Optical Instruments

• Optics is the study of visible light and the ways it can be used to extend

human vision and do other tasks. Knowledge of light was needed for the

invention of optical instruments such as microscopes, telescopes, binoculars

and cameras, in addition to optical fibers. These instruments use mirrors and

lenses to reflect and refract light and form images.

• Optical instruments are devices which are used to help the human eye views

small or distant objects more clearly.

Simple Microscope

• It consists of a biconvex lens which may be hand-held or placed in a simple

frame. It is sometimes referred to as a magnifying glass of small focal length.

The idea is to get an erect, magnified and virtual image of the object. For this

the object is placed between F and P on one side of the lens and viewed from

other side of the lens. There are two magnifications to be discussed for two

kinds of focusing

(i) Near point focusing – The image is formed at near point, i.e. 25 cm for

normal eye. This distance is also known as least distance D of distinct

vision. In this position, the eye feels comfortable but there is little strain on

the eye. This is shown in Figure (a) below

(ii) Normal focusing – The image is formed at infinity. In this position the eye

is most relaxed to view the image. This is shown in Figure (b). below

Magnification in near point focusing

• The near point focusing is shown in Figure (a) below.

• Object distance 풖 is less than 풇. The image distance is the near point D. The

magnification 풎 is given by the relation,

• With the help of lens equation,

as

the magnification can further be written

, but

,

therefore;

• Substituting for v with sign convention, i.e., 풗 = −푫, we get

• Therefore;

This is the magnification for near point focusing (it is the

maximum magnification).

Magnification in normal focusing (angular magnification)

• The normal focusing is shown in Figure (b). We will now find the magnification

for the image formed at infinity. If we take the ratio of height of image to height of

object (

) to find the magnification, we will not get a practical relation, as

the image will also be of infinite size when the image is formed at infinity.

• Hence, we can practically use the angular magnification. The angular

magnification is defined as the ratio of angle 휽_풊subtended by the image with

aided eye to the angle 휽_풐subtended by the object with unaided eye.

• For unaided eye shown in figure (b) (i) above,

• For aided eye shown in figure (b)(ii) above,

• The angular magnification,

• Therefore, angular magnification,

focusing. (It is the minimum magnification)

NB;

. This is the magnification for normal

• The magnification for normal focusing is one less than that for near point

focusing. But the viewing is more comfortable in normal focusing than near point

focusing. For large values of

, the difference in magnification is usually small.

• In the case of the simple microscope or magnifying glass, the size of the image

formed depends on the focal length of the lens used. The shorter the focal

length the more powerful the microscope becomes. Worked examples

1. A diamond is viewed with a jeweler's loupe that has a focal length of 5.0 cm.

Where must the diamond be placed to provide a virtual image at infinity?

What will be the angular magnification of this simple magnifier?

ANS;

• The diamond should be placed at the focal point of the lens (the jeweler's

loupe), that is, 5.0 cm from the lens to provide a virtual image at infinity.

• The magnification is given by;

2. Aman with a near point of 25 cm reads a book with small print using a magnifying

glass, a convex lens of focal length 5 cm.

(a) What is the closest and the farthest distance at which he should keep the

lens from the page so that he can read the book when viewing through the

magnifying glass?

(b) What is the maximum and the minimum angular magnification (magnifying

power) possible using the above simple microscope?

ANS; 푫 = ퟐퟓ 풄풎, 풇 = ퟓ풄풎

NB;

• For closest object distance, u; the image distance, v is, –25 cm. (near point

focusing)

• For farthest object distance, u’; the corresponding image distance, v’ is, 풗 =

∞ (normal focusing)

(a) For closest and farthest distance

• For closest object distance; consider

•

cm

• For furthest distance; consider

•

cm

(b) Maximum and minimum Magnification;

• Magnification in near point,

• Magnification at normal focusing,

Uses

• It is used to view specimen in the laboratory

• It is used by watchmakers to obtain a magnified view of small parts of the watch.

• It is used by jewelers to get a magnified view of the fine parts of jewelry.

• A simple microscope is called the hand lens or the reading lens to obtain an

enlarged view of hard-to-read small alphabets.

• It is used by a dermatologist (a skin specialist) to examine various skin

diseases.

• It is used in studying soil particles (Pedology)