Physics 1020, Homework #9
Due Thursday, April 3 at 11AM
1. (1 pt.) The lens of your eye resembles that of a camera -- light from the scene in front of you focuses
to a real image on your retina.
Unlike a camera, the lens of your eye can actually change its focal length by changing its curvature using
muscles in your eye. The more curved the lens's surface is, the more strongly it bends light together and
the shorter the focal length. The lens's variable focal length allows you to focus the real image of a
particular object onto your retina without having to change the distance between the lens and the retina.
Explain why focusing on a distant object requires less lens curvature than focusing on a nearby object.
(You might want to draw a picture to help understand this.)
2.
(0.5 pt.) The lens of a nearsighted person has trouble becoming flat enough to form an in-focus, real
image of a distant object on the retina. When a nearsighted person looks at a distant object, the
location where the real image formed by the lens of their eye focuses is:
a. in front of the retina, that is between the lens and the retina
b. on the retina surface
c. behind the retina
3. (1 pt.) Explain why the glasses for a nearsighted person use diverging lenses (lenses that bend light
rays apart) to help them see distant objects.
4.
(0.5 pt.) A farsighted person needs glasses to read. The lens of a farsighted person has trouble
becoming curved enough to form an in-focus, real image of a nearby object on the retina.
When a farsighted person looks at a nearby object, the location where the real image formed by the
lens of their eye focuses is:
a. in front of the retina, that is between the lens and the retina
b. on the retina surface
c. behind the retina
5.
(1 pt.) Explain why the reading glasses for a farsighted person use converging lenses to help them see
nearby objects.
6. (1 pt.) The iris of your eye changes diameter to control how much light reaches your retina. The
brighter the light, the smaller the iris's opening. Explain the physics of why your eye's depth of focus
is greater in bright light than in dim light.
7. (0.5 pt) When you look towards the west at the Boulder Flatirons, you see the sky at the top of your
field of view and the flatirons and meadow at the bottom. Green Mountain is to the right (north) and
Bear Mountain is to the left (south). How are these features oriented in the real image that the lens of
your eye forms on your retina?
a. image has the sky at the top, the meadow at the bottom, Green Mountain on the right, and Bear
Mountain on the left
b. image has the sky at the top, the meadow at the bottom, Bear Mountain on the right, and Green
Mountain on the left
c. image has the meadow at the top, the sky at the bottom, Green Mountain on the right, and Bear
Mountain on the left
d. image has the meadow at the top, the sky at the bottom, Bear Mountain on the right, and Green
Mountain on the left
8. (1 pt.) A camera uses a converging lens to form a real image of the scene on the light-sensitive film.
Your camera has a 35-80 mm variable focal length (zoom) lens. You want to take a photograph of a
distant mountain, and you select your focal length to be 45 mm for the composition you want. How
far should the lens be from the film surface for the mountains to be in focus (in mm)?
9. (0.5 pts.) After you capture the mountains on film, you want a picture of your two friends smiling
faces. You need to move the lens in order for the picture to be in focus. Which way should you move
the lens?
a.. toward the film
b. away from the film
10. (1 pt.) Explain the physics principles and reasoning behind your answer above.
11. (1 pt.) If your friends are 1.5 meters away from you and you set the focal length on the camera to
50mm, how far should the lens be from the film for the picture to be in focus (in mm)?
12. (1 pt.) The sun is setting and the light is dim. You increase the aperture (open the iris) of your lens to
compensate. Explain why this will brighten the image on the film. What are you sacrificing by
increasing the aperture? Be sure to include the physics principles and your reasoning for (all) your
essay answers.
(Next 4 problems). A traditional 35mm camera records an image on a film negative that is 24mm high by
36 mm across and has an effective resolution of about 2000 x 3000 pixels. By contrast a typical digital
camera records an image on a CCD (charged-coupled device) detector array that is much smaller. For the
purposes of this problem, consider a CCD array measures 4.4mm high by 6.6mm across and has a
maximum resolution of 912 x 1216 pixels.
13. (0.5 pts.) You want this digital camera to capture the same image on its detector as the 35 mm camera,
that is if you can stand 30 ft away from a big tree and get the image of the whole tree onto the film,
you want to be able to stand 30 ft away from the tree and get the image of the whole tree onto the
smaller CCD array. What needs to be different in the digital camera for the camera to behave this
same way, with the image in focus on the detector? Draw a picture to help you.
a. the focal length of the lens needs to be larger than 35 mm
b. the focal length of the lens needs to be larger than 35 mm and the lens needs to be closer
to the CCD detector
c. the focal length of the lens needs to be larger than 35 mm and the lens needs to be farther
from the CCD detector
d. the focal length of the lens needs to be smaller than 35 m
e. the focal length of the lens needs to be smaller than 35 mm and the lens needs to be
closer to the CCD detector
f. the focal length of the lens needs to be smaller than 35 mm and the lens needs to be
further from the CCD detector
14. (1 pt.) Explain the physics principles and reasoning behind your answer above.
15. (1 pt.) A color filter is placed in front of each pixel on the CCD array in order to record information
about the color of the light as well as the brightness. What would the recorded image look like
without these color filters? As always, be sure to include the physics principles and reasoning behind
your answer.
16. (1 pt.) Explain how the filters in front of the CCD pixels record the color of the image and how it is
similar to how our eye sees color?
17. (1 pt) When light from your desk lamp passes through a 50.0-mm focal length lens, it forms a sharp
real image on a sheet of paper located 50.5 mm from the lens. How far is the desk lamp from the
lens?
18. (0.5 pt) When using a magnifying glass to focus the Sun onto a piece of paper, the distance from the
lens to the paper should be
a. less than the focal length of the lens.
b. equal to the focal length of the lens.
c. greater than the focal length of the lens.
d. not enough information is given to determine.
19. (0.5 pt) In a CD player, light from a laser diode enters a plastic disc and reflects from a layer of shiny
aluminum. Before it enters the plastic, the laser light has a wavelength of 780 nanometers and a frequency
of 385,000,000,000,000 Hz. Once this light is inside the plastic, its wavelength
a. stays the same but its frequency becomes higher.
b. becomes shorter but its frequency stays the same.
c. becomes longer but its frequency stays the same.
d. stays the same but its frequency becomes lower.
20. (0.5 pt) What is the main reason a DVD can hold more information than a CD, aside from the number
of layers of data storage medium?
a. DVD’s use polarized light.
b. CD’s are diffraction limited
c. DVD’s use shorter-wavelength light than CDs
d. CD light is circularly polarized
21. (1 pt) You are working for a company that designs CDs. A co-worker and you are having an argument
about light. You claim that you measure a particular wavelength of light in a vacuum to have a certain
value and when your co-worker goes to calculate the wavelength of light in plastic they get a different
value. Is your co-worker’s value higher or lower than yours? Please explain the discrepancy.
22. (1 pt) You are in San Diego on vacation and are diving. Underwater, you notice that if you do not
look at your watch directly there is an angle where the surface looks just like a mirror. Please explain.
Extra credit (1 pt)
Each week you should review both your answers and the answer key from the previous week's homework.
Usually you will get to correct an essay from the prior week. Select one problem for which you had the
wrong answer FROM ANY OF THE PRIOR WEEKS INCLUDING THE EXAM.
1.Identify the question you are correcting,
2.state (copy) your original incorrect answer, and
3.explain where your original reasoning was incorrect, the correct reasoning for the problem, and how it
leads to the right answer.
If you had no incorrect answers, which problem(s) did you find most useful to your understanding.