Reflection and Refraction
Investigation
25B
25B Reflection and Refraction
How does light behave when its path is changed?
Looking in a mirror we see a twin of ourselves
reversed left-to-right. A fish underwater appears
in a different place from where the fish really is.
Both of these illusions are caused by the
bending of light rays. This Investigation
explores reflection and refraction, two
processes that bend light rays. You will also use
refraction to calculate the focal point of a lens.
A
Materials
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Optics with Light & Color
Protractor & metric ruler
Index card
Thin tipped white board marker
Graph paper (2 pieces)
Sheet of blank white paper
Observing the law of reflection
1
2.
3.
4.
5.
Set a sheet of graph paper on your lab table.
Slide the laser into a light stand, turn the laser on and put it on the graph paper.
Align the laser so the beam follows one horizontal line across the paper.
To line up the beam with the horizontal line on the
graph paper, place an index card on its side onto the
paper. Keep its long side flat on the graph paper and
slowly slide the card over until you can just start to see
the beam on the very edge of the card. Follow the
vertical edge of the index card down and make a mark
on the paper. The mark tells you where the beam is,
and you can use this method to trace the path of the
beam in all the Optics with Light & Color activities.
6. Follow the beam as it moves across the graph paper
and make sure it lines up with one of the horizontal
lines on the paper.
7. Set the mirror on the graph paper so the light beam
from the laser hits its shiny front at an angle. The
mirror should be placed so its long side is down on the
paper.
8. Draw a line on the graph paper marking the front face of the mirror.
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Investigation
25B
Reflection and Refraction
9. Use a pencil and the index card to trace the light rays going toward and away from
the mirror. Label it ray #1.
10. Draw small arrows every couple of inches indicating the direction the beam is
traveling.
B
Thinking about what you observed
a.
A diagram showing how light rays travel is called a
ray diagram. Lines and arrows on a ray diagram
represent rays of light.
b.
Look at your ray diagram showing the surface of the
mirror and the light rays before and after the mirror.
c.
Which is the incident ray? Label it on your ray
diagram.
d. Which is the reflected ray? Label it on your ray
diagram.
C
The law of reflection
a.
Move the laser and the mirror to a new location on the graph paper. Make sure the beam
hits the mirror at a slightly different angle and repeat steps 7–10. Be sure to label each
ray. Continue shining and tracing the beam from a total of 4 different locations. Label the
rays 1–4.
b.
For each ray diagram, draw a line
perpendicular to the mirror surface at the
point where the rays hit. This line is called
the normal line.
c.
Use a protractor to measure the angle
between the normal and the incident and
reflected rays. Record your measurements
in Table 1.
d. Write down your own statement of the law
of reflection, describing the relationship
between the angles you measured.
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Reflection and Refraction
Investigation
25B
e. A laser shines at a mirror at an angle of incidence of 75 degrees. Predict its angle of
reflection. After predicting, test your prediction. Were you right?
Table 1: Angles of incidence and reflection
Diagram #1
Diagram #2
Diagram #3
Diagram #4
Angle of incidence
Angle of reflection
D
Light rays going through a prism
A prism is a solid piece of glass with polished surfaces. Prisms are useful for investigating how
light bends when it crosses from one material into another, such as from air into glass or glass
into air.
1. Set the laser on the left side of the paper. Turn the laser on. Face the laser so it is
shining horizontally across the paper.
2. Place the prism in the middle of the paper (as shown in the picture above) into the
laser beam so the beam comes out the opposite short side.
3. Rotate the prism in the beam and observe where the beam comes out.
4. Keep rotating the prism until you can see the beam refracted and reflected at the
same time.
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Investigation
E
25B
Reflection and Refraction
Sketching what you observed
a.
Draw at least one ray diagram showing a laser beam that is
refracted after passing through the prism. The refracted ray
is the ray that comes out of the prism at a different angle
than it entered.
b.
Draw a ray diagram showing a laser beam that is reflected.
c.
Draw a ray diagram showing a laser beam that is both
refracted and reflected.
F
Seeing reflection and refraction at the same time
Both refraction and reflection often occur when light hits a boundary between materials such
as the boundary between glass and air. The amount of light reflected or refracted depends on
the angle at which you are looking relative to the surface.
1. Take a piece of graph paper about the size of a business card and draw a line about
5 centimeters from one edge, dividing the rectangle in half. Draw the letter A on one
side of the line and the letter B on the other side.
2. Fold the paper on the line and wrap it around one of the corners of the prism that is
not a right angle.
3. Look into the prism. Move your head up and down to change the angle at which you
look.
G
Thinking about what you observed
a.
Draw a diagram showing the path of the light when you see the letter A.
b.
Draw a diagram showing the path of the light when you see the letter B.
c.
Is the image in the prism always reflected or refracted or can there be both reflection and
refraction at the same time?
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Reflection and Refraction
H
Investigation
25B
Refracting light through a lens
1. Divide a new sheet of graph paper in half horizontally. For the first part of this
activity use the top half of the new sheet of graph paper. Place the laser on the edge of
the paper and shine the laser so it follows a horizontal grid line across the paper.
2. Place the light blue lens 10 cm to the right of the laser with the slot facing up. Line the
lens up vertically using the grid lines on the graph paper. It is important that the beam
is perpendicular to the lens. Make sure the beam of the laser is lined up with the middle
of the lens. There are lines on the side of each lens indicating the middle of the lens.
3. Trace around the base of the lens so it can be removed and put back in place in case
you need to move it to complete ray tracing.
4. Shine the laser through the lens so the beam passes off-center, almost at the very
outer edge of the lens. Set the block with the mirror about 30 cm on the right side of
the lens so the beam hits it after passing through the lens. Use the side of the block
with the graph paper on it so you can clearly see where the beam hits the block.
5. Trace the laser beam before and after it passes through the lens. Be sure to always
carefully mark the points that the beam exits the laser, enters the lens, exits the lens,
and then hits the block. Connect all these points to see the path of the beam.
6. Realign the laser with a different horizontal grid line parallel to the original beam
and closer to the center of the lens. Again, trace the path of the beam before and after
it passes through the lens.
7. Realign the beam so it passes directly through the center and trace the beam again.
8. Trace two more beams passing through the lens on the other side of the center of the
lens for a total of five beams. Label the beams 1-5 on both sides of the lens.
9. Repeat steps 1–8 with the dark blue lens using the bottom half of your graph paper.
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Investigation
I
25B
Reflection and Refraction
Thinking about what you observed
a.
Feel the glass surface with your fingers and note the shape of the lenses. How are they
different?
b.
Draw a quick sketch of the shape of each lens itself with no stand from a side view. Label
each lens.
c.
Describe the paths of the rays before and after they traveled through the light blue lens.
Include the words refract, converge and diverge in your description.
d. What is the focal point of a lens? Mark the focal point on the first ray diagram.
e.
What is the focal length of the lens? Measure the focal length of the light blue lens.
f.
Describe the paths of the rays before and after they traveled through the dark blue lens.
Include the words refract, converge and diverge in your description.
g.
How are the two lenses different?
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Reflection and Refraction
Investigation
25B
h. One lens is referred to as a diverging lens, and the other a converging lens. They are also
sometimes referred to as convex or concave. Research these terms and explain which is
which.
J
Making an image with a lens
Certain types of lenses can make an image of a distant light source. The image forms about
one focal length away from a lens when the object is far away.
1. Find a wall at least 5 meters away from a lamp or sunlit window. Tape a piece of
white paper to the wall to create a screen for seeing the image.
2. Get the light blue lens. Hold the lens at different distances parallel to your screen
and window or light source. Try distances between 15 and 25 centimeters.
3. Move the lens until you see a sharp image of the lamp or window on the screen. An
image is produced when your lens is about one focal length away from the screen.
4. Measure the distance from the lens to the wall and record in Table 2. Use this
technique to determine the focal lengths for both lenses.
Table 2: Focal lengths of lenses
Focal length (cm)
Light blue Lens
Dark blue lens
Images can be smaller or larger than the object that created them. Images can also be right
side up or inverted.
a.
Was the image created by the light blue lens smaller or larger than the object?
b. Was the image right side up or was it inverted?
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Investigation
K
25B
Reflection and Refraction
Projecting an image with a lens
You can think about a lens as collecting a cone of light from each point on an object.
For a perfect lens all the light in the cone is bent so it comes together at a point again to make
the image. This is how movie projectors take an image on film and project it onto a screen.
1. Place one of the lights near the edge of the graph paper. Shine it horizontally.
2. Take the light blue lens and set it on the graph paper 35 cm away from the light.
3. Shine the light at a distant light colored wall at least 5 meters away. If one is not
available, affix a piece of paper to the wall as your projection screen. Slowly move the
lens toward the light until you see a sharp image of the “F” on the wall or screen.
Have one group member check the projected image closely while the lens is slowly
moved to find the exact place the lens needs to be to make it come into focus.
At what distance from the light does the lens produce a sharply focused image?
4. Take a paper card and use it to block some of the light from the lens. The card should
be 2–3 cm from the lens and on the same side as the light.
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Reflection and Refraction
L
a.
Investigation
25B
Thinking about what you observed
Describe the characteristics of the image formed by the lens. Characteristics include
whether the image right-side-up, inverted, larger, or smaller.
b. Discuss with your class why blocking part of the lens makes the image dimmer, even
though you still see the entire image.
c.
CHALLENGE! The thin lens formula is used to
calculate the exact focal length of a lens.
Calculate the focal length of your light blue lens
using the thin lens formula. You will need to
measure the distance from the lens to your
projection screen or wall where the image
became sharply in focus. How does it compare
with the other 2 focal lengths of the light blue lens you observed? Was the method from
section 8 or 10 more accurate compared to the focal length you just calculated using the
thin lens formula?
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