Chapter 25 Optical Instruments 25.1 Conceptual Questions
1) The focal length of the lens of a simple digital camera is 40 mm, and it is originally focused on a person 25 m away. In what direction must the lens be moved to change the focus of the camera to a person 4.0 m away?
A) It does not make any difference.
B) sideways from the CCD sensors
C) away from the CCD sensors
D) towards the CCD sensors
2) If a camera lens aperture is set to f/4, then
A) the radius of the lens aperture is ¼ the focal length.
B) the focal length of the lens is 4 mm.
C) the diameter of the lens aperture is ¼ the focal length.
D) the radius of the lens aperture is 4 times its focal length.
E) the diameter of the lens aperture is 4 times its focal length.
3) If a camera lens lets in an amount of light Iwhen it is set to f/4, then how much light will it let in when it is set to f/8?
4) The closest object that a typical young person with normal vision can focus on clearly is closest to
A) 1 cm.
B) 10 cm.
C) 25 cm.
D) 50 cm.
E) 75 cm.
5) If the lens in a person's eye is too highly curved, this person is suffering from
B) spherical aberration.
D) chromatic aberration.
6) If the back of a person's eye is too close to the lens, this person is suffering from
B) spherical aberration.
D) chromatic aberration.
7) Nearsightedness is usually corrected with
9) A little known fact is that both Robinson Crusoe and Friday wore eyeglasses. As it so happens, Robinson Crusoe was farsighted while Friday was nearsighted. Whose eyeglasses did they use whenever they wanted to start a fire by focusing the sun's rays?
A) Robinson Crusoe's
C) Both would work equally well.
D) Both actually worked, but Friday's was a little bit better.
E) Neither's worked, but fortunately they were in possession of matches.
10) What type of lens is used to make a magnifying glass?
C) Either type would work equally well.
11) The angular magnification of a magnifying glass is largest when the image is at
A) the far point of the eye.
B) the focal point of the lens.
C) the near point of the eye.
E) The magnification depends on the magnifying glass lens, not where the person puts his eye.
12) In a compound microscope
A) both the objective and the eyepiece form real images.
B) magnification is provided by the objective lens and not by the eyepiece. The eyepiece merely increases the resolution of the image viewed.
C) magnification is provided by the objective and not by the eyepiece. The eyepiece merely increases the brightness of the image viewed.
D) the magnification is m1 + M2, where m1 is the lateral magnification of the objective and M2 is the angular magnification of the eyepiece.
E) the image of the objective serves as the object for the eyepiece.
13) A simple compound microscope normally uses
A) a short focal length objective and a shorter focal length eyepiece.
B) a short focal length objective and a longer focal length eyepiece.
C) a long focal length objective and a shorter focal length eyepiece.
D) a very long focal length objective and a longer focal length eyepiece.
14) For relaxed viewing with a microscope or a telescope, the eyepiece is adjusted to place the image at
A) the near point of the eye.
B) the pupil of the eye.
C) the focal point of the eyepiece.
E) the focal point of the objective lens.
15) Jack and Mary view the same microorganism through the same compound microscope. Mary's near point distance, NM, is twice as large as Jack's near point distance, NJ. If Mary sees the microorganism with magnification MM, with what magnification does Jack see it?
A) 2 MM
C) 4 MM
E) 8 MM
16) You have a choice between two lenses of focal lengths fa and fb, where fb = 2fa, to use as the objective lens in building a compound microscope. If the magnification you obtain using lens a is Ma, what will be the magnification when using lens b?
A) Mb = 2 Ma
B) Mb = 4 Ma
C) Mb = 8 Ma
D) Mb = Ma/4
E) Mb = Ma/2
17) Which one of the following is a characteristic of a compound microscope?
A) The objective is a diverging lens.
B) The eyepiece is a diverging lens.
C) The final image is real.
D) The image formed by the objective is virtual.
E) The image formed by the objective is real.
18) Which one of the following statements is correct?
A) The image formed by the objective of a microscope is smaller than the object.
B) The image formed by the objective lens of a telescope is larger than the object.
C) The image formed by the eyepiece is real for both a microscope and a telescope.
D) The image formed by the objective lens of a microscope is larger than the object.
19) You are given two converging lenses to build a compound microscope. Lens A has focal length 0.50 cm and lens B has focal length of 3.0 cm. Which one of the two lenses would you use for the objective?
A) Lens A, because it has the shorter focal length.
B) Lens B, because it has the longer focal length.
C) It makes no difference which lens I use for the objective.
D) None, because the objective should be a diverging lens.
20) To maximize the magnification of a refracting telescope, what lenses should you choose?
A) The objective lens and eyepiece should both have small focal lengths.
B) The objective lens and eyepiece should both have large focal lengths.
C) The objective lens should have a small focal length and the eyepiece should have a large focal length.
D) The objective lens should have a large focal length and the eyepiece should have a small focal length.
E) The objective lens should have a large focal length, but it doesn't matter what the focal length of the eyepiece is.
21) Which one of the following is normally not a characteristic of a simple two-lens refracting astronomical telescope?
A) The angular size of the final image is larger than that of the object.
B) The final image is virtual.
C) The objective forms a virtual image.
D) The final image is inverted.
22) A simple refracting telescope provides large magnification by employing
A) a short focal length objective and a short focal length eyepiece.
B) a short focal length objective and a long focal length eyepiece.
C) a long focal length objective and a short focal length eyepiece.
D) a long focal length objective and a long focal length eyepiece.
23) You have a choice between two lenses of focal lengths fa and fb, where fb = 2fa, to use as the objective lens in building a refracting telescope. If the magnification you obtain using lens a is Ma, what will be the magnification when using lens b?
A) Mb = 2 Ma
B) Mb = 4 Ma
C) Mb = 8 Ma
D) Mb = Ma/4
E) Mb = Ma/2
24) A refracting telescope has a magnification M. If the focal length of the objective lens is doubled and the eyepiece focal length is halved, what is the new magnification?
25) Given perfect lenses, what is the main reason that a telescope with a large-diameter objective lens can produce a sharper image than one with a small-diameter lens?
A) A larger lens can gather more light than a smaller lens.
B) A larger lens can focus more accurately than a smaller lens.
C) There is less diffraction with a larger lens than with a smaller lens.
D) A larger lens can focus more wavelengths of light than a smaller lens.
E) A larger lens has a longer focal length than a smaller lens.
26) If a metal sheet containing a tiny hole is heated (without damaging it) and therefore expands, what happens to the angular location of the first-order diffraction maximum?
A) It moves toward the centerline.
B) It moves away from the centerline.
C) It doesn't change.
27) Light of wavelength 500 nm illuminates a round 0.50-mm-diameter hole. A screen is placed 6.0 m past the slit. What is the width of the central bright circle on the screen?
A) 15 mm
B) 260 μm
C) 7.3 mm
D) 3700 μm
28) Light from a He-Ne laser of wavelength 633 nm passes through a circular aperture. The resulting pattern is observed on a screen 4.0 m behind the aperture, where the width of the central bright circle is 1.1 cm. What is the diameter of the hole?
A) 560 μm
B) 9.8 μm
C) 32,000 μm
D) 4700 μm
29) A round 0.85-mm-diameter hole is illuminated by infrared light of wavelength 2.5 μm What is the angle of the first dark fringe in the resulting pattern?
A) 0.21 degrees
B) 3.6 × 10-3 degrees
C) 1.2 × 10-2 degrees
D) 4.8 × 10-2 degrees
1) A 35-mm digital camera using a standard 50.0 mm lens is focused on a 1.80 m tall person who is standing 3.25 m from the lens. You now refocus the camera on a 15.0 cm tall flower that is 75.0 cm from the lens.
(a) In refocusing, by how much did you move the lens? Did you move it toward the film or away from it?
(b) What is the height of the flower's image on the CCD sensor?
Answer: (a) 2.79 mm, away from the film (b) 10.7 mm
2) A slide projector needs to focus the image of a 24.0-mm × 36.0-mm slide onto a square screen 1.50 m on each side. You want to fill the screen as completely as possible without any of the image falling beyond the screen. Due to space constraints in the projector, the slide must be placed 10.5 cm from the lens.
(a) What magnification does the projector produce?
(b) How far from the lens should the screen be placed?
(c) What focal length projector lens is needed?
Answer: (a) 41.7, (b) 4.38 m, (c) 10.3 cm
3) A 35-mm camera equipped with a 40-mm focal length lens is used to photograph a tree 17 m tall. If a 32-mm high image of the tree on the CCD sensor is required, how far should the camera lens be from the tree?
A) 21 m
B) 22 m
C) 23 m
D) 24 m
E) 26 m
4) The focal length of the lens of a simple film camera is 40.0 mm. By what amount should the distance between the lens and the film be increased or decreased to change the focus from a person who is 25 m from the lens to one who is 4.0 m from the lens?
A) 0.2 mm
B) 0.3 mm
C) 0.4 mm
D) 0.5 mm
E) 0.7 mm
5) A simple digital camera lens with focal length perfectly focuses the image of an object on a CCD sensor located a distance away from the lens. The object now moves away from the lens and the new distance between the lens and sensor is
(a) By what amount did the object change its distance from the lens?
(b) What is the new magnification?
A) (a) 0.08 cm
B) (a) 0.03 cm
C) (a) 990.00 cm
D) (a) 12.22 cm
6) A 35-mm single lens reflex (SLR) digital camera is using a lens of focal length 35.0 mm to photograph a person who is 1.80 m tall and located 3.60 m from the lens.
(a) How far is the CCD sensor from the lens when the person is in focus?
(b) How tall is the person's image on the CCD sensor?
Answer: (a) 3.53 cm (b) 1.77 cm
7) A zoom lens for a camera is adjusted to change its focal length from 38 mm to 304 mm. If the same amount of light is to be admitted to the lens in both cases, what is the final f-number if the original f-number was f/2.0?
8) A 35-mm camera equipped with a 45-mm focal length lens is used to photograph a tree 15 m tall. If the aperture of the lens is set at f/2, what is the aperture diameter?
A) 23 mm
B) 14 mm
C) 18 mm
D) 27 mm
E) 32 mm
9) A lens of focal length 45 mm is mounted on a 35-mm camera. The lens aperture is set at f/2.0 and the shutter speed set at 1/2000 s. The photographer wants to keep the same exposure (amount of light), but wants to change the f-number to f/8.0 for greater depth of field. What should be the new shutter speed to maintain the same exposure?
A) 1/1000 s
B) 1/500 s
C) 1/4000 s
D) 1/8000 s
E) 1/125 s
10) A camera lens has a focal length of 50.0 mm and an aperture setting of f/4.00. What is the diameter of the aperture of this lens?
A) 12.5 mm
B) 10.1 mm
C) 25.0 mm
D) 35.4 mm
E) 200 mm
11) The focal length of a camera lens is 40 mm and its aperture diameter is 10 mm. What is the f-number of this lens?
12) For an eye that is 2.4 cm in diameter, what must be the range of focal lengths of the lens to focus objects from the near point (25 cm) to infinity on the retina?
Answer: 2.2 cm ≤ f ≤ 2.4 cm
13) A person's near point is 25 cm, and her eye lens is 2.7 cm away from the retina. What must be the focal length of this lens for an object at the near point of the eye to focus on the retina?
A) -3.4 cm
B) -2.4 cm
C) 2.4 cm
D) 3.4 cm
E) 2.6 cm
14) A person's eye lens is 2.8 cm from the retina, and his near point is at 25 cm. What must be the focal length of his eye lens so that an object at the far point of the eye will focus on the retina?
A) -2.8 cm
B) 2.8 cm
C) -2.4 cm
D) 2.4 cm
E) 2.2 cm
15) What power (in diopters) of corrective contact lens is required to correct the vision of a myopic eye whose far point is at 170 cm?
Answer: -0.59 diopters
16) A person is nearsighted with a far point of 75.0 cm.
(a) What focal length contact lens is needed to give him normal vision?
(b) What is the power of the corrective lens?
Answer: (a) -75.0 cm (b) -1.33 diopters
17) A nearsighted person has a near point of 12 cm and a far point of 17 cm. If the corrective lens is 2.0 cm from his eye, (a) what lens power will enable this person to see distant objects clearly, and (b) what then will be the new near point of his eye?
Answer: (a) -6.7 diopters (b) 30 cm in front of the lens
18) A person uses corrective glasses of power -8.5 D that are worn 2.0 cm from her eye.
(a) Is she nearsighted or farsighted?
(b) What is her far point without glasses?
Answer: (a) nearsighted (b) 14 cm
19) A nearsighted man wears contact lenses of -3.00 diopters. With these lenses, his corrected near point is 25.0 cm. What is his uncorrected near point?
Answer: 14.3 cm
20) A person's hyperopic (farsighted) eye can focus on objects that are 75 cm away and farther. What power and what focal length contact lenses should be prescribed so this person can see objects 25 cm and farther from the eye?
Answer: 2.7 diopters, 38 cm
21) A nearsighted person has a far point of 50 cm for his right eye and 150 cm for his left eye. For each eye, what power contact lens will allow the person to see gorgeous sunsets?