk lamp with 100 watt soft white bulb (“slide projector” light source)
Transparency-film “slide”
Hand-held magnifying glass
Reading glasses
Calculator
2 Meter sticks
Clear plastic ruler
4 Large binder clips (support legs for pinhole projector and screen)
Small rubber band
Exploration: 15 minutes - Group Lab Work
The students first trace three rays from an object through a pinhole and predict that
the pinhole should produce an inverted real image. They then observe the images
produced by a glowing filament through a small, medium and large pinhole and
compare the brightness and focus of the three images. Next they explore the depth
of field and magnification by varying the distance of the lamp to the projector.
Analysis: 5 minutes – Lecture
The concepts of reflection, refraction, refractive index, focal length, depth of field
and magnification are discussed. Real and virtual images are also discussed.
Exploration: 20 minutes – Group Lab Work
The students first measure the focal length of a thin converging lens by measuring
the image distance for a distant lamp. Next they predict what will happen when they
place the lamp near the focal point of the lens and then observe that placing the
lamp near the focal point of the lens will project the image of the lamp filament on a
distant wall. Finally they design and test a “slide projector” made from a 100 watt
desk lamp, a transparency film slide and a magnifier.
Analysis: 5 minutes – Lecture
The Thin Lens Equation is introduced and related to the thin-lens Exploration results.
The f/number and diopter scale are defined.
Application: 25 minutes – Group Lab Work
The students apply the thin lens equation for the magnifying glass with a screen to
lamp distance of 1.5 meters and verify that there are two locations that produce an
image of the lamp and that the focal length predicted by the thin lens equation is in
reasonable agreement with that obtained by measuring the image distance for the
distant lamp. Next they compute the f/number and diopter magnification for the
magnifying glass and the f/number for a pinhole camera. They then observe that
the virtual magnified image of a magnifying glass is produced for objects inside the
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
focal point of the lens. Finally, they measure the diopter magnification for a pair of
reading glasses, again using the distant lamp method.
Suggested Demos:
6A61.10 Pin Hole Camera
Home-made shoe box pinhole camera with five-LED flashlight.
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. Images where the light actually emanates from the location of the image are
called real images. Images where the light only appears to emanate from the
image location but does not actually do so are called virtual images. The
image of a lamp filament projected by a thin converging lens is real. The
image in a plane mirror is virtual.
3. Ray tracing demonstrates that pinholes can produce an inverted, real image.
Small pinholes produce well focused, dim images. Large pinholes produce
bright, poorly focused images. The depth of field for a point pinhole is
infinite, and real pinholes have a very large depth of field. The f/number of a
pinhole or lens is defined as the ratio of its focal length to its diameter. The
focal length of a pinhole is the distance from the pinhole to the image screen.
A true pinhole (geometric point) therefore has an “infinite” f/number (f/∞)
(owing to a zero diameter) and real pinholes have very large f/numbers.
4. Convex lenses can focus parallel light rays to a common focal point and can
therefore produce real images. Concave lenses always produce diverging
rays and can only produce virtual images.
5. 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.
6. 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 and is described by Snell’s Law.
7. The Thin Lens Equation relates the image distance (i), object distance (o) and
focal length (f) for a thin lens and is stated as: 1/i + 1/o = 1/f. This
equation predicts that light from very distant objects (0→∞) will converge to
form a real image at the focal point of a converging lens and that
interchanging the image and object distances will again result in a focused
image. It also predicts that an object placed at the focal point of the lens
(o=f) will project an image “to infinity” (1/i→0 so i→∞), that is, to a very
distant screen (slide projector). Real images produced by a thin converging
lens are inverted (“upside-down”) Objects viewed with image distances less
than the focal length of the lens will produce an upright (“right-side-up”)
virtual image of the object—this how a converging lens is used as a
“magnifying glass”. The thin lens equation also works for diverging lenses
which have a negative focal length.
8. The magnification for any thin lens or pinhole is defined as –(i/o) where the
minus sign predicts that real images are inverted and virtual images (which
have negative image distances) are upright.
9. 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. 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.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109