Physical Science 1422
Department of Physics and Geology
Ray Tracing
Equipment Needed
Light Source with Mask
Prism
Concave Lens
Spectroscope
Neon Light Source
Qty
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1
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1
Equipment Needed
Triangular Mirror
Protractor
Convex Lens
Color Maker Box
Hydrogen Light Source
Qty
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1
Background
When light strikes a polished glass surface, some of the light bounces off the surface, and some will
enter the glass. The light that bounces off the surface is said to have been “reflected”, or to have
undergone reflection. The light that enters the glass is said to have been “refracted”, or to have
undergone refraction.
When we examine the properties of lenses and mirrors, it is important to first realize that mirrors
work on the principle of reflection, and that lenses (and prisms) work on the principle of refraction.
Lenses and mirrors are often used to magnify or reduce images. To do this the light will be either
focused to a real focal point (like in a concave mirror or a converging lens) or spread away from a
virtual focal point (like in a convex mirror or a diverging lens).
Reflection
All light reflects according to the Law of Reflection, regardless if
it is reflected off polished surfaces, mirrors, or even water. The Law
of Reflection states that the angle of reflection “θr” equals the angle
of incidence “θi”. The angle of incidence “θi” is the angle measured
between the incident ray and the normal N. The normal N is an
imaginary line perpendicular to the reflective surface. The law holds
true even for curved mirrors.
Refraction
When light strikes a transparent material at an angle and some of the
light goes into the material, the path of the light is refracted or bent. The
speed of light in materials slows down and is less than what it is in a
vacuum. This has the effect of bending the path of light as the light enters
a medium and slows down. The more the light slows down, the more it
bends towards the normal line. Light will also refract or bend when
coming out of a medium and speed up. However, in this case it would
refract or bend away from the normal line.
In some materials, different wavelengths (or colors) of light travel at
different speeds. This leads to the slower wavelengths of light being bent
more than the faster ones due to refraction. The net effect is a spreading of
the spectrum. We call this effect dispersion. To enhance dispersion, the
side of glass where the light enters is placed at an angle to the side it exits.
When the glass is cut in a triangle to enhance dispersion we call it a prism.
Figure 3 Illustration of a prism
Light
Light is an electromagnetic wave. This type of wave consists of an electric and a magnetic field
that move simultaneously through space. The electric and magnetic fields are perpendicular to each
other and to the direction the wave is traveling.
This is why light is a transverse wave. The color of light depends on its wavelength (or if you
prefer – on its frequency). Red light has the longest wavelength (and the smallest frequency), and
violet light has the shortest wavelength (and the largest frequency). White light contains light waves of
all wavelengths. If the wavelength of an electromagnetic wave becomes greater than 7.5 x 10-7 meters
or less than 3.8 x 10-7 meters, our eyes can no longer see it.
In a vacuum, all electromagnetic waves travel at the speed of light (3 x 108 m/s). Electromagnetic
radiation is generated whenever electric charges are accelerated. For example, radio waves are produced
when electrons are oscillated back and forth in an antenna.
The “Color” of an Object
An object can absorb light, transmit it and/or reflect it (and/or emit it). How light interacts with a
particular material can depend on its frequency. Some materials absorb light of one color and reflect
light of another color. When our eyes see a blue shirt, this is because the shirt reflects blue light and
absorbs all other colors. A yellow banana appears yellow because it reflects the two primary colors red
and green and absorbs blue light.
SAFETY REMINDER
•
Follow the directions for using the equipment.
For You to Do
The purpose of this laboratory activity is to study the behavior of light as it encounters mirrors,
lenses and prisms. The main concepts to investigate are wave reflection, wave refraction and dispersion
by using ray tracing.
PROCEDURE:
I. Reflection
1) Plane Mirror
1. Place the triangular mirror on top of the shape as indicated.
2. Use the mask to select a single ray of light. This ray will be the incident ray.
3. Draw the reflected ray (don’t forget to put arrowheads on all of your rays)
4. Use a dashed line to indicate the normal line.
5. Label the Angle of Incidence and the Angle of Reflection.
6. Measure and record the Angle of Incidence and Angle of Reflection.
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2) Concave Mirror
1. Use the mask to select five rays of light.
2. Draw the incident and reflected rays (don’t forget the arrowheads).
3. Label the focal point. Indicate if it is real or virtual.
3) Convex Mirror
1. Use the mask to select five rays of light.
2. Draw the incident and reflected rays (don’t forget the arrowheads).
3. Label the focal point. Indicate if it is real or virtual.
II. Refraction
4) A Converging Lens (convex lens)
1. Place the convex lens on top of the shape on the paper.
2. Use the mask to select five rays of light.
3. Draw the incident and refracted rays (don’t forget the arrowheads)
4. Label the focal point. Indicate if it is real or virtual.
5) A Diverging Lens (concave lens)
1. Place the concave lens on top of the shape on the paper.
2. Use the mask to select five rays of light.
3. Draw the incident and refracted rays (don’t forget the arrowheads).
4. Label the focal point. Indicate if the focal point is real or virtual.
III. Dispersion
6) A Prism
1. Place the prism on top of the shape on the paper.
2. Use the mask to select a single ray of light. This ray will be the incident ray.
3. Draw the refracted ray (don’t forget the arrowheads)
4. Show the spectrum and indicate the order of colors.
IV. Combining colors
1) Place the Color Maker Box on your table, turn it on and slowly turn up the white knob on the
box to a reading of about 8.
2) Use the spectroscope to see the colors of the spectrum given off by the lamp.
3) Keep the white light on and turn on the red knob. See the colors using the spectroscope.
4) Keep the white and red lights on and turn the green knob on. See the colors using the
spectroscope.
5) Keep the white, red and green lights on and turn the blue knob on. See the colors using the
spectroscope.
6) Answer the questions of the lab report section.
7) Turn off all the knobs.
8) Place a white sheet of paper in front of the color maker box about 3 inches from the light source
as a screen.
9) Set the knobs to the different combinations listed on the lab report section and write what color
each combination forms.
10) Turn off the color maker.
11) Use the spectroscope to look at the colors that form the spectrum of the lamps provided by your
instructor and record the colors in the space provided.
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V. General Questions on Light
Answer the questions in the lab report section.
VI. Wave Properties of Light
With the aid of the chart below, order different types of light by their wavelength (from shortest to
longest). Next, order them by their frequency (from smallest to largest). Record your results the lab
report section.
Figure 4: Electromagnetic Spectrum
Record your results in the Lab Report section.
Credits:
Figure 3: http://gallery.hd.org
Figure 4: http://en.wikipedia.org
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