NEET Physics Notes Magnetostatics EMI and AC, EM waves-Ray Optics-Human Eye
The human eye is one of the most valuable and sensitive sense organs of human being. The cornea, acqueous humour, crystalline lens and vitreous humour together form a lens system which forms an inverted and real image on retina for the objects situated in front of eye. The optic nerve transmits this image to the brain which makes it erect and analyses it.
The eye lens has power of accommodation to adjust its focal length, so as to focus objects situated at different distances from eye at the retina.
The least distance of distinct vision (D) or near point of an eye is generally 25 cm. Far point of a normal eye is at infinity.Ciliary muscles play an important role in changing focal length of eye lens.
Persistence of vision is i.e. if time interval between two consecutive light pulses is lesser than 0.1 s, eye cannot distinguish them separately.Resolving limit for eye is
For myopic (near sighted) eye near point is at 25cm but far point comes closer to eye from ∞ causes of near sightedness are
- decrease in focal length of eye lens, or
- elongation of eye ball. The defect can be rectified by use diverging lens.
If a myopic person cannot see objects situated beyond a distance x, then he should use lens of focal length f = – x. So, concave lens is used to correct this. A person suffering from hypermetropia or long sightedness can see distant objects clearly but cannot see nearby objects distinctly. It means that his near point has shifted away from 25 cm to distance x. Possible causes of hypermetropia are
- increase in focal length of eye lens, or
- shortening of eye ball.
To rectify hypermetropia, the person should use a converging lens of focal length , so that it forms virtual image of an object situated at distance (D = 25 cm) at near point x of defective eye
So, convex lens is used for its correction.
Presbyopia is due to weakness of ciliary muscles due to advancing age. Generally, defective eye suffers from near sightedness as well as long sightedness. To rectify the defect bifocal lenses are used.
Astigmatism is due to non-uniformity of curvature of the cornea. The defective eye cannot concentrate simultaneously along horizontal as well as vertical. To rectify this defect, cylindrical lenses are used.
Colour of Objects
An opaque object appears to be of that colour which it selectively reflects. A red rose appears red because out of white light, it reflects light of red colour only and absorbs light of all other colours.
Defects of Images
The two types of defects which are occurred commonly, given below
- Spherical Aberration
This defect is present in spherical mirrors as well as in lenses, whose aperture is comparatively large.
In this defect paraxial rays bend less on refraction/reflection and meet farther away from the lens/mirror. But marginal rays bend more and meet nearer.
In mirrors, the spherical aberration can be almost eliminated by taking a paraboloidal mirror instead of a spherical mirror.
In a lens, we can minimise the aberration by
(a) using plano-convex lens with plane surface on the side of more convergent/divergent light beam.
(b) by using a combination of two lenses separated by a distance d = fx- f2.
- Chromatic Aberration
This defect is present only in lenses.
Chromatic aberration is the inability of a lens to focus rays of different colours/ wavelengths at a single point. If a white light beam is incident on a lens, then violet rays are deviated the most and red rays the least. Hence, a blurred coloured image is formed on the screen.
Longitudinal chromatic aberration of a lens is measured by the difference in the focal lengths of red and violet light. It is found that longitudinal chromatic aberration where ω = dispersive power of lens material and f = mean focal length of the lens.
Chromatic aberration can be removed by preparing a combination of two thin lenses of different materials
An optical instrument is used to enhance and analyzes the light waves. The light waves are in the form of photons, hence optical instruments also determine the characteristics properties of light waves.
It is an, optical instrument which forms a magnified image of a small nearby object and thus, increases the visual angle subtended by the image at the eye, so that the object is seen to be bigger and distinct.
i. Simple Microscope (Magnifying Glass) It consists of a single convex lens of small focal length and forms a magnified image of an object placed between the optical centre and the principal focus of the lens. If the image is formed
If final image is formed at infinity, then
D = normal viewing distance (25 cm)
f= focal length of magnifying lens.
It consists of two lenses of small focal length and small apertures. Also, the focal length and aperture of objective lens are smaller than that of eyepiece. The image formed by the objective lens is real, inverted and magnified. This image acts as the object for the eyepiece and the final image is highly magnified, virtual and inverted w.r.t. the original object.
Telescope is an optical instrument which increases, the visual angle at the eye by forming the image of a distant object at the least distance of distinct vision, so that the object is seen distinct and bigger.
It consists of an objective lens of large focal length fe and large aperture.
The eyepiece consists of a convex lens of small aperture and small focal length fe. Distance between the two lenses is set as,
In normal adjustment, the final image is formed at infinity and magnifying power of the telescope is
In practical adjustment, the final image is formed at the near point of the observer’s eye. In this arrangement,
Resolving Power of an Optical Instrument
Resolving power of an optical instrument is its ability to produce distinct images of two points of an object (or two nearby objects) very close together. Resolving power of an optical instrument is inverse of its limit of resolution. Smaller the limit of resolution of a device, higher is its resolving power. Limit of resolution of a normal human eye is 1′.
The minimum distance (or angular distance) between two points of an object whose images can be formed distinctly by the lens of an optical instrument, is called its limit of resolution.
Resolving Power of a Telescope
If the aperture (diameter) of the telescope objective be the D, then the minimum angular separation dθ between two distant objects, whose images are just resolved by the telescope, is
and resolving power of the telescope,
Resolving Power of a Microscope
The least distance (d] between two points, whose images are just seen distinctly by a microscope, is given by
where,λ= wavelength of light used to illuminate the object,
nm = refractive index of the medium between the object and the objective lens, and θ= semi angle of the cone of light from the point object.
The term is generally called the numerical aperture of the microscope, the objective.
Resolving power of the microscope,
To see the objects which are too small for the naked eye, we usually use the optical instrument named as microscope. Investigating such as small objects is called microscopy! Microscopic means invisible to the eye, unless aided by a microscope.