LIGHT- Reflection ,Refraction class10


LIGHT AND IT’S PROPERTY: Light is the form of energy which produce the sensation of sight, it travels in electromagnetic waves which requires no medium so it can travel through vacuum in Straightline, it has dual nature as wave nature as well as particle nature, it produces shadow, speed is 3×10^8 meter/sec. It experiences, interference and diffraction.

REFLECTION: When a ray /beam of light strikes on smooth and polish surface after obeying some laws it goes back in the same medium. we can also say the bouncing back of light from the polish and smooth surface in the same medium like mirror.

LAWS OF RELECTION: i)the angle of incidence is equal to angle of reflection,  i=r                                                                    ii) The incident ray, reflected ray and normal all lie in the same fig.


IMAGE: when two or more rays of light actually meet or appear to be meeting it form image. It is of two type first real image   when two or more rays actual meet at a point it forms real image which can be obtained on screen, inverted images formed on cinema screen other type of image is virtual it is formed when two or more rays appear to meet but actually do not meet this image cannot be taken on screen it is always erect image formed on plane mirror or convex mirrorplane mirror


IMAGE OF PLANE MIRROR: The image formed by plane mirror is virtual, erect, same size of the object, distance between object and mirror is same between mirror and image behind the mirror and lateral inversion (image position of left side is seen on right in the mirror and right position appear on left side in the mirror)                                                                     SPHERICAL MIRRORS : The mirrors whose reflecting surface is curved  are called spherical mirrors ,they are of two type i) convex mirror whose reflecting surface is bulging one or outward these kinds mirror are divergent mirror in which rays spread after reflection the image formed is virtual ii) Concave mirror whose reflecting surface is hump one or inward it is convergent mirror which allow the rays to meet at a point after reflection most of images  formed are real                                              spherical            FUNDAMENTAL FACTS  i) Principal axis is the line joining the pole and center of curvature) Pole (p)  it is the center of spherical mirror iii) Aperture is the effective diameter of spherical mirror iv) Center of curvature (C)  is the center of hollow glass sphere from where mirror has been taken  v)Radius of curvature (R) is the radius of the hollow sphere from where has been taken. vi)Focus (F) it is the point on principal axis where the parallel ray of light after reflection meet or appear to be meeting   vii) Focal length (f) it is the distance between pole and focus of the mirror

Relation    f= R/2    focal length = radius of curvature /2

Magnification =height of image / height of object                          m = h’/h  = -v/u

if m=1  size of image is equal to size of object , m<1 size of image is smaller than size of object  m>1  size  of image is larger than size of object .

1/f = 1/v+1/u                 f= focal length v= distance of image to pole u =distance of object to pole

SIGN CONVENTION: Types of mirror a) Concave Mirror U is negative, v, in case of real image positive in case of virtual positive , R  is  negative , height of object positive ,height of image is negative for real positive  for virtual image. b) Convex Mirror U is negative, v is positive for only real image,  , height of object ,height of  virtual image is  positive.



RULES FOR MAKING RAY DIAGRAM BY CONCAVE AND CONVEX MIRROR.                                                                                    Concave Mirror: i) When a ray parallel to principal axis  after reflection  passes through focus ii) A ray from focus after reflection goes parallel to principal axis iii) A ray passes through center of curvature   goes back to the same path after reflection  iv) A ray incident obliquely to the pole of principal axis of concave mirror is reflected back with same angle/equal angle.concave rule4

 concave rays rules


CONVEX MIRROR :  i) A ray to parallel axis after reflection   appear to be passing through principal focus. ii) A ray which   is passing through the focus of mirror after reflection it goes parallel to principal axis  iii) A ray directed towards the center of curvature retraces its path . iv) A ray incident  obliquely to principal pole of principal axis of convex mirror reflected obliquely with same angle. covexrule1

covex rule2

 convex rule 3


a) when object is at  infinity image formed at focus which real, inverted and diminished

concave 1

b) When an object is beyond center of curvature (C) the image formed between focus (f) and C  it is real ,inverted ,smaller than object

concave im 2

c) When object is at center of curvature (c) the image also formed at C  which is real inverted  and equal in size ,

 concave im3

d) When object lies between focus (f) and centerof curvature (C) image formed  beyond C which is real inverted and bigger size than object .

concave im6

e) When object is at focus image formed at infinity which is real inverted and enlarged.concave im 5


f) When object lies between focos (f) and Pole (p) the image formed behind the mirror upright/erect enlarged and virtual

concave im 4


RAY DIAGRAMS OF COVEX MIRROR : i) when object is at infinity  image formed at focus ,it is virtual, erect point size.

ii) when object is placed  near and in front of mirror image formed behind the mirror small virtual upright.

convex im

USES : Concave Mirrors are used in torches, search light, vehicles headlight, it is used by dentist to see large image of teeth of patients, shaving mirror ,large mirror in solar furnace.                      Convex Mirror are used as rear view mirrors in vehicles because they always give erect ,diminish image and have wider field of view, it is also used at blind turns or on the point of merging traffic ,used as security mirror.

REFRACTION :It is the process of bending of light when it passes from one medium to another .This is due to change of speed of light in different medium is different. It is maximum in air 3.10^8 m/s

some practical example as  a) Bottom of swimming pool appear higher than real depth. b) A pencil partially immersed in  water appears to be bent at interface of water and air c) lemons placed in glass tumbler appears bigger

Refraction through glass slab : When a ray of light incident on the glass slab at that point normal is drawn the angle between incident ray and normal is angle of incidence as it passes the refracted ray bends towards normal forming the angle of refraction with normal than finally the ray again comes out in form of emergent ray. The extent of bending of ray of light at opposite face of the slab is equal and opposite.  If incident ray is traced ahead the parallel distance between traced incident ray and emergent ray is called lateral displacement which depends upon refractive index and thickness of glass.


glass slab

LAWS OF REFRACTION: The incident ray, refracted ray, normal to the interface of two medium at point of incidence all lie at same plane.      The ratio of sine of angle of incidence (sine i) to the sine of angle of refraction (sine r) bears the constant ratio It is also called Snell’s Law.        sine i/sine r   


Refractive index = velocity of light in air / velocity of light medium                                       

n12 =v1/v2

means refractive index of 2nd medium with respect to 1st.  If we want to calculate the refractive index of  1st medium with respect to 2nd we have to calculate it’s reciprocal                       n21 = 1/ n12

TYPES OF MEDIUM : There two types of medium rare and denser when ray of light passes from rare medium to denser it bends towards normal and if ray of light passes from denser to rare than it bends away from normal.medium

LENS : There are spherical lens which  are the transparent medium bounded by two  surfaces which at least  one is curved It is of two kinds a) Convex lens  they are thin at edges thick at center and convergent in nature b) Concave lens they are thick at edges and thin at center and divergent in nature.



CONVEX LENS: A ray parallel to principal axis after refraction passes through focus on the other side .Aray passing from focus after refraction goes parallel ot principal axis and if it passes  through optical center it goes without any deviation.

convex rule

CONCAVE LENS: If a ray parallel to principal axis after refraction appears to be passing through focus on same side, if it is passing through focus ray appears to diverge from principal focus of same side and it passes through optical center without any deviation.

concave rule


SIGN CONVENTION: It is as in Convex and Concave

1/f = 1/v- 1/u    f is focal length for convex lens it is positive for concave it is negative; u is the object distance from optical center negative in all cases v image distance from optical center if to the opposite side of object, it is positive otherwise negative.

Magnification: It is the ratio of height of image to height of object

m = height of image/height of object m= v/u .

m =image distance / object distance from optical center.


CONVEX LENS:  a) When an object is at infinity  image formed at focus real inverted and diminished.

convex im 1

b)When object lie  beyond 2f the image formed between f and 2f on the other side of lens real inverted and smaller than object

 convex im2

c) When object lies at 2f image is also formed at 2f   image is real ,inverted  equal  in size .

convex im3

d) when object lies between f and 2f  image formed beyond 2f   image is real, inverted and bigger in size

convex im4

e) When object lies at f image formed at infinity  real inverted and enlarge .

convex im 5

f) when object lies between f and optical center of lens image formed behind the object magnified and virtual and erect.

convex im6

CONCAVE LENS: a) When object is at infinity  image formed at f  it is virtual erect and diminished.

concave im 1

b) When object  between infinity and optical center the image is formed between f  and optical center towards the same side of object smaller in size ,erect and virtual.


concave im 2

POWER OF LENS ; It is reciprocal of focal length taken in meters   power of lens = 1/ focal length in meters                                     unit of power is dioptre =D = m^-                                                   for convex lens power of lens is +ive for concave power of lens is -ive. power of lens is inversely proportional to thickness of lens  P= P1+P2+P3——.

( note : figure taken NCERT BOOK )




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