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Light Reflection and Refraction Lab
Using PhET Simulation
I) Introduction:
When a light ray strikes a smooth interface separating two transparent materials
(like air, glass, or water), the wave is partly reflected and partly refracted (or
transmitted) into the second material. For an example of this, imagine you are
outside looking at a restaurant window. You can probably see both the inside of the
restaurant (from the refracted light) and some of the street behind you (from the
reflected light). Similarly, a person in the restaurant can see some of the street
scene, as well as a reflection of the other people in the restaurant.
The goal of this pre-lab is to understand how light is reflected and refracted, and
what the general relationships are between the two. You will be using the Bending
Light simulation found at ng the Run Now button.
II) Initial Observations:
First, let’s get acquainted with the PhET sim that we will be using. The red button on
the laser turns the light on.
What do you notice about the angles of the reflected and refracted light?
Briefly, give a qualitative description of the following features:
What happens to the reflected and refracted rays as you change the
angle of the incident light beam?
Eventually the refracted ray will make an angle of 90° with the surface
normal. If the angle of incidence is increased beyond that angle, then
refraction does not occur. All of the light incident on the interface is reflected
back into the incident medium.
What does changing the index of refraction do to the refracted and reflected
light? Be specific.
There is an angle at which the light will not pass into the other material and will start to
be reflected at the surface. This is call the critical angle of refraction.
III) Quantitative Analysis:
Now let’s try to find a mathematical expression for what we are seeing. First, notice
that there is a protractor and intensity meter that can be found in the lower right of
the sim. You can grab each by moving your pointer over top of them and clicking on
your mouse while moving them in place.
For convenience (and by convention), try measuring angles as starting from the
vertical axis and beam. (The same goes for the refracted beam – but now you will
want to measure from the z-axis to the beam.)
1) With air as the top medium and water as the bottom medium, use your
protractor to adjust your laser to so it creates an angle of incidence of
30o. What is the angle of reflection? What is the angle of refraction?
Using Snell’s Law equation, what is the mathematical solution for 2?
2) As you increase the angle of incidence (move laser lower), what
happens to the angle of refraction? Does it move toward the normal or
away from the normal? What happens to its intensity?
A
3) Change the top medium to water and the bottom medium to air. Once
again, set the incident angle at 30o. What is the angle of refraction?
Compare your answer to the angle of refraction in question 1. Did the
angle of refraction increase (move away from the normal) or decrease
(move toward the normal)?
4) At what angle of incidence does the refracted light travel perpendicular
to the normal, along the boundary layer? What do we call this angle?
You won’t be able to see this in the simulation, but you can get close.
Calculate this angle using Snell’s Law.
5) With your angle of incidence set at 30o again, select ‘Mystery A’ as the
medium in the bottom layer. What do you notice about the angle of
refraction compare to air as the bottom medium? Does ‘Mystery A’ have
a higher or lower indices of refraction than that of water? Solve for the
indices of refraction of ‘Mystery A’ using Snell’s Law.
Mystery A has a higher indices of refraction than water. The refraction is 0.96.
6) Knowing that the speed of light through air is approximately 2.99x108
m/s, what is the speed of light through glass? Through ‘Mystery A’?
.
7) What happens to the intensity of the reflected and refracted beam as
the angle of incidence increases and decreases? What is the value of the
refracted beam when the laser is perpendicular to the boundary layer?
8) Consider a simple ray diagram for the objects and lenses below. Determine
the images and describe if they are real or virtual, erect or inverted, and
enlarged or reduced.
9) True or false and explain why.
10)
A virtual image cannot be displayed on a screen. T/F
Why?
11)
Aberrations occur only for real images. T/F
Why?
12)
A diverging lens cannot form a real image from a real object. T/F
Why?
13)
The image distance for a positive lens is always positive. T/F
Why?
14)
A negative image distance implies that the image is virtual. T/F
Why?
15)
An object 4.0 cm tall is placed 20 cm in front of a thin lens of power 20
diopters. Draw a precise ray diagram to find the position and size of the
image and check your results using the thin-lens equation.
16)
Describe the cause of chromatic aberration in a lens and how it affects
the image collected.
Consider your own eyesight. Can you detect any indication of spherical
aberration? If so, describe what you see.