OPTICAL INSTRUMENTS
These are devices which are used to help the human eye to view small or distant objects more
clearly.
Optical instruments use refraction or reflection or both to form different types of images.
Optical instruments use combination of lenses and/or mirrors to produce an enhanced image of an
object.
Common optical instruments include the following:
1. Microscopes (simple and compound microscope)
2. Telescopes
3. Cameras
4. Projectors
1. MICROSCOPES (SIMPLE AND COMPOUND MICROSCOPE)
a) SIMPLE MICROSCOPES (MAGNIFYING GLASS)
❖ The simple microscope (magnifying glass) consists of a biconvex lens which may be hand – held
or placed in a simple frame.
MODE OF ACTION ON A SIMPLE MICROSCOPE
When the lens is held close to the eye and the object to be viewed is placed at a distance shorter than
the focal length (f) of the lens, a virtual, upright and magnified image of the object is formed.
The object must be placed at a distance between the lens and its focal point for clear image to be
formed.
The image appears clearest when it is about 25cm from the eye. This distance (thus 25cm) is called near point (D). The nearer the object is to the lens, the further and larger the image is formed.
NOTE:
NEAR POINT (D): This is the distance whereby an image appears clearest (sharpest).
NATURE/PROPERTIES OF IMAGE FORMED BY A SIMPLE MICROSCOPE
➢ Image is virtual
➢ Image is erect/upright
➢ Image is magnified
MAGNIFICATION OF A SIMPLE MICROSCOPE
The simple microscope magnifies an object by increasing the angle subtended at the eye by the object.
Where by: α and β = Angle subtended at the eye by an object
hi = Height of image
h0 = Height of object
D = Near point = 25cm
❖ The magnification produced by a simple microscope depends on the focal length of the lens.
❖ Lenses of small focal length give greater magnification than those with big focal length.
TYPES/CATEGORIES OF MAGNIFICATION PRODUCED BY A SIMPLE MICROSCOPE
a) Lateral magnification
b) Linear magnification
c) Angular magnification
