Physics 9/19
Ray optics
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NAME: ______________________________
SECTION NUMBER: ___________________
TA: _________________________________
LAB PARTNERS: _______________________
RAY OPTICS LAB:
Reflection, refraction and lenses
Introduction
In this lab you will explore the world of optics which deals with the propagation of light
through different media and how one can manipulate light using combinations of lenses for imaging
purposes. The idea is for you to develop an intuitive understanding of the behavior of light in
different situations and an appreciation for how common imaging tools work.
Important Background Information
Reflection and Refraction:
When a beam of light enters a transparent material such as glass or water, its speed decreases
from that in vacuum by a factor n called the index of refraction. The index of refraction for most
transparent materials ranges between 1 and 2. A ray of light incident on the boundary between two
transparent materials undergoes both reflection and refraction. The angles of incidence, reflection and
refraction are determined by the following laws.
θi θ’
θr
Law of reflection: The angle of incidence is equal to the angle of reflection i.e. θI = θ’
Law of refraction (Snell’s Law): ni sinθI = nr sin θr . Here ni is the refractive index of the material from
which the light is incident and nr is the refractive index of the material into which the light is refracted.
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Thin lenses
A thin lens is one where the thickness of the lens is small compared to other relevant dimensions
such as the radii of curvature of the lens faces. Lenses can be converging or diverging. If parallel rays
(e.g. from a far away source) pass through a converging lens they will be brought to a focus at some
distance behind the lens. The distance between the center of the lens and the focal point is called the
focal length of the lens. A diverging lens on the other hand will spread the rays out. The rays will
appear to emanate from a point in front of the lens called the virtual focus. The negative of the distance
between this point and the lens center is the focal length of the diverging lens. Thus a diverging lens
has a negative focal length.
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Draw two diagrams showing parallel rays passing through a converging lens and a diverging
lens. Make sure to mark the focal points in both cases.
If an object is placed in front of a lens of focal length f at a distance u then the lens will form an image
at a distance v. These three quantities are related by the thin-lens equation
1/f = 1/u + 1/v
Note that f can be positive (converging lens) or negative (diverging lens). The image distance v can
also be positive. The image is then called a real image and it can be captured on a screen. If v is
negative the image is called virtual because it cannot be caught on a screen. Light then appears to come
from an image located on the same side of the lens as the object. This is what you see when looking
through a diverging lens.
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Procedure
Reflection and refraction
1.
Attach a piece of graph paper to the cork board. Now, place the glass slab on it and trace out
its outline on the paper. Stick two pins in the board, both on the side of the glass slab furthest from
you as shown by the black circles in Diagram 1, such that the line joining them intersects the long side
of the slab at an angle. Now look at the pins through the long side of the slab nearest you. While
doing this stick two more pins into the board, both on the side of the slab closer to you as shown by the
white circles in Diagram 1, so that all four pins appear to be in a straight line. Remove the slab.
Diagram 1: The first 2 pins are represented here in black. The first 2 pins should
be placed on the side of the glass slab which is furthest from you. The second 2
pins, represented here in white, should be placed on the long side of the glass
slab which is nearest you.
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Are all four pins in a straight line? Why or why not?
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Consider the ray incident on the slab along the line joining the first two pins. What is the angle
of incidence?
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Explain why this ray exits the other side of the slab along the line joining the third and fourth
pins.
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What is the angle of refraction?
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Knowing the angle of incidence and refraction, calculate the refractive index of the glass.
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Now, rotate the cork board 180˚ so the first two pins are nearest you. Try to see the reflection of
the first two pins on the long side of the glass slab nearest you. It is helpful to place the black card
against the long side of the glass slab which is furthest from you. Stick in two more pins that seem to
be aligned and in a straight line with the image of first two pins on the long side of the glass slab
nearest you. Use the line joining these two pins to compute the angle of reflection. Refer to Diagram 2
for clarification.
Diagram 2: The first 2 pins are represented here in black. The first 2 pins should
be on the side of the glass slab nearest you by rotating the cork board 180˚. To
better see the reflection, place the black card against the long side of the glass slab
which is furthest from you. Look at the reflection of the first 2 pins on the long
edge of the glass slab which is nearest you. Place the second 2 pins, represented
here in white, in line with the image of the first 2 pins on the long edge nearest
you.
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Is the law of reflection satisfied?
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Ray optics
Replace the glass slab with the rectangular plastic container with water in it and repeat the
previous procedures. Compute the refractive index of water.
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Thin lenses
2.
Mount the lens marked focal length, f = 10 cm in the lens holder and carefully remove the light
source from the optics track. If you are not sure how to do this, ask your TA to show you.
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Now try to focus some distant light source (for example, a window, an object outside the
window such as a tree, or a light at the other end of the room) on to the screen behind the lens.
The track will need to be pivoted so the lens is facing the distant source. What is the distance
between the screen and lens?
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Assuming the object is at infinity use the thin lens equation and the above image distance to
compute the approximate focal length of the lens. Why is this approximate?
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Now, re-attach the light source to the track; the base of the light source snaps onto the track
with a slight pull to the side. The figure “4” should be facing the lens. Obtain a focused image
on the screen on the other side of the lens. Carefully measure the object and image distances
and use the thin lens formula to calculate the focal length. Does this agree with your previous
estimate?
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Use the image produced above as the object for a second converging lens. Where does the
image form? Measure the distances and use this to compute the focal length of the second lens.
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Now use a diverging lens as the second lens. What do you see? How would you measure the
focal length of a diverging lens? (bonus)
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