A First Course in Physics · Ch. XX, §§ 552–564 · Chapter 20: Formation of Images
65. Cameras, the eye, magnifiers, telescopes and microscopes
Goal: Your student can explain the pinhole and lens camera, the projector and the eye, and find the magnifying power of a simple lens, a telescope and a compound microscope.
You'll need
- The book, open to these pages
- A shoebox or tube, aluminum foil, a pin, wax paper and tape (a pinhole camera)
- Two magnifying glasses of different strengths (for a simple telescope)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
The pin-hole camera; the photographic camera; the projecting lantern; the eye and how it focuses; apparent size and the visual angle; the distance of most distinct vision (25 cm); the magnifying power of a lens (25/f), an astronomical telescope (F/f) and a compound microscope (25L/Ff); the terrestrial telescope, opera glass, stereoscope and Zeiss binocular. Eleven questions follow.
The lesson, step by step
Hook question
SayHow is your eye like a camera, and how does it focus on things near and far?
Read together
Read §§ 552–564, studying Figs. 457–466. The eye:
The eye is essentially a camera in which the cornea C (Fig. 461), the aqueous humor I, and the crystalline lens O act as one single lens which forms an inverted image P'Q' on the retina
Try it: a pinhole camera
Cut a hole in one end of a box and tape foil over it; prick a small pinhole. Cut out the other end and tape wax paper over it. Point the pinhole at a bright window or a lamp with the wax paper toward you (cover your head and the box with a dark cloth): an upside-down picture appears (§ 552). Try a bigger hole: brighter but blurrier.
Try it: a two-lens telescope
Hold the weaker magnifying glass at arm's length toward a distant tree or building, and the stronger one near your eye. Slide the near one back and forth until the view is sharp: the image is magnified and upside down, just like the astronomical telescope in Fig. 465 (magnifying power = F/f). Never point it at the Sun.
Explain the magnifying powers
A simple lens: 25/f (f in cm), so a 2.5 cm lens magnifies 10 times (§ 558). A telescope: focal length of the objective ÷ focal length of the eyepiece (§ 559). A microscope: 25 × tube length ÷ (objective focal length × eyepiece focal length) (§ 560). In inches, use 10 in. instead of 25 cm.
What we know now
The Yerkes 40-inch refractor (§ 559) is still the largest lens telescope ever used for astronomy, but today's great telescopes use huge mirrors, some over 8 m across, and the James Webb Space Telescope's mirror is 6.5 m. Most cameras now record images on electronic sensors instead of photographic plates.
Say it back
Your student explains how the eye focuses on near and far objects (§ 555) and what glasses do for a shortsighted eye (question 11).
Questions
Work questions 1–11 on book page 442.
Afterwards
Make the pinhole camera bigger (a large box you can put your head into) and watch the street outside on a sunny day.
Tip
Safety: never look at the Sun through a lens, and never point a magnifying glass at the Sun near paper, skin or anything that can burn; a lens can focus sunlight hot enough to start a fire.
Answers to the book's problems
1. Farther away, so the whole figure's image is small enough to fit on the plate. 2. We judge size by taking into account how far away things are. 3. 40 (10 in. ÷ ¼ in.). 4. 360. 5. The 6-in. lens: the image is as much larger than the slide as the screen distance is greater than the slide distance, which is nearly the focal length. 6. 160 (10 × 8 ÷ (½ × 1)). 7. Its eyepiece inverts the image a second time. 8. 4. 9. Nearer to the objective. 10. 0.16 in. 11. Concave lenses for shortsightedness, convex lenses for longsightedness.
Worked out for this site from the scan. If a number in your copy differs, trust the book.