Optical instruments

Myopia is the defect of an eye which cannot see the distant (object at infinity) clearly. An eye suffering from this defect forms the image of distant object before retina.

Hypermetropia: In this defect, a person can see the distant objects clearly but not the objects nearer to the eye.

Accommodation is the phenomenon of adjusting the focal length of eye lens in order to focus the image of the object at the retina.

Visual angle: When an object is viewed, the angle subtended by the object at the eye is known as visual angle.

Simple microscope is an optical instrument used to view near objects. It is simply a magnifying glass of converging lens of short focal length.

Angular magnification is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended at the eye by the object when placed at the same distances as the image.

Telescope is an optical instrument which is used to see the distant objects.

Camera: A photographic camera consists of a converging lens system at one end of a light proof box and light sensitive film at the other end. A real inverted image of the object is formed on the film.

Useful Informations

• Ciliary muscles control the focal length of the eye lens.
• The image on the retina is an inverted image.
• Pres Biopia is also called old age defect.
• To get a sharp and bright image in a simple microscope achromatic lens in used.
• A modern compound microscope gives a magnification of about 2500 times.
• The aperture of the eye piece should be small so that whole light may enter the eye.
• Reflecting telescope are free from chromatic and spherical aberration.
• Microscope in which ultraviolet is used to examine the object is known as ultra microscope.
• Least distance of distinct vision for a child is less than 25 cm.
• An eye can suffer from miopia, hypermetropia as well as astigmatism simultaneously.
• Magnification of a simple microscope is limited (maximum 10).
• Rayleigh law of scattering is invalid if scatterer size is greater than the wavelength of the incident light.

Key formula

1. f=-x
2. $\frac{1}{f}$=$\frac{1}{D}-\frac{1}{X}$
3. Simple microscope : $M=\frac{\beta }{\alpha }=\frac{D}{u}$
4. Image at D: M=-$\left(1+\frac{D}{f}\right)$
5. Image at infinity : M=$\frac{D}{f}$
6. Compound microscope : M=-$\frac{v_o}{u_o}$$\left(\frac{D}{f_e}+1\right)$
7. M=$\frac{v_o}{u_o}$$\frac{D}{f_e}$, image formed at infinity
8. Astronomical telescope : M=$\frac{\beta }{\alpha }$=$\frac{f_o}{f_e}$, L=$(f_o+f_e)$, normal adjustment
9. Image at least distance : M=$\frac{f_o}{f_e}$$\left(1+\frac{f_e}{D}\right)$; L=$(f_o+u_e)$

fig: Compound Microscope