Physics 1230
Homework 7
Due Monday June 20, 2016
When transmitted, the light rays bend, which we call refraction. The rules for reflection
(reflected angle is equal to incident angle) and refraction (Snell’s Law) allow us to
predict how rays travel. The cases of spherical mirrors and lenses and how images are
formed are particularly useful.
1) [6 pts] Changing index of refraction
I suggest doing this with the PhET "Bending Light" simulation, at
http://phet.colorado.edu/en/simulation/bending-light. However, you can also use
knowledge from class and reading to do this problem.
a) Set the simulation so that light travels from air to water, as shown below.
What happens to the angle of refraction as you increase the index of
refraction, "n" of the second material? Sketch rays for different values of
index on the diagram.
Physics 1230
HW7.1
Summer 2016
Physics 1230
Homework 7
Due Monday June 20, 2016
b) Now set the simulation so that light travels from water into air. Explain some
examples where you go from some light reflecting and some transmitting to all
the light reflecting (Total Internal Reflection). In other words, have some
sentences like, “The angle of incidence was at (insert number of degrees) and
there was reflection and transmission. Then I changed (insert what you did) and
suddenly the light was totally internally reflected.” Come up with at least two
examples. In one of them, change an angle. In the other, change something else.
c) Briefly summarize the behavior you have observed in parts (a) and (b). If
you were to try describe to a fellow student what is going on, what would you
say?
Physics 1230
HW7.2
Summer 2016
Physics 1230
Homework 7
Due Monday June 20, 2016
2) [4 pts] Draw a diagram showing dispersion of white light (from a slit) as it goes
through a prism. You only need to show rays corresponding to red and blue light.
Label the angle of incidence and the angles of refraction. You don’t need to find the
numerical values of these angles, but the direction of their refraction toward or away
from the normal should be correct of course.
3) [4 pts] Where does observer O perceive the fish to be? A greater, the same, or smaller
depth than it really is? Draw rays to back up your claim. The fish is in water and the
observer in air.
Physics 1230
HW7.3
Summer 2016
Physics 1230
Homework 7
Due Monday June 20, 2016
4) (10 pts) Locating the image: Ray tracing
Use a ruler to draw ray diagrams to locate the images of the following objects. Use a
ruler to make the distances (object distance, focal length) accurate. For each ray diagram,
do the following:
 Measure the image distance. Is it positive or negative?
 State whether the image real or virtual, upright or inverted?
 What is a practical application of this lens? (Think about what the image looks
like).
See the lecture notes for sample diagrams,. Be careful of your sign conventions –
concave lenses have a negative focal length, and image distances are negative for
images on the same side of the lens as the object. Measure all distances from the center
of the lens (not the edge of the glass). It's easiest if you draw the object as 1-2 cm tall.
(a) An object that is 7 cm from a convex lens of 3 cm focal length.
(b) An object that is 5 cm from the same lens.
Physics 1230
HW7.4
Summer 2016
Physics 1230
Homework 7
Due Monday June 20, 2016
(c) An object that is 2 cm from the same lens.
(d) An object that is 7 cm from a concave lens of focal length -3 cm.
5) (6 pts) Locating the image: The Lens equation
Now, let's do some calculations to back up the ray diagrams. All questions use the
information from problem #4. Use should use the Lens Equation discussed in class.
(a) Using the lens equation, calculate the image distance for #4a (An object that is 7
cm from a convex lens of 3 cm focal length.). Does your answer using the lens
equation match your answer from ray tracing in #4a?
Physics 1230
HW7.5
Summer 2016
Physics 1230
Homework 7
Due Monday June 20, 2016
(b) Using the lens equation, calculate the image distance for #4d (an object that is 7
cm from a concave lens of focal length -3 cm.) Does it match your answer from
#4d? (Be careful of the sign conventions for concave lenses).
Physics 1230
HW7.6
Summer 2016