Purpose of this Minilab

1
Purpose of this Minilab

• Apply the basics of ray tracing to learn about
  reflection and refraction of light.

2
Activity 1 Light Reflection at Plane Surfaces
Angle of incidence
Angle of reflection
Index of refraction of the two materials
ni
nt
Angle of transmission (refraction)
3
..the laws.
Law of Reflection
Snells Law of Refraction
Incident, reflected, and transmitted ray lie in
one plane.
Verify the law of reflection using a plane
mirror. Verify your homework result on a 90?
plane mirror.
4
Checking the law of reflection with a plane mirror
Polar graph paper
45?
Qr
0?
90?
Qi
Light Source
45?
135?
90?
180?
135?
Mirror
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Measuring refraction
Polar graph paper
45?
0?
90?
Use Snells law to determine nplastic.
Qi
Light Source
45?
135?
Light must hit the center of the flat side
Qt
90?
180?
135?
Semicircular lens
nplastic
6
Measuring angle of total internal reflection
Polar graph paper
45?
0?
90?
45?
135?
Light must hit the center of the flat side
Light Source
Qcrit
90?
180?
135?
Semicircular lens
7
Snells Law for Critical Angle
1
8
Light beam displacement by plane parallel plate
Light Source
Q
Q
t
d
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Light beam displacement by plane parallel plate
Polar graph paper
90?
Light Source
135?
45?
180?
Q
0?
Q
Let the beam hit the rectangle in center of the
polar paper
t
135?
45?
d

• Trace light ray on polar graph paper.
• Outline location of rectangular plastic on
  paper.
• Measure angles Q and Q.
• Measure widths d and t.

90?
10
Light beam displacement by plane parallel plate

• Use one incident angle Q (and corresponding Q
  and d and t) ? calculate n.
• Use this calculated n to predict the
  displacement d for a different incident angle.
• (Hint You will also need to use Snells Law for
  this calculation.)
• Verify experimentally d for the new angle.

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Activity 2 Reflection and Refraction at
Spherical Surfaces
Getting the radius R of a concave mirror
Concave mirror, reflecting side here.
R
x
D
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Alternative method to get R ..
Polar graph paper
90?
135?
45?
Move mirror until curvature matches the curvature
on polar graph paper. then measure R as shown.
180?
0?
R
135?
45?
90?
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Finding the focal point of the concave mirror
Regular graph paper Trace the rays and determine
f.
Light Source
parallel rays
f
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Finding the focal point of the convex mirror
Regular graph paper Trace the rays and determine
f.
Extend the light rays backward to where they seem
to come from.
Light Source
Virtual image (isnt really there).
parallel rays
f
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Imaging with the convex mirror
Regular graph paper Trace the rays and determine
f.
Here is our object point
Light Source
P
S
Semicircular lens
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Thin Lens Equation (how to calculate focal length
from the radii of a lens and its index of
refraction)
Each lens has two interface with the air (1 and
2). Interface 1 is the one that is encountered
by the light when entering the lens. Interface 2
is the one that is encountered by the light when
exiting the lens.
Interface 1 has radius R1.
Interface 2 has radius R2.
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Thin Lens Equation (how to calculate focal length
from the radii of a lens and its index of
refraction)
Sign rules for R1
R1 negative
R1 positive
R2 positive
R2 negative
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Example of using the lens equation
A double concave lens (concave on interface 1
and also on 2) with both radii being 5cm and the
index of refraction n1.65

• R1 - 5 cm and R2 5 cm

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The Imaging Equation for Lenses and Mirrors
S Object Distance P Image Distance f Focal
Length
For Mirrors
where R Radius of Mirror
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Sign Rules For Lenses and Mirrors
f
Means a positive number
Convex Lens Concave Lens
- Convex Mirror - Concave Mirror
Most objects are real.
Real objects S is positive Virtual
objects S is negative Real images
P is positive Virtual images P
is negative
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Example of signs for f, S, and P
Convex mirror f is negative
Real object
Light Source
Virtual image
P
S
positive
negative
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Using the Desk Lamp
Lamp Plug (black) must be plugged into dimmer
plug. Dimmer plug (white) must be plugged into
power outlet.
Dimmer
On/Off switch of lamp