physics chp 9 light - reflection and refraction

light basics about reflection magnification factor spherical mirrors concave and convex pole,focus,centre of curvature,principal axis,focal length,radius of curvature. ray digram for concave and convex mirror

Light

light is a form of energy that causes sensation of vision

there are various phenomenon of light

reflection - the phenomenon of bouncing back of light in the same medium on striking a highly polished surface.

Refraction is the phenomenon of light bending as it passes from one medium to another, causing a change in its speed and direction.

regular reflection

when the rays of light strike a highly polished surface and bounce back in the same medium this type of reflection is known as regular reflection

irregular reflection

when the rays of light strike a rough surface and the reflected rays spread in random direction then this type of reflection is known as irregular reflection

basics related to reflection

ray of light

the representation of direction of light is known a ray of light

To represent a ray of light, draw a straight line with an arrowhead perpendicular to the surface it strikes.

beam of light

the collection of a large number of rays of light is called a beam of light

there are two types of beam of light

converging - when the rays of light approach at a point

diverging - when the rays of light move away from each other from a point

mirror

consider a rectangular glass slab and coat one of its side by using silver foil. After coating paint that part with a dark colour. This part will now become non reflecting and the front part will become reflecting

source of light

the media from which light is produced is known as the source of light

there are two sources of light

luminous - the source of light that produces its own light ex- sun,stars

non luminous - the source of light that does not have its own light ex- moon,human body

the incident ray

the ray of light which strikes the reflecting part of the mirror is known as the incident ray

point of incidence

the point at which the incident ray strikes the reflecting part of the mirror is known as the point of incidence

reflected ray

the ray of light that bounces back after striking the reflected part of mirror is called the reflected ray

angle of incidence

the angle made by the incident ray and normal is called the angle of incidence

angle of reflection

the angle made by the reflected ray and normal is known as the angle of reflection

the laws of reflection

1st - the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane

2nd - angle of incidence and the angle of reflection are always equal [ regular reflection should occur ]

image

optical appearance of an object is called image

an image at the point where the reflected rays intersect

there are types of image

real image - it is the type of image which is formed by the converging of reflected rays. Real image is always formed in front of the mirror and always formed at the point where the rays intersect

virtual image - it is

magnification factor

magnification factor is defined as the ration of height of image and height of object

m = h1/ h

magnification factor can be greater than 1 smaller than 1 and equal to 1

if the height is image formed is greater than the height of object then m is greater than 1

if the height of image formed is equal to the height of object then m is equal to 1

if the height of image formed is smaller than the height of object then m is more than 1

m can be negative and positive

if the image formed is real and inverted then m will be negative

if the image formed is virtual and erect then m will be positive

spherical mirrors

when the reflecting part of the mirror is curved then it is called a spherical mirror

there are two types of spherical mirrors

concave - when the reflecting part of the mirror is curved inwards then it is known as a concave mirror

convex - when the reflecting part of the mirror is curved outwards then it is known as a convex mirror

pole

the centre point on a spherical mirror.

centre of curvature

the centre of the hollow spherical glass from which the spherical mirror is obtained.

principal axis

the line that passes through the centre of curvature and pole.

radius of curvature

the distance between centre of curvature and pole

aperture

the effective area of a spherical mirror is called its aperture.

there are two types of aperture

linear aperture - the linear distance between the two endpoints of spherical mirror

angular aperture - the angle made between the endpoints of spherical mirror and the centre of curvature

focus of concave mirror

that point on the principle axis from which the reflected rays pass through

why is concave mirror called converging mirror

it is because the rays of light after reflection move towards one point

focus of convex mirror

the point on principle axis where the rays of light tend to intersect

why is the convex mirror called diverging mirror

it is because the rays of light appear to move away from each other after striking the convex mirror

focal length

the length between pole and focus and centre of curvature.

rules for drawing ray digram of concave mirror

1st - when a ray of light is incident parallel to the principle axis then after reflection it will pass through focus

2nd - when a ray of light is incident through focus then after reflection it will be parallel to principle axis

3rd - when a ray of light is incident through the centre of curvature then it will pass through centre of curvature without and deviation after reflection

4th - when the ray of light is incident on pole at a particular the ray will form the same angle after reflection [pole is the angle of incidence]

ray digram for concave mirror when object is placed at infinity In front of a concave mirror.

the image will be formed at focus.

it is real and inverted

m = negative

m = less than 1 [the size of image is highly diminished]

ray digram for concave mirror when the object is placed beyond c

the image is formed between c and f

it is real and inverted

m = negative

m = less than 1 [the image is diminished]

ray digram for concave mirror when the object is placed at c in front of the concave mirror

this image is not possible in a spherical mirror as the spherical mirrors are curved and this image is supposed to form at c of the same size,real and inverted and this is only possible in the plain mirror.

ray diagram of an object placed between c and f in front of concave mirror

the image is real and inverted

the image formed is beyond c

it is enlarged

m = negative

m = greater than 1

ray diagram for an object which is placed on focus

the image will be formed at infinity

the image will be highly enlarged

it is real and inverted

m = negative

m = more than 1

ray diagram for object which is placed between f and p

the image formed will be virtual and erect

the image will be formed behind the mirror

it will be enlarged

m = positive

m = more than 1

[it is the only time concave mirror will form an virtual and erect image]

why is concave mirror used as makeup or shaving mirror

concave mirror forms an enlarged virtual and erect image therefore this enlarged image helps the artists during shaving and makeup

rules for drawing ray diagrams of convex mirror

1st - if a ray of light is incident parallel to principle axis then after reflection it will appear to pass through focus.

2nd - if a ray of light is incident through focus so after reflection it will emerge parallel to the principle axis.

3rd - when a ray of light is incident through the centre of curvature then it will pass through it without any deviation

4th - when a ray of light is incident at the pole at a particular angle then the reflected ray will form the same angle.

ray diagram when the object is placed at infinity in front of a convex mirror

the image will be formed at focus

it will be highly diminished

it will be virtual and erect

m = positive

m = less than 1

ray diagram when object is placed between infinity and pole in front of a convex mirror

the image will be formed between the pole and focus

it will be diminished

it will be virtual and erect

m = positive

m = less than 1

sign convention of concave mirror

everything at the left will be negative

everything at the right will be positive

everything above the principle axis will be positive

and everything below the principle axis will be negative

the focal length [f] will be negative

object distance [u] will be negative

image distance [v] will be negative

height of the object [h] will be positive

height of the image [h1] will be negative

radius of curvature [R] will be negative

magnification factor will be negative [as the image is real and inverted]

sign convention of convex mirror

focal length [f] will be positive

object distance [u] will be negative [the only negative sign in the convex mirror]

image distance [v] will be positive

height of object [h] will be positive

height of image [h1] will be positive

radius of curvature [R] will be positive

magnification factor [m] will be positive [ the image formed is virtual and erect]

mirror formula

it is the formula which shows the relationship between the distance of image,distance of object and focal length

the sum of reciprocal of object distance and image distance gives the reciprocal of focal length

1/v + 1/u = 1/f

how is magnification factor related to image distance and object distance

m = h1/h = -v/u

the formula of radius of curvature

r = 2f

refraction of light

whenever light changes its medium it then changes its direction which means it gets deviated from the original path and this concept is called the refraction of light

the speed of light also changes when the medium changes

two types of medium

there are two types of medium depending upon the speed of light in particular medium

optically rarer medium - the medium in which light travels faster ex- vacuum

optically denser medium - the medium in which light travels slower ex- diamond

why does refraction of light occur

whenever light enters obliquely from one medium to another then due to the change of speed of light in the second medium it changes its path therefore the phenomenon of refraction of light occurs.

rules of ray diagram of refraction

1st - when a ray of light enters from a rarer medium to a denser medium then the ray of light bends towards the normal

2nd - when a ray of light enters from a denser medium to rarer medium then the ray of light bends away from the normal