PLC Activity 10
Practice Exam 2.2
How to get credit for this activity
First, sign-in at the computer and sign-out when the activity is complete. Show your work
and results to a PLC tutor so they can check your work and initial the signoff sheet. Be
prepared to answer questions about the activity or your results.
Choose (i) four from problems 1 – 6 and (ii) answer 3 of 5 from problem 7!
Due: see webpage for due date
Problem 1
A spotlight on a boat is 2.5 m above the water, and the light strikes the water at a point
that is 8.0 m horizontally displaced from the spotlight (see the drawing). The depth of the
water is 4.0 m. (a) Draw an accurate ray diagram. Does the ray bend towards or away
from the normal? Explain. (b) Determine the distance, which locates the point where the
light strikes the bottom. Answer: 12.1 m
Problem 2
The owner of a van installs a rear-window diverging lens that has a focal length of 30
cm. When the owner looks out through the lens at a person standing directly behind the
van, the person appears to be just 0.240 m from the back of the van, and appears to be
0.34 m tall. (a) How far from the van is the person actually standing, and (b) how tall is
the person? (c) Sketch a ray diagram of the situation.
Answer: 120 cm, 170 cm
Problem 3
Two very narrow slits are spaced 1.80 μm apart and are placed 35.0 cm from a screen.
What is the distance between the first and second dark lines of the interference pattern
when the slits are illuminated with coherent light (550 nm) (Hint: The angle is not small.)
Answer: 0.1264 m
Problem 4
A plastic film with index of refraction 1.85 is put on the surface of a car window to
increase the reflectivity and thus to keep the interior of the car cooler. The window glass
has index of refraction 1.52. (a) What minimum thickness is required if light with
wavelength 550 nm in air reflected from the two sides of the film is to interfere
constructively? (b) It is found to be difficult to manufacture and install coatings as thin as
calculated in part (a). What is the next greatest thickness for which there will also be
constructive interference? (c) Draw an accurate ray diagram for the situation.
Answer: 149 nm, 297 nm
Problem 5
A spaceship of rest length 130 m races past a timing station at a speed of 0.740c. (a)
What is the length of the spaceship as measured by the timing station? (b) What time
interval will the station clock record between the passage of the front and back ends of
the ship? Answer: 87.4 m, 0.394 𝛍s
Problem 6
A space traveler takes off from Earth and moves at speed 0.9900c toward the star Vega,
which is 26.00 ly distant. How much time will have elapsed by Earth clocks (a) when the
traveler reaches Vega and (b) when Earth observers receive word from the traveler that
she has arrived? (c) How much older will Earth observers calculate the traveler to be
(measured from her frame) when she reaches Vega than she was when she started the
trip? Answer: 26.26 y, 52.26 y, 3.71 y
Problem 7
A 5.00-grain aspirin tablet has a mass of 320 mg. For how many kilometers would the
energy equivalent of this mass power an automobile? Assume 12.75 km/L and a heat of
combustion of 3.65 ×107 J/L for the gasoline used in the automobile.
Answer: 1.01  107 km
Problem 8
Use short concise sentences to answer 3 of the 5 following questions using physics
principles. Make sure to explain your reasoning for each answer
a. The figure shows the viewing screen in a double-slit experiment with
monochromatic light. Fringe C is the central maximum. What will
happen to the fringe spacing if the (i) wavelength of the light is
decreased?; (ii) distance to the screen is decreased?; (ii) slit
separation is decreased?
b. Explain in detail the type of lens the stick figure is holding? What is the
object and where is it located?
c. The laws of optics also apply to electromagnetic waves invisible to the
eye. A satellite TV dish is used to detect radio waves coming from orbiting satellites.
Why is a curved reflecting surface (a “dish”) used? The dish is always concave,
never convex; why? The actual radio receiver is placed on an arm and suspended in
front of the dish. How far in front of the dish should it be placed??
d. A very thin soap film (n = 1.33) whose thickness is much less than a wavelength of
visible light, looks black; it appears to reflect no light at all. Why? By contrast, an
equally thin layer of soapy water (n = 1.33) on glass (n = 1.50) appears quite shiny.
Why is there a difference?
e. The average life span in the United States is about 70 years. Does this mean that it
is impossible for an average person to travel a distance greater than 70 light-years
away from the earth? (A light-year is the distance light travels in a year.) Explain.
f.
When a monochromatic light source moves toward an observer, its wavelength
appears to be shorter than the value measured when the source is at rest. Does this
contradict the hypothesis that the speed of light is the same for all observers?
Explain.