Q -1 name some phenomena associated with light.
ans – 1) image formation by mirror
2) twinkling of stars
3) formation of rainbow
4) bending of light by a medium
Q-2 how can you say that light travels in straight line?
ans- the fact that a small source of light casts a sharp shadow of an opaque object points to the straight-line path of light.
Q-3 state the laws of reflection.
ans – 1) the angle of incidence is equal to the angle of reflection.
2) the incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
Q – 4 are the laws of reflection applicable to spherical surfaces?
ans – yes
Q-5 state the properties of the image formed by a plane mirror.
ans 1) virtual and erect
2) equal in size to that of object
3) formed as far behind the mirror as the object is in front of it.
4) laterally inverted
Q- 6 what are spherical mirrors?
ans – the mirrors whose reflecting surfaces are spherical are called spherical mirrors.
Q-7 what is concave mirror?
ans – a spherical mirror whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.
Q – 8 define convex mirror.
ans- a spherical mirror whose reflecting surface is curved outwards is called a convex mirror.
Q-9 define pole.
ans – the centre of the reflecting surface of a spherical mirror is called pole.
Q – 10 where does pole lie?
ans – pole lies on the surface of the mirror.
Q-11 how is ‘pole’ denoted?
ans – pole is denoted by the letter P.
Q – 12 what is centre of curvature of a spherical mirror?
ans –the centre of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the centre of curvature of the spherical mirror.
Q – 13 how is centre of curvature represented?
ans – it is represented by the letter C.
Q-14 where does centre of curvature of a spherical mirror lie?
ans – the centre of curvature of a concave mirror lies in front of it. it lies behind the mirror in case of a convex mirror.
Q-15 what is the radius of curvature of a spherical mirror?
ans- the radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature of the spherical mirror.
Q-16 how is radius of curvature denoted?
ans – radius of curvature is denoted by the letter R.
Q -17 what is principal axis of a spherical mirror?
ans – the imaginary line passing through the pole and the centre of curvature of a spherical mirror is called the principal axis.
Q -18 which ray of light is normal to the spherical mirror at its pole?
ans – Principal axis
Q -19 what happens if the sunlight is directed towards a sheet of paper through a concave mirror for a long time? explain.
ans – the paper at first begins to burn producing smoke.eventually it may catch fire
-this is because the sunlight is converged at a point,as a sharp, bright spot by the mirror. this spot if light is the image of the sun on the sheet of paper.
-the heat produced due to the concentration of sunlight ignites the paper.
Q – 20 define principal focus of a concave mirror.
ans – the point on the principal axis at which the rays parallel to principal axis intersect after reflecting from a concave mirror, is called the principal focus of the concave mirror.
Q – 21 define principal focus of a convex mirror.
ans – the point on the principal axis, from which the rays parallel to principal axis appear to come after reflection from a convex mirror,is called principal focus for a convex mirror.
OR
the rays parallel to the principal axis appear to come from a point on the principal axis, after reflecting from a convex mirror.this point is called the principal focus of a convex mirror.
Q -22 how is principal focus denoted?
ans- it is denoted by the letter F.
Q-23 what is focal length of a spherical mirror? How is it represented?
Ans – the distance between the pole and the principal focus of a spherical mirror is called the focal length. It is represented by the letter f.
Q – 24 what is aperture of a spherical mirror?
Ans – the diameter of the reflecting surface of a spherical mirror is called its aperture.
Q- 25 what is the relationship between radius of curvature and focal length of a spherical mirror?
ans- for spherical mirror of small aperture, the radius of curvature is equal to twice the focal length. i.e. R = 2f.
Q -26 make a table for all possible image formations by a concave mirror showing the position, size and nature of the image.
ans -
OBJECT POSITION / IMAGE POSITION / IMAGE SIZE / IMAGE NATUREat infinity / at the focus F / highly diminished, point sized. / real and inverted
beyond C / between F and C / diminished / real and inverted
at C / at C / same size / real and inverted
between F and C / beyond C / enlarged / real and inverted
at the F / at infinity / highly enlarged / real and inverted
between P and F / behind the mirror / enlarged / virtual and erect
Q -27 how many rays originate from a point source of light?
ans – infinite
Q – 28 infinite number of rays originate from a point source of light. then why are only 2 – 3 lines drawn in a ray diagram?
ans – for the sake of clarity of the ray diagram.
Q-29 what is the relationship between the ray parallel to the principal axis and principal focus?
ans – a ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.
Q -30 when will a ray emerge parallel to the principal axis after relection from a spherical mirror?
ans – a ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.
Q -31 when will a ray of light reflected back in the same path from a spherical mirror?
ans – a ray passing through the centre of curvature of a concave mirror or directed towards the centre of curvature of a convex mirror, after reflection, is reflected back along the same path.
Q -32 why do the rays rays of light passing through centre of curvature of a concave mirror, or directed towards the centre of curvature of a convex mirror, reflect back along the same path.
ans – the light rays come back along the same path because the incident rays fall on the mirror along the normal to the reflecting surface.
Q – 33 what happens to a ray of light incident obliquely to the principal axis towards a pole of a spherical mirror?
ans – a ray obliquely to the principal axis, towards a point P on the concave mirror or a convex mirror, is reflected obliquely. the incident ray and the reflected ray make equal angles with the principal axis.
Q – 34- draw ray diagrams for image formation by a concave mirror when the object is at infinity, beyond C, at C, between C and F, at F , and between F and P.
ans – ( learn from ncert science t.b. class 10 pg 166.
Q -35 which mirror is used in search lights and why?
ans – concave mirror is used in search light to get powerful parallel beams of light.
Q – 36 what are the uses of concave mirror?
ans 1) concave mirrors are commonly used in torches, searchlights and vehicles headlights to get powerful parallel beams of light.
2) they are often used as shaving mirrors to see larger images of the face.
3) the dentists use concave mirrors to see large images of the teeth of patients.
4) large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Q – 37 what will be the nature size and position of image of an object after refraction through a convex mirror from infinity, and between infinity and P ?
ans –
Object position / image position / image size / image natureat infinity / at F, behind the mirror / highly diminished / virtual and erect
between infinity and pole / between P and F, behind the mirror / diminished / virtual and erect
Q – 38 which mirror is fixed at the Agra Fort to view the full image of a tall tomb opposite to it?
ans – convex mirror
Q – 39 which mirrors are used to wing mirrors in vehicles?
ans – convex mirrors
Q- 40 what are the uses of convex mirror?
ans 1) convex mirrors are commonly used as rear-view(wing) mirrors in vehicles.
2) they are used in large shops where the shop-keeper can view the whole shop sitting at one place.
3) in double decker buses also they are used for the conductor to see the upper floor by standing on the lower floor.
Q – 41 why are convex mirrors used as wing mirrors?
ans – convex mirrors are preferred as wing mirrors because they give erect images and i wider field of view to facilitate safe driving.
Q – 42 what do you mean by New Cartesian Sign Convention?
ans - New Cartesian Sign Convention refer to a set of sign conventions which are followed while dealing with the reflection of light by spherical mirrors.
Q – 43 what is the origin in spherical mirrors?
ans – pole
Q – 44 what is taken as x-axis in the New Cartesian Sign Convention?
ans – principal axis
Q – 45 state the conventions in New Cartesian Sign Convention.
ans -1) the object is always placed to the left of the mirror. this implies that the light from the object falls on the mirror from the left-hand side.
2) all distances parallel to the principal axis are measured from the pole of the mirror.
3) all the distance measured from the right of the origin ( along the + x-axis) are taken as positive while those measured to the left of the origin ( along – x –axis) are taken as negative.
4) distances measured perpendicular to and above the principal axis ( along + y-axis) are taken as positive.
5)distances measured perpendicular to and below the principal axis ( along - y-axis) are taken as negative.
Q -46 what is the application of the New Cartesian Sign Convention?
ans – these conventions are used to obtain mirror formula and solve related numerical problems.
Q – 47 what do you mean by image distance?
ans –in a spherical mirror, the distance of the object from its pole is called the object distance ( u ) .
Q -48 what do you mean by image distance?
ans – the distance of the image from the pole of the mirror is called the image distance.
Q – 49 what is the relationship between image distance, object distance and focal length?
ans – 1/v + 1/u = 1/f , where v is the image distance, u is the object distance and f is the focal length of the mirror. (this is called mirror formula.)
Q -50 What is magnification?
ans - magnification gives the relative extent to which the image of an object is magnified with respect to the object size.
OR
magnification is defined as the ration of the height of the image to the height of the object.
Q -51 how is magnification expressed and represented?
ans – it is expressed as the ratio of the height of the image to the height of the object. it is usually represented by the letter m.
if h is the height of the object and h’ is the height of the image, the magnification m produced by a spherical mirror is given by
m = h’
h
Q – 52 what is the relationship between h, h’, u and v for a mirror ?
ans - h’ = - v
h u
what is the relation between m, v and u for a mirror?
ans - m = -v/u
Q – 53 in which case will the height be positive and when will it be negative:
1) h, when object is above principal axis.
2) h’ when the image is virtual
2) h’ when image is real
ans – 1) positive
2) positive
3) negative
Q -54 give examples of refraction in daily life.
ans - 1) bottom of a tank or a pond containing water appears to be raised.
2) when a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab.
3) a pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air and water.
4) a lemon kept in water in a glass tumbler appears to be bigger than its actual size, when view from the sides.
Q – 55 what is refraction of light?
ans – when travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. this phenomenon is called refraction of light.
Q -56 explain the refraction of light through a glass slab, by using a diagram.
ans – ( learn from ncert pg 173)
Q – 57 why does refraction occur?
ans – refraction is due to change in the speed of light as it enters from one transparent medium to another.
Q-58 state the laws of refraction of light.
ans – 1 )the incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
2)the ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media.
Q – 59 state Snell’s law of refraction.
ans - the ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media.
if i is the angle of incidence and r is the angle of refraction then
sin i = constant
sin r
Q -60 what is refrative index?
ans – the extent of the change in direction of a ray of light that takes place in a given pair of media is expressed in terms of the refractive index.
Q – 61 what is the speed of light in vacuum?
ans – 3 x 108 m/s.
Q – 62 what is absolute refractive index?
ans – absolute refractive index of a medium the the ratio of speed of light in air to the speed of light in the medium.
nm speed of light in air______= c
speed of light in the medium v
Q – 63 what is the refractive index of (1) water (2)air (3) vacuum and (4) crown glass?
ans – 1) 1.33
2) 1.0003
3) 1
4) 1.52
Q – 64 optically denser medium may not posses greater mass density. justify.
ans – kerosene is optically denser than water, although its mass density is less than water.
Q- 65 what is a lens?
ans – a transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens.
Q- 66 what is a double concave lens?
ans – a double concave lens is bounded by two spherical surfaces, curved inwards. it is thicker at the edges than at the middle.
Q – 67 what is convex lens?
ans – a lens bounded by two spherical surfaces bulging outwards is called convex lens. it is thicker at the middle as compared to the edges.
Q – 68 what is converging lens?
ans –a converging lens, or a convex lens,is a lens bounded by two spherical surfaces bulging outwards. it converges the light rays after refration.
Q – 69 what is diverging lens?
ans- diverging lens, or concave lens, is a lens bounded by two spherical surfaces, curved inwards. it diverges the light rays after refraction.
Q -70 what do you mean by centres of curvature of lens?
ans – the centres of the sphere of which the spherical surfaces of a lens form a part , are called centres of curvature of lens.
Q -71 how are the centres of curvature of lens represented?
ans – the centre of curvature on the left hand side is denoted by C1 and the one on the right side of lens is represented by C2.
Q – 72 what is principal axis of a lens?
ans – an imaginary line passing through the two centres of curvature of a lens is called its principal axis.
Q -73 what is optical centre?
ans – the central point of a lens is its optical centre.
Q – 74 how is optical centre represented?
ans – optical centre is represented by the letter O.
Q -75 when will a ray of light not deviate even after passing through a lens?
ans –a ray of light through the optical centre of a lens passes without suffering any deviation.
Q -76 what is aperture of a lens?
ans – the effective diameter of the circular outline of a spherical lens is called its aperture.
Q – 77 what are thin lenses with small apertures?
ans – such lenses whose aperture is much less than its radius of curvature are called thin lenses with small apertures.
Q -78 what happens when the sunlight is passed through a convex lens on a paper?
ans - the paper begins to burn producing smoke. it may even catch fire after a while.
- this is because, the sunlight constitutes parallel rays of light. this rays are concentrated at a sharp bright spot formed on the paper. the concentration of the sunlight at a point generates heat which causes the paper to burn.
Q -79 what is principal focus of a convex lens?
ans – the point on the principal axis at which the rays of light parallel to the principal axis meet after falling on the lens, is called the principal focus of the convex lens.
Q – 80 what is the principal focus for a concave lens?
ans – the point on the principal axis from which the rays of light parallel to the principal axis appear to diverge after refration through the concave lens is called the pricipal focus of a concave lens.
Q – 81 how many foci does a (1) mirror have, (2) lens have?
ans – a mirror has 1 focus while a lens has 2 foci.
Q- 82 how are foci represented in a lens?
ANS - the focus on the left side of the lens is represented by F1 and the focus on the right side of the lens is represented by F2.
Q – 83 what is the focal length of a lens?
ans – the distance of the principal focus from the optical centre of a lens is called its focal length.
Q – 84 how will you find the focal length of a mirror or a lens?
ans – the distance between the position of the mirror/lens and the point at which the image is formed, when the object is kept at infinity, gives the approximate focal length.