6.5 - Optical Instruments

Magnifying Glass and Uses of Optical Instruments

A magnifying glass uses a convex lens with the object inside its focal length to form a virtual, upright, magnified image.
Gemologists use magnifying glasses to identify and evaluate gemstones; ophthalmologists use them to examine eyes.
Microbiologists use microscopes to examine microorganisms; geologists use them to identify rock and mineral specimens.
Astronomers use telescopes to study celestial objects.

Uses two convex lenses with short focal lengths: the objective lens has focal length $f_{o}$ and the eyepiece lens has focal length $f_{e}$ , with $f_{o} < f_{e}$ .
The separation of the lenses is greater than $f_{o} + f_{e}$ .
Place the object between $F_{o}$ and $2 F_{o}$ . The objective forms the first image, $I_{1}$ , which is real, inverted and magnified.
$I_{1}$ lies between the eyepiece’s focal point and optical centre and acts as the object for the eyepiece.
The eyepiece functions as a magnifying glass, producing final image $I_{2}$ , which is virtual, magnified and inverted relative to the original object.
Adjust the eyepiece so that the final image is at the observer’s near point, approximately 25 cm, for clear vision.

Uses two convex lenses: an objective with a long focal length, $f_{o}$ , and an eyepiece with a short focal length, $f_{e}$ .
Parallel rays from a distant object converge in the objective’s focal plane, forming first image $I_{1}$ : real, inverted and diminished.
$I_{1}$ acts as the object for the eyepiece. The final image, $I_{2}$ , is virtual, magnified and inverted relative to the original object.
At normal adjustment, the focal planes coincide, the final image is at infinity and lens separation is:

L = f_{o} + f_{e}

M = \frac{f _{o}}{f _{e}}

A longer objective focal length relative to the eyepiece focal length gives greater magnification. A larger objective diameter produces a brighter image.

Thin smartphones and small closed circuit television (CCTV) cameras use small convex lenses.
The lens forms a real, inverted, diminished image on the sensor, enabling clear photographs and video recordings.
The minimum distance from the lens centre to the sensor equals the camera lens’s focal length.
Since focal length cannot be zero, it limits the overall thickness of the smartphone or CCTV camera.