The ray of light which falls on the mirror surface is called incident ray.
The point at which the incident ray strikes the mirror is called the point of incidence.
The ray of light which is sent back by the mirror is called the reflected ray.
The ‘normal’ is a line drawn at right angles to the mirror surface at the point of incidence.
The angle between incident ray and normal is called the angle of incidence.
The angle between reflected ray and normal is called the angle of reflection.
Laws of reflection of light:
Ø The incident ray, the reflected ray, and the normal {at the point of incidence}, all lie in the same plane.
Ø The angle of reflection is always equal to the angle of incidence.
Regular Reflection and Diffuse Reflection of Light
Regular Reflection
In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction.
Regular reflection of light occurs from smooth surfaces like that of a plain mirror.
Images are formed by regular reflection of light.
Diffused Reflection
In Diffuse reflection, a parallel beam of incident light is reflected in different directions.
The diffuse reflection of light takes place from rough surfaces
A sheet of paper produces diffuse reflection of light.
No image is formed in diffuse reflection of light.
Characteristics of image formed by a plane mirror
Ø The image formed by a plane mirror is virtual & Erect.
Ø The image formed in a plane mirror is the same distance behind the mirror as the object is in front of it.
Ø The image formed in a plane mirror is of the same size as the object.
Ø The image in a plane mirror is laterally inverted.
Periscope : A Periscope is a device through which a person can se the objects that are out of the direct line of sight.
For example, by using a periscope, we can see the objects on the other side of a high wall which cannot be seen by us directly. Periscope has two plane mirrors arranged parallel to one another.
Each plane mirror, however, makes an angle of 450 with the side of the tube.
Some of the uses of periscopes are given below:
Ø Periscope is used to see over the heads of a crowd
Ø A Periscope is used by soldiers sitting in bunker to observe the enemy activities outside {over the ground}.
A Periscope is used by a navy officer sitting in a submarine to see ships over the surface of water in the sea.
Multiple Images: When two plane mirrors are kept inclined at an angle, they can form multiple images of an object. If two plane mirrors are inclined at an angle q, then the number of images formed in them is given by the formula:
No. of images formed = (3600/ q) - 1
Kaleidoscope : The kaleidoscope is an instrument which produce multiple reflections of coloured glass pieces {or coloured plastic pieces} and create beautiful patterns.
Construction: The Kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 600 to one another forming a hollow prism, and fitted into a cardboard tube.
One end of the cardboard tube is closed by an opaque disc {cardboard disc} having a small hole at its centre. The other end of cardboard tube is closed with circular discs of glass. The inner disc being of transparent glass {clear glass } and the outer disc of ground glass {translucent glass}.
A number of small pieces of different coloured glass {or plastic } and having different shapes are kept between the two glass discs {which can move around freely in the space between the two glass discs}. When we hold the kaleidoscope tube towards light and look inside it through the small hole, we see beautiful patterns of coloured glass.
Patterns formed by Kaleidoscope
Dispersion of light
The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. The seven colours of the spectrum of white light are: violet, indigo, blue, green, yellow, orange and Red.
The Human Eye
The main parts of the human eye are : Cornea, Iris, Pupil, Ciliary muscles, Eye lens {which is a flexible convex lens}, Retina and Optic nerve.
Cornea is made of a transparent substance
The light coming from an object enters into the eye hrough cornea. Functionof cornea is to protect the eye.
Iris is the colored part of the eye. The iris has a hole at its centre which is called pupil. Pupil appears like a dark spot in the centre of iris because no light is reflected from it. Iris controls the amount of light entering the eye by adjusting the size of pupil.
The eye-lens is a convex lens made of a transparent and flexible material.
The eye-lens is held in position by ciliary muscles. The function of ciliary muscle is to hold the lens & changes its size according to the need.
The retina is a screen on which the image is formed in the eye.
The optic nerve carries the image formed on retina to the brain in the form of electrical signals. Rods and cones
Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light.
Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light. Cones also cause the sensation of colour of objects in our eyes.
Blind spot
At the junction of optic nerve and retina in the eye, there are no light sensitive cells {no rods or cones }due to which no vision is possible at spot. This is called blind spot.
Persistence of vision
The image of an object seen by our eyes persists {or remains}on the retina for about 1/16th of a second even after the object has disappeared from our view. The abilily of an eye to continue to see the image of an object for a very shot duration even after the object has disappeared from view, is called persistence of vision.
Defects of the Eye:
a) Myopia (short-sightedness) : In this defect the person is able to see the near by objects but not able to see the far-off objects.
This defect can be corrected by using spectacles containing Concave lens.
b) Hypermetropia (Far-sightedness) : In this defect the person is able to see the far-off objects but not able to see the near by objects.
This defect can be corrected by using spectacles containing Convex lens.c) Cataract : Cataract develops when the eye lens of person becomes cloudy (or even opaque) due to the formation of membrane over it. Cataract decreases the vision of the eye gradually. It can even lead to total loss of vision of the eye.
The opaque lens is removed from the eye of the person by surgical operation & a new artificial lens is inserted in its place.
Refraction of Light
The deviation in the path of light when it passes from one medium to another medium of different density is called refraction.
The twinkling of stars is due to atmospheric refraction of starlight. Since the atmosphere bends starlight towards the normal the apparent position of the star is slightly different from its actual position. Hence the star appears slightly higher than its actual position.
Refraction and Reflection of Light
IO incident ray OR' reflected ray OR refracted ray
The diagram shows how the light gets refracted when it is traveling from one optical medium to another. Like reflection, refraction of light takes place according to certain laws.
Terms which are commonly used to explain the phenomenon of refraction.
Incident Ray The ray of light striking the surface of separation of the mediums through which it is traveling is known as the incident ray.
Point of Incidence The point at which the incident ray strikes the surface of separation of the two mediums is called the point of incidence.
Normal The perpendicular drawn to the surface of separation at the point of incidence is called the normal.
Refracted Ray The ray of light which travels into the second medium, when the incident ray strikes the surface of separation between the mediums 1 and 2, is called the refracted ray.
Angle of Incidence (i) The angle which the incident ray makes with the normal at the point of incidence, is called angle of incidence.
Angle of Refraction (r) The angle which the refracted ray makes with the normal at the point of incidence, is called angle of refraction.
Cause of Refraction A ray of light refracts or deviates from its original path as it passes from one optical medium to another because the speed of light changes.
Laws of Refraction
The incident ray, the refracted ray and the normal to the surface at the point of incidence all lie in one plane.
For any two given pair of mediums, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. The above law is called Snell's law after the scientist Willebrod Snellius who first formulated it Thus, Sin i /Sin r = a constant = m
Where m is the refractive index of the second medium with respect to the first medium.
We know that the phenomenon of refraction is taking place because the speed of light changes when it is traveling from one optical medium to another. Thus we can define refractive index in terms of the speed of light in the two media. The refractive index of glass with respect to air is given by the relation
m = speed of light in air/speed of light in glass
If the medium 1 is air or vacuum, the refractive index of medium 2 is referred to as the absolute refractive index. Critical angle is that angle of incidence for which a ray of light while moving from a denser to a rarer medium just grazes over the surface of separation of the two media (that is, angle of refraction = 90o).
Total Internal Reflection
If the angle of incidence of a ray of light traveling from a denser medium to a rarer medium is greater than the critical angle for the two media, then the ray is reflected into the denser medium and this phenomenon is described as total internal reflection.
The conditions to be satisfied for total internal reflection to take place are
The ray of light must travel from a denser medium to a rarer medium.
The angle of incidence must be greater than the critical angle for those two mediums. The refractive index of a medium depends on the following factors:
1. the nature of the medium 2. the color or wave length of the incident lightPoints to remember
1.) There can be only one reflected ray for a given single incident ray falling on a plane mirror.
2.) A ray of light which is incident normally on a mirror is reflected back along the same path because the angle of incidence for such a ray of light is 0o the angle of reflection is also 00.
3.) Diffuse reflection is caused by the roughness in the reflecting surface of an object.
4.) Laws of reflection are valid for both regular reflection & irregular reflection.
5.) If a person is standing at a distance of 1 metre in front of a plane mirror, then distance between the person & his image will be 2 metres.
6.) In an image formed by plane mirror, the left side of the object appears on the right side in the image whereas the right side of the object appears on the left side in the image.
This change of sides of an object & its mirror image is called Lateral inversion.
7.) As the angle between the two plane mirrors decreases, the number of images formed increases.
When the angle between the two plane mirrors becomes 0 deg ie. When the two mirrors are parallel to each other then infinite number of images is formed.
8.) An interesting feature of a kaleidoscope is that we can never see the same patterns again. Every time a new pattern is formed. Kaleidoscope are used by designers of wall papers & fabrics, as well as by artists to get ideas for new patterns.
9.) Rainbow in the sky is natural phenomenon showing the dispersion of sunlight. Rainbow is produced by the dispersion of sunlight by the tiny rain drops suspended in the atmosphere (which act as tiny prisms made of water).
10.) The adjustment of the size of pupil takes some time. That’ when we go from a bright light to a darkened cinema hall, at firstwe cannot see our surroundings clearly.
11.) The range of vision of a normal human eye is from infinity to about 25 centimeters.
