Focal Length

• To study deviation of light in diverging and converging lenses
• To learn about the focal length of a lens

Optics has a long history when it comes to astronomy and space exploration. Galileo (1564-1642) was one of the first to use a small telescope to look at the Moon and other objects in the night sky (the first to report his observations anyway). He discovered...

Sunspots and that the planets were disks, not points of light like stars.

Better ground based telescopes and special optical instruments such as spectrometers enabled people to discover many secrets of the universe, and ground based telescopes and scientific optical instruments continue to be used and improved even today.

But since 1957, optics has also been an important part of the Space Age, starting with ground-based telescopes used to track rockets and satellites. Soon after there were Earth satellites with cameras, telescopes, and other optical instruments, many to look back at Earth, and many more to look at the stars and planets. A little later the US and the then Soviet Union also started to send spacecraft out of Earth orbit to visit the Moon, Venus, Mars, and eventually all the planets except Pluto (a NASA probe is on its way to Pluto now, due to arrive in 2015).

The Hubble Space Telescope is perhaps the most famous space-based optical system, and several engineers at ORA actually worked on the design and other aspects of the repair optics (ORA even won a NASA award for this and other space-related work).

Lens. A lens is an optical device which transmits and refracts light, concentrating or diverging the beam. Manufactured lenses are typically made of glass or transparent plastic. Lenses are characterized by a focal length and curvature (or radius).

Curvature. The curvature is defined as 1/R, where R is the radius of curvature.

Converging and diverging lenses. In general we can summarize the types of lenses as converging or diverging ones (we call f the focal length and R the radius).

For example, observing an object through a biconvex converging lens, it is possible to get a rough estimate of the focal length of a converging lens.

Moving the lens away from the object, note the distance at which the object starts to become blurred. This distance is the focal length of the lens.

Optical power (dioptric power or refractive power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. For two or more thin lenses close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens. Similarly, the optical power of a single lens is roughly equal to the sum of the powers of each surface.

• Inclined plane, with 3 different track terminals — 5491.11
• Holder for 50 mm diameter lenses and mirrors — 4414.02
• Slider for holder — 4417.01
• Prism table — 4418.29

• What happens if we use two lenses together, one converging and the other one diverging?
• What do you think will happen if the diverging one has the same focal length as the converging one?