10th Class Science Human Eye and Colourful World Light and Human Eye

Light and Human Eye

 

Light is an electromagnetic wave which do not require a material medium for their propagation. Light is composed of particles which travel in a straight line at very high speed. Light has a dual nature i.e. waves and particles. Speed of light is different in different mediums. Speed of light in vacuum is \[3\times {{10}^{8}}\text{ }m/s\].

 

Reflection of Light

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The process of sending back the light rays which fall on the surface of an object, is called reflection of light.

 

Rules for obtaining images formed by concave mirrors

 

• A ray of light which is parallel to the principal axis of a concave mirror, passes through its focus after refection from the mirror.
• A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path.
• A ray of light passing through the focus of a concave mirror becomes parallel to the principal axis after reflection.
• A ray of light which is incident at the pole of a concave mirror is reflected back making the same angle with the principal axis.
• F = R/2, where R = radius of curvature and F = focal length

 

Images Formed by Concave Mirrors

 

 

		object Position	Image Position	Nature of Image
(a)		at infinity	at the focus F	real and point-sized
(b)		between infinity and the centre of curvature C	between F and C	real, smaller than the object, inverted
(c)		at C	at C	real, enlarged, inverted
(d)		between C and F	between C and infinity	real, enlarged inverted
(e)		at F*	at infinity	real, infinitely large, inverted
(f)		between the pole	behind the P and F	virtual, enlarged mirror erect

 

 

Rules for obtaining images formed by convex mirrors

 

• A ray of light which is parallel to the principal axis of a convex mirror, appears to be coming from its focus after reflection from the mirror.
• A ray of light going towards the centre of curvature of convex mirror is reflected back along the same path.
• A ray of light going towards the focus of a convex mirror becomes parallel to the principal axis after reflection.
• A ray of light which is incident at the pole of a convex mirror is reflected back making the same angle with the principal axis.

 

 

Images Formed by Convex Mirrors

 

		Object Position	Image Position	Nature of Image
(a)		between infinity and the pole	between the focus and the pole	virtual, smaller and erect
(b)		at infinity	at the focus F	virtual, point sized

 

 

Mirror Formula

\[\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\]

Where,

v = distance of images from mirror

u = distance of object from mirror

f = focal length of the mirror

 

Magnification Produced by Mirrors

The ratio of the height of image to the height of object is known as linear magnification.

\[m=\frac{{{h}_{2}}}{{{h}_{1}}}\]

Where, m = magnification

\[{{h}_{2}}\]= height of image

\[{{h}_{1}}\]= height of object

The linear magnification produced by a mirror is equal to the ratio of the image distance to the object distance with a minus sign.

\[m=\frac{-v}{u}\]

So, \[\frac{{{h}_{2}}}{{{h}_{1}}}=\frac{-v}{u}\]

 

Refraction of Light

The bending of light when it goes from one medium to another medium is called refraction of light. The refraction of light occurs due to the change in the speed of light on going from one medium to another.

 

Laws of Refraction of Light

1. The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
2. For a given colour of light and for a given pair of media, the ratio of sine 01 angle of incidence to the sine of angle of refraction is always a constant.

\[\frac{\sin i}{\sin \,r}=\,cons\tan t\]

 

Refractive Index

Refractive index is ratio of speeds of light in the two mediums. The refractive index of medium 2 with respect to medium 1 is equal to the ratio of speed of light in medium 1 to the speed of light in medium 2.

 

 

Refraction of Light by Spherical Lenses

The working of a lens is based on the refraction of light rays when they pass through it. A lens is a piece of transparent glass bound by two spherical surfaces. There are two types of lenses: convex lens and concave lens.

 

Rules for Obtaining Images Formed by Convex Lens

 

• A ray of light which is parallel to the principal axis of a convex lens, passes through its focus after refraction through the lens.
• A ray of light passing through the optical centre of a convex lens goes straight after refraction through the lens.
• A ray of light passing through the focus of a convex lens becomes parallel to its principal axis after refraction through the lens.

 

Images Formed by a Convex Lens

 

 

		Position of the Object	Position of the Images	Nature of Size
(a)		At infinity	at F2	real, point-size
(b)		between infinity and 2F1	between F2 and 2F2	real, smaller inverted
(c)		at 2F1	at 2F2	real same ? sized inverted
(d)		between 2F1 and F1	between infinity	real, enlarged inverted
(e)		at	F1	at infinity real, infinitely large, inverted
(f)		between F1 and O	on the side of the object	virtual, larger erect

 

Rules for Obtaining images Formed by Concave Lenses

 

• A ray of light which is parallel to the principal axis of a concave lens, appears to be coming from its focus after refraction through the lens.
• A ray of light passing through the optical centre of a concave lens goes straight after passing through the lens.
• A ray of light going forwards the focus of a concave lens, become parallel to its principal axis after refraction through the lens.

 

Images Formed by a Concave Lens

 

	Position of the Object	Position of the Images	Nature of Size
	at infinity	at F2	virtual, point sized
	between infinity and O	between F2 and O	virtual, erect and smaller

 

 

Lens Formula

\[\frac{1}{v}-\frac{1}{u}=\,\frac{1}{f}\]

Where, v = image distance

u= object distance

f = focal length

 

 

Magnification produced by lenses

\[Magnification\,=\frac{height\,of\,image}{height\,of\,object}\]

\[m\,=\,\frac{{{h}_{2}}}{{{h}_{1}}}\]                                          (i)

Where, m = magnification

\[{{h}_{2}}\]= height of image

\[{{h}_{1}}\]= height of object

The linear magnification produced by a lens is equal to the ratio of image distance to

\[m\,=\frac{v}{u}\]                                         (ii)

Where, m = magnification

v = image distance

u = object distance

from (i) and (ii)

\[m\,=\frac{{{h}_{2}}}{{{h}_{1}}}=\frac{v}{u}\]

 

Power of a Lens

The power of a lens is defined as the reciprocal of its focal length in metres.

\[P\,=\,\frac{1}{F}\]

Where, P = Power of the lens

F = Focal length of the lens

The unit of the power of a lens is dioptre, which is denoted by the letter D.

 

Eye

The human eye is one of the most important living material which enables us to see things around us.

The main parts of the human eye are: Cornea, Iris, Pupil, Ciliary muscles, eye lens, retina and optic nerve.

 

Working of Human Eye

The light rays coming from the object kept in front of us enter our eyes through the cornea, pass through the pupil of the eye and fall on the eye-lens.

The eye-lens is a convex lens, so it converges the light rays and produces a real an inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision.

 

Defects of Vision

There are mainly three common refractive defects of vision. These are myopia or near- sightedness, Hypermetropia or farsightedness and Presbyopia. These defects can be corrected by lenses.

 

Myopia (Short-sightedness)

A person suffering from myopia can see nearby objects clearly but cannot see distant objects. This defect may arise due to thikning of eye-lens or due to eye-ball being too long. This defect can be corrected by using a concave lens of suitable power.

 

Hypermetropia (Far-sightedness)

It is also known as far-sightedness. A person with hypermetropia can see distant object clearly but cannot see nearby objects distinctly. This defect arises either because the focal length of the eye lens is too long or the eyeball has become too small. This defect can be corrected by using a convex lens of appropriate power.

 

Presbyopia (Old-sight)

The people find difficult to see nearby objects. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens with ageing.

 

Dual Eye Defect

Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bifocal lenses. A common type of bifocal lenses consists of both concave convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.