Download Lab #8 Ray Optics

Ray Optics
Physical Science 101
11/96
Name ______________ Section ________
Partner’s Name _____________________
Purpose:
The purpose of this lab is to study the laws of reflection and refraction for flat surfaces and
to find out how converging lenses and converging mirrors can be used to form real images.
Equipment:
ceiling tile, light source, 5 x 8 note card, object light, frosted glass, paper, plane mirror,
prism, protractor, straight pins, and thin lens
Part 1 Images from a Plane Mirror
Purpose: to understand what an optical image is.
Procedure:
Locate the position of the image of a pin in front of a mirror by following the steps below.
1.
Place a sheet of paper on the ceiling tile. Put the plane mirror on the paper with the object
pin 2 to 3 cm in front of the mirror. Draw a line outlining the surface of the mirror on the
paper.
2.
Place an alignment pin anywhere a little to the left of the object and closer to the mirror.
Look into the mirror at the image of the object pin along a line of sight through the first
alignment pin on the left (see head position on left side of figure below). Place another
alignment pin along your line of sight so that it, the first alignment pin, and the image are
all along a straight line (see figure below). Repeat this process for two alignment pins on
the right side of the object pin.
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Ray Optics
Figure 2
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Mirror and Alignment Pins
3.
Remove the mirror from the paper.
4.
Draw a straight line through the left alignment pins that extends behind the mirror. Draw a
second straight line through the right alignment pins that also extends behind the mirror.
Make a circle where these two lines intersect and label this the image.
5.
Measure the distance to the image from the reflective surface of the mirror, di, and the
distance to the object from the mirrors reflective surface, do.
di = ________________
do = ________________
Questions
1.
Are di and do equal?
2.
If you stand 10 ft in front of a plane mirror, how far away will your image appear to be
from you?
3.
Real images are formed when the rays of light really come together on the screen and
virtual images are where the light appears to come from. Is the image formed by a plane
mirror real or virtual?
4.
In you walk towards a mirror with a speed of 2 m/s how fast do you and your image
approach each other?
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Part 2 Light Refraction through a Prism
Purpose: to show that white light is composed of a spectrum of colors and that each color of
light interacts with the prism material differently.
When light travels through a transparent object its speed is reduced relative to its speed in a
vacuum. This change in speed can cause bending or refraction of light. When you look
into a pool of water things appear at different positions than they actually occupy. The
light coming to your eye from the underwater object is bent.
A very ‘colorful’ phenomena has to do with the fact that different colors of light have
different speeds in transparent objects. The following steps will help you determine what
color of light moves fastest, slowest, and how this determines what rainbows look like. The
index of refraction is related to the refraction angle of the incident light beam. The greater
the angle of refraction, the greater the index of refraction.
Procedure:
1
Shine a ray of light from
the
. light source through a
prism.
.
.
The
. figure to the right is a
top view looking down
from above the table.
2.
Hold a card up on the other side of the prism and adjust the prism until you can see a
rainbow on the card.
3.
Sketch below the rainbow and indicate the colors.
Questions
1.
What color is refracted through the largest angle by the prism? Smallest?
2.
What color has the largest index of refraction? Smallest?
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Ray Optics
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Ray Optics
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Part 3 Lenses and Concave Mirrors
Section a
Thin Lenses
Purpose: To see how converging lenses and mirrors can be used to form real images.
Procedure:
1.
Take a lens, some frosted glass, and a meter stick outside.
2.
Once outside hold the lens in one hand and the frosted glass in the other between your eyes
and the lens. Point the lens at something far away like the chime tower or a tree. Adjust
the distance between the screen and the lens until you get a clear well focused image of the
distant object on the screen. If the object is far enough away then the distance between the
screen and the lens will be equal to the focal length of the lens.
3.
Have your partner carefully measure this distance while you keep the image in focus.
f = ___________ cm
4.
The thin lens equation provides another way of determining the focal length of a lens,
(Method 2). It can be written as,

or
5.
(Method 1)
where
f is the focal length
o is the distance from the object to the lens
is the distance from the image to the lens
(see Figure)
Go back inside and set up the optical bench as shown in Figure 2 using the lens that you
took outside. Adjust the distances between the object (the light with an arrow), the lens,
and the screen (wall) until you get a clear image.
Figure 2
Optical Bench
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Ray Optics
6.
11/96
Measure the distance from the object, o, and the distance from the image, i. Then calculate
the focal length, f (show your work).
image distance: i = _______________ cm
object distance: o = _______________ cm

= ___________ cm
7.
(Method 2, calculation)
Compare the two values you have obtained for the focal length. Find the percent difference
between the measurement and the calculation of the focal length.

_________________
Questions
1.
Was the image formed by the lens real or virtual?
2.
Was the image formed by the lens upright or inverted?
3.
What happens to the image when the top half of the thin lens is covered up? Look
carefully and note all subtle changes
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Ray Optics
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Section b Concave Mirror
Concave mirrors can also be used to find real images on a screen via reflection and the

same equation for focal length,
, works for the mirror.
Procedure
Mirror
Screen
Source
object distance
image distance
Figure 3
Optical Bench
1.
Set up the optical bench as shown in Figure 3 using the concave side of a spherical mirror.
Adjust the distances between the object (the light with an arrow), the mirror, and the screen
until you get a clear image on the screen.
2.
Measure the distance from the object, o, and the distance from the image, i. Then calculate
focal length, f (show your work).
image distance: i = _______________ cm
object distance: o = _______________ cm

= ___________ cm
Questions
1.
Was the image formed by the converging mirror real or virtual?
2.
Was the image formed by the converging mirror upright or inverted?
3.
What happens to the image when the top half of the converging mirror is covered up?
Look carefully and note all subtle changes
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