AY Honors/Optics/Answer Key
1. Define and/or draw a diagram of the following:
- a. Focal length
- b. Positive lens
- c. Negative lens
- d. Two kinds of distortion
- e. Longitudinal color
- f. Lateral color
- g. Spherical aberration
- h. Achromatic lens
- i. Refraction of light
2. Explain how light behaves when it strikes or traverses water, oil, feldspar, and a mirror.
3. Name and draw diagrams of three kinds of positive lenses and three kinds of negative lenses.
4. What should be the minimum distance of light source from the lens when testing for focal length?
5. Find the focal length of at least four lenses, one being a negative lens.
6. Explain by diagram why an image from a positive lens makes an image reversed and inverted.
7. Show with diagrams how a prism works. State the angles at which the colors appear and disappear.
8. Show and demonstrate what happens when light strikes one-way glass.
9. Construct one optical instrument using mirrors or lenses, such as a periscope, a slide or opaque projector, or a simple telescope.
10. Explain what is meant by the term 6[math]\displaystyle{ \times }[/math]35 and 7[math]\displaystyle{ \times }[/math]50 as applied to binoculars.
The numbers used for describing binoculars give the power followed by the diameter in milimeters. This means that 6[math]\displaystyle{ \times }[/math]35 have a magnifying power of 6x and have a diameter of 35.
The power represents how much bigger the object appears than with the unaided eye. You can also think of how many times closer it appears to be. The diameter lets you know how much light is gathered by the lens. A 50mm objective lens gathers much more light than a 35mm objective lens. Because the amount of light goes up by area we can calculate the light gathering difference as follows:
Area of lens= π (radius of lens)[math]\displaystyle{ ^2 }[/math]
| Binocular Specification | Magnifying Power | Diameter of objective (mm) | Area of objective [math]\displaystyle{ mm^2 }[/math] | Light gathering power (compared to unaided eye) |
| Unaided Eye | 1x | 8 | 50.265 | 1x |
| 10x25 | 10x | 25 | 490.8738 | 9.7x |
| 6x35 | 6x | 35 | 962.11275 | 19.14x |
| 7x50 | 7x | 50 | 1,963.495 | 39.06x |
The 50mm objective lens collects 2.0408... times more light than a 35mm objective lens. If you are in a dark environment, you will get twice as much light with the 7x50 binoculars than with a 6x35. The only real disadvantage is that the 7x50 binoculars are heavier to carry as well because of the increase in size.
