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Optics Review Questions
PROBLEMS
1. A slab of glass with an index of refraction of 1.5 is submerged in water with an index of refraction of 1.33.
Light in the water is incident on the glass. Find the angle of refraction if the angle of incidence is (a) 60°, (b) 45°,
(c) 30°.
2. An object placed 8 cm from a concave spherical mirror produces a virtual image 10 cm behind the mirror. (a)
If the object is moved back to 25 cm from the mirror, where is the image located? (b) Is it real or virtual?
3. (a) The speed of light in a material can be calculated by dividing the speed of light in air (3 x 10 8 m/s) by the
index of refraction for that material. Calculate the speed of light in water (n = 1.33).
4. Know how to draw the ray diagrams for convex/concave lenses and mirrors.
ADDITIONAL QUESTIONS
16. Explain how you could determine the focal length of a magnifying glass if you were outside on a sunny day.
17. A 10-cm tall flower is 40 cm from a converging lens with a focal length of 15 cm. (a) How far from the lens
will the image of the flower be in focus? (b) How large will that image be?
18. Galileo attempted to measure the speed of light with the following experiment. He recruited a friend (let’s
call him Albert). Galileo and Albert each took a lantern up to the top of a different mountain. Galileo turned his
lantern on; as soon as Albert saw the light from Galileo’s lantern, he turned his lantern on. Galileo measured the
time from when he first turned on his lantern until he saw the light coming from Albert’s lantern. What do you
think the results of this experiment were? What could Galileo conclude from this experiment? What problems do
you see in this experimental design?
20. The index of refraction for flint glass is 1.66 for blue light and 1.61 for red light. A beam of white light hits the
glass at an incident angle of 45°. (a) Find the angle of refraction for the blue light. (b) Find the angle of refraction
for the red light.
21. Find the critical angle for light traveling from glass (n = 1.5) to air.
22. A silver coin sits on the bottom of a swimming pool that is 4 m deep. A beam of light reflected from the coin
emerges from the pool making an angle of 20° with respect to the water’s surface and enters the eye of an
observer. (a) Draw a ray from the coin to the eye of the observer. (b) Extend this ray, which goes from the waterair interface to the eye, straight back until it intersects with a vertical line drawn through the coin. What is the
apparent depth of the swimming pool to the observer?
24. Under what condition will a concave mirror produce an upright image? A virtual image? An image smaller
than the object? An image larger than the object? Repeat the questions for a convex mirror.
26. For the following object distances and focal lengths of thin lenses in air, find the image distance and
magnification and state whether the image is real or virtual and upright or inverted. (a) do = 40 cm, f = 20 cm; (b)
do = 10 cm, f = 20 cm; (c) do = 40 cm, f = -20 cm; (d) do = 10 cm, f = -30 cm.
28. The glass of a converging lens has an index of refraction of 1.6. When the lens is in air, its focal length is 30
cm. If immersed in water, will its focal length be greater than, less than, or equal to 30 cm?
29. You wish to see an image of your face for applying makeup or shaving. If you want the image to be upright,
virtual, and magnified 1.5 times when your face is 30 cm from the mirror, should you use a convex or concave
mirror? What should its focal length be?
30. A glass light pipe in air will totally internally reflect a light ray if its angle of incidence is at least 39 degrees.
What is the minimum angle for total internal reflection if the pipe is in water (n = 1.33)?
31. A diverging lens has a focal length of -0.4 m. Find the image location for an object placed 2 m from the lens.
ANSWERS
1. (a) 50; (b) 39; (c) 26
2. (a) 5.4 cm; (b) virtual
3. (a) 2.26 x 108 m/s; (b) 4.29 x 1014 Hz; (c) 527 nm
17. (a) 24 cm; (b) 6 cm
19. 5.4 min
20. (a) 25.2; (b) 26.0
21. 42
22. (b) 1.46 m
25. location: 4 m; diameter: 3.7 cm
26. (a) 40 cm; 1; real, inverted; (b) -20 cm; 2; virtual,
upright; (c) -13.3 cm; 0.33; virtual, upright; (d) -7.5 cm;
0.75; virtual, upright
27. (a) 20 cm to the right, 5 cm tall; (b) 30 cm to the
right, 10 cm tall; (c) 2
28. greater
29. concave, 90 cm
30. 56.8
31. –0.33 m