PHYSICS || OPTICAL ISTRUMENTS

OPTICAL INSTRUMENTS

Simple Microscope

The Structure of the Simple Microscope

Describe the structure of the simple microscope

A magnifying glass, an ordinary double convex lens with a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type. When an object is placed nearer such a lens than its principal focus, i.e., within its focal length, an image is produced that is erect and larger than the original object. The image is also virtual; i.e., it cannot be projected on a screen as can a real image.

The Mode of Action of a Simple Microscope

Describe the mode of action of a simple microscope

The image formed by magnifying glass or simple microscope is virtual and erect object place between principal focus (f) and convex lens.

• The normal district vision
• The position of the lens is usually adjusted so that V is about 25cm, which is the shortest distance of distinct vision.

Using the equation of lens (Lens formula).

I/U + I/V = I/F

Adopting the 'real is positive' sign convention we obtain:

V = (-Ve) since the image is virtual.

I/U – I/V = I/F

V= 25 –(Normal district vision)

I/U – I/25 =I/F

I/U = I/F + I/25

(I/U)=-1 (25 + F )

25F

U = 25F/F+25

The above formula shows the means of obtaining the distance of object, U.

Magnication (M) of simple microscope

Magnification is the ratio of the image distance to the object distance.

M = Image distance, V

Object distance, U

Hence

M = v/u …………………..(i)

From V = 25cm (distance of district vision)

From U = 25f/(f+25) ……………………… (ii)

Insert eqn (ii) into (i)

M = V/ (25f/(f+25)

M = 25/(25f/f+25)

M = 25/f + 1

Example 1

A simple microscope with lens of focal length 5cm is used to read division of a scale 0.5mm in size. How large will the division be seen through the simple microscope?

Data given

• Focal length, f = 5cm
• Required to find magnification, M

Soln:

From

M = (25/f + 1)

= (25/5+1)

=(5+1)

= 6

The magnification of lens = 6

Let the size of the object be ho and that of the image be hi. Then:

M = h1/H ……………(i)

H1 = 6h

The Height , h = (0.5mm)

H1 = 6 (0.5mm)

HI = 3mm

Hence, each division will appear to have a size of 3.0mm viewed through the simple microscope.

A Simple Microscope

Construct a simple microscope

Parts of simple microscope