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39-42
9 2 1 Sign convention To derive the relevant formulae for reflection by spherical mirrors and refraction by spherical lenses, we must first adopt a sign convention for measuring distances In this book, we shall follow the Cartesian sign convention
9
40-43
2 1 Sign convention To derive the relevant formulae for reflection by spherical mirrors and refraction by spherical lenses, we must first adopt a sign convention for measuring distances In this book, we shall follow the Cartesian sign convention According to this convention, all distances are measured from the pole of the mirror or the optical centre of the lens
9
41-44
1 Sign convention To derive the relevant formulae for reflection by spherical mirrors and refraction by spherical lenses, we must first adopt a sign convention for measuring distances In this book, we shall follow the Cartesian sign convention According to this convention, all distances are measured from the pole of the mirror or the optical centre of the lens The distances measured in the same direction as the incident light are taken as positive and those measured in the direction opposite to the direction of incident light are taken as negative (Fig
9
42-45
In this book, we shall follow the Cartesian sign convention According to this convention, all distances are measured from the pole of the mirror or the optical centre of the lens The distances measured in the same direction as the incident light are taken as positive and those measured in the direction opposite to the direction of incident light are taken as negative (Fig 9
9
43-46
According to this convention, all distances are measured from the pole of the mirror or the optical centre of the lens The distances measured in the same direction as the incident light are taken as positive and those measured in the direction opposite to the direction of incident light are taken as negative (Fig 9 2)
9
44-47
The distances measured in the same direction as the incident light are taken as positive and those measured in the direction opposite to the direction of incident light are taken as negative (Fig 9 2) The heights measured upwards with respect to x-axis and normal to the Rationalised 2023-24 Ray Optics and Optical Instruments 223 principal axis (x-axis) of the mirror/lens are taken as positive (Fig
9
45-48
9 2) The heights measured upwards with respect to x-axis and normal to the Rationalised 2023-24 Ray Optics and Optical Instruments 223 principal axis (x-axis) of the mirror/lens are taken as positive (Fig 9
9
46-49
2) The heights measured upwards with respect to x-axis and normal to the Rationalised 2023-24 Ray Optics and Optical Instruments 223 principal axis (x-axis) of the mirror/lens are taken as positive (Fig 9 2)
9
47-50
The heights measured upwards with respect to x-axis and normal to the Rationalised 2023-24 Ray Optics and Optical Instruments 223 principal axis (x-axis) of the mirror/lens are taken as positive (Fig 9 2) The heights measured downwards are taken as negative
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48-51
9 2) The heights measured downwards are taken as negative With a common accepted convention, it turns out that a single formula for spherical mirrors and a single formula for spherical lenses can handle all different cases
9
49-52
2) The heights measured downwards are taken as negative With a common accepted convention, it turns out that a single formula for spherical mirrors and a single formula for spherical lenses can handle all different cases 9
9
50-53
The heights measured downwards are taken as negative With a common accepted convention, it turns out that a single formula for spherical mirrors and a single formula for spherical lenses can handle all different cases 9 2
9
51-54
With a common accepted convention, it turns out that a single formula for spherical mirrors and a single formula for spherical lenses can handle all different cases 9 2 2 Focal length of spherical mirrors Figure 9
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52-55
9 2 2 Focal length of spherical mirrors Figure 9 3 shows what happens when a parallel beam of light is incident on (a) a concave mirror, and (b) a convex mirror
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53-56
2 2 Focal length of spherical mirrors Figure 9 3 shows what happens when a parallel beam of light is incident on (a) a concave mirror, and (b) a convex mirror We assume that the rays are paraxial, i
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54-57
2 Focal length of spherical mirrors Figure 9 3 shows what happens when a parallel beam of light is incident on (a) a concave mirror, and (b) a convex mirror We assume that the rays are paraxial, i e
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55-58
3 shows what happens when a parallel beam of light is incident on (a) a concave mirror, and (b) a convex mirror We assume that the rays are paraxial, i e , they are incident at points close to the pole P of the mirror and make small angles with the principal axis
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56-59
We assume that the rays are paraxial, i e , they are incident at points close to the pole P of the mirror and make small angles with the principal axis The reflected rays converge at a point F on the principal axis of a concave mirror [Fig
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57-60
e , they are incident at points close to the pole P of the mirror and make small angles with the principal axis The reflected rays converge at a point F on the principal axis of a concave mirror [Fig 9
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58-61
, they are incident at points close to the pole P of the mirror and make small angles with the principal axis The reflected rays converge at a point F on the principal axis of a concave mirror [Fig 9 3(a)]
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59-62
The reflected rays converge at a point F on the principal axis of a concave mirror [Fig 9 3(a)] For a convex mirror, the reflected rays appear to diverge from a point F on its principal axis [Fig
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60-63
9 3(a)] For a convex mirror, the reflected rays appear to diverge from a point F on its principal axis [Fig 9
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61-64
3(a)] For a convex mirror, the reflected rays appear to diverge from a point F on its principal axis [Fig 9 3(b)]
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62-65
For a convex mirror, the reflected rays appear to diverge from a point F on its principal axis [Fig 9 3(b)] The point F is called the principal focus of the mirror
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63-66
9 3(b)] The point F is called the principal focus of the mirror If the parallel paraxial beam of light were incident, making some angle with the principal axis, the reflected rays would converge (or appear to diverge) from a point in a plane through F normal to the principal axis
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64-67
3(b)] The point F is called the principal focus of the mirror If the parallel paraxial beam of light were incident, making some angle with the principal axis, the reflected rays would converge (or appear to diverge) from a point in a plane through F normal to the principal axis This is called the focal plane of the mirror [Fig
9
65-68
The point F is called the principal focus of the mirror If the parallel paraxial beam of light were incident, making some angle with the principal axis, the reflected rays would converge (or appear to diverge) from a point in a plane through F normal to the principal axis This is called the focal plane of the mirror [Fig 9
9
66-69
If the parallel paraxial beam of light were incident, making some angle with the principal axis, the reflected rays would converge (or appear to diverge) from a point in a plane through F normal to the principal axis This is called the focal plane of the mirror [Fig 9 3(c)]
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67-70
This is called the focal plane of the mirror [Fig 9 3(c)] FIGURE 9
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68-71
9 3(c)] FIGURE 9 3 Focus of a concave and convex mirror
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69-72
3(c)] FIGURE 9 3 Focus of a concave and convex mirror The distance between the focus F and the pole P of the mirror is called the focal length of the mirror, denoted by f
9
70-73
FIGURE 9 3 Focus of a concave and convex mirror The distance between the focus F and the pole P of the mirror is called the focal length of the mirror, denoted by f We now show that f = R/2, where R is the radius of curvature of the mirror
9
71-74
3 Focus of a concave and convex mirror The distance between the focus F and the pole P of the mirror is called the focal length of the mirror, denoted by f We now show that f = R/2, where R is the radius of curvature of the mirror The geometry of reflection of an incident ray is shown in Fig
9
72-75
The distance between the focus F and the pole P of the mirror is called the focal length of the mirror, denoted by f We now show that f = R/2, where R is the radius of curvature of the mirror The geometry of reflection of an incident ray is shown in Fig 9
9
73-76
We now show that f = R/2, where R is the radius of curvature of the mirror The geometry of reflection of an incident ray is shown in Fig 9 4
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74-77
The geometry of reflection of an incident ray is shown in Fig 9 4 Let C be the centre of curvature of the mirror
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75-78
9 4 Let C be the centre of curvature of the mirror Consider a ray parallel to the principal axis striking the mirror at M
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76-79
4 Let C be the centre of curvature of the mirror Consider a ray parallel to the principal axis striking the mirror at M Then CM will be perpendicular to the mirror at M
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77-80
Let C be the centre of curvature of the mirror Consider a ray parallel to the principal axis striking the mirror at M Then CM will be perpendicular to the mirror at M Let q be the angle of incidence, and MD Rationalised 2023-24 Physics 224 be the perpendicular from M on the principal axis
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78-81
Consider a ray parallel to the principal axis striking the mirror at M Then CM will be perpendicular to the mirror at M Let q be the angle of incidence, and MD Rationalised 2023-24 Physics 224 be the perpendicular from M on the principal axis Then, ÐMCP = q and ÐMFP = 2q Now, tanq = MD CD and tan 2q = MD FD (9
9
79-82
Then CM will be perpendicular to the mirror at M Let q be the angle of incidence, and MD Rationalised 2023-24 Physics 224 be the perpendicular from M on the principal axis Then, ÐMCP = q and ÐMFP = 2q Now, tanq = MD CD and tan 2q = MD FD (9 1) For small q, which is true for paraxial rays, tanq » q, tan 2q » 2q
9
80-83
Let q be the angle of incidence, and MD Rationalised 2023-24 Physics 224 be the perpendicular from M on the principal axis Then, ÐMCP = q and ÐMFP = 2q Now, tanq = MD CD and tan 2q = MD FD (9 1) For small q, which is true for paraxial rays, tanq » q, tan 2q » 2q Therefore, Eq
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81-84
Then, ÐMCP = q and ÐMFP = 2q Now, tanq = MD CD and tan 2q = MD FD (9 1) For small q, which is true for paraxial rays, tanq » q, tan 2q » 2q Therefore, Eq (9
9
82-85
1) For small q, which is true for paraxial rays, tanq » q, tan 2q » 2q Therefore, Eq (9 1) gives MD FD = 2 MD CD or, FD = CD 2 (9
9
83-86
Therefore, Eq (9 1) gives MD FD = 2 MD CD or, FD = CD 2 (9 2) Now, for small q, the point D is very close to the point P
9
84-87
(9 1) gives MD FD = 2 MD CD or, FD = CD 2 (9 2) Now, for small q, the point D is very close to the point P Therefore, FD = f and CD = R
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85-88
1) gives MD FD = 2 MD CD or, FD = CD 2 (9 2) Now, for small q, the point D is very close to the point P Therefore, FD = f and CD = R Equation (9
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86-89
2) Now, for small q, the point D is very close to the point P Therefore, FD = f and CD = R Equation (9 2) then gives f = R/2 (9
9
87-90
Therefore, FD = f and CD = R Equation (9 2) then gives f = R/2 (9 3) 9
9
88-91
Equation (9 2) then gives f = R/2 (9 3) 9 2
9
89-92
2) then gives f = R/2 (9 3) 9 2 3 The mirror equation If rays emanating from a point actually meet at another point after reflection and/or refraction, that point is called the image of the first point
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90-93
3) 9 2 3 The mirror equation If rays emanating from a point actually meet at another point after reflection and/or refraction, that point is called the image of the first point The image is real if the rays actually converge to the point; it is virtual if the rays do not actually meet but appear to diverge from the point when produced backwards
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91-94
2 3 The mirror equation If rays emanating from a point actually meet at another point after reflection and/or refraction, that point is called the image of the first point The image is real if the rays actually converge to the point; it is virtual if the rays do not actually meet but appear to diverge from the point when produced backwards An image is thus a point-to-point correspondence with the object established through reflection and/or refraction
9
92-95
3 The mirror equation If rays emanating from a point actually meet at another point after reflection and/or refraction, that point is called the image of the first point The image is real if the rays actually converge to the point; it is virtual if the rays do not actually meet but appear to diverge from the point when produced backwards An image is thus a point-to-point correspondence with the object established through reflection and/or refraction In principle, we can take any two rays emanating from a point on an object, trace their paths, find their point of intersection and thus, obtain the image of the point due to reflection at a spherical mirror
9
93-96
The image is real if the rays actually converge to the point; it is virtual if the rays do not actually meet but appear to diverge from the point when produced backwards An image is thus a point-to-point correspondence with the object established through reflection and/or refraction In principle, we can take any two rays emanating from a point on an object, trace their paths, find their point of intersection and thus, obtain the image of the point due to reflection at a spherical mirror In practice, however, it is convenient to choose any two of the following rays: (i) The ray from the point which is parallel to the principal axis
9
94-97
An image is thus a point-to-point correspondence with the object established through reflection and/or refraction In principle, we can take any two rays emanating from a point on an object, trace their paths, find their point of intersection and thus, obtain the image of the point due to reflection at a spherical mirror In practice, however, it is convenient to choose any two of the following rays: (i) The ray from the point which is parallel to the principal axis The reflected ray goes through the focus of the mirror
9
95-98
In principle, we can take any two rays emanating from a point on an object, trace their paths, find their point of intersection and thus, obtain the image of the point due to reflection at a spherical mirror In practice, however, it is convenient to choose any two of the following rays: (i) The ray from the point which is parallel to the principal axis The reflected ray goes through the focus of the mirror (ii) The ray passing through the centre of curvature of a concave mirror or appearing to pass through it for a convex mirror
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96-99
In practice, however, it is convenient to choose any two of the following rays: (i) The ray from the point which is parallel to the principal axis The reflected ray goes through the focus of the mirror (ii) The ray passing through the centre of curvature of a concave mirror or appearing to pass through it for a convex mirror The reflected ray simply retraces the path
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97-100
The reflected ray goes through the focus of the mirror (ii) The ray passing through the centre of curvature of a concave mirror or appearing to pass through it for a convex mirror The reflected ray simply retraces the path (iii) The ray passing through (or directed towards) the focus of the concave mirror or appearing to pass through (or directed towards) the focus of a convex mirror
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98-101
(ii) The ray passing through the centre of curvature of a concave mirror or appearing to pass through it for a convex mirror The reflected ray simply retraces the path (iii) The ray passing through (or directed towards) the focus of the concave mirror or appearing to pass through (or directed towards) the focus of a convex mirror The reflected ray is parallel to the principal axis
9
99-102
The reflected ray simply retraces the path (iii) The ray passing through (or directed towards) the focus of the concave mirror or appearing to pass through (or directed towards) the focus of a convex mirror The reflected ray is parallel to the principal axis (iv) The ray incident at any angle at the pole
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100-103
(iii) The ray passing through (or directed towards) the focus of the concave mirror or appearing to pass through (or directed towards) the focus of a convex mirror The reflected ray is parallel to the principal axis (iv) The ray incident at any angle at the pole The reflected ray follows laws of reflection
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101-104
The reflected ray is parallel to the principal axis (iv) The ray incident at any angle at the pole The reflected ray follows laws of reflection Figure 9
9
102-105
(iv) The ray incident at any angle at the pole The reflected ray follows laws of reflection Figure 9 5 shows the ray diagram considering three rays
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103-106
The reflected ray follows laws of reflection Figure 9 5 shows the ray diagram considering three rays It shows the image A¢B¢ (in this case, real) of an object AB formed by a concave mirror
9
104-107
Figure 9 5 shows the ray diagram considering three rays It shows the image A¢B¢ (in this case, real) of an object AB formed by a concave mirror It does not mean that only three rays emanate from the point A
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105-108
5 shows the ray diagram considering three rays It shows the image A¢B¢ (in this case, real) of an object AB formed by a concave mirror It does not mean that only three rays emanate from the point A An infinite number of rays emanate from any source, in all directions
9
106-109
It shows the image A¢B¢ (in this case, real) of an object AB formed by a concave mirror It does not mean that only three rays emanate from the point A An infinite number of rays emanate from any source, in all directions Thus, point A¢ is image point of A if every ray originating at point A and falling on the concave mirror after reflection passes through the point A¢
9
107-110
It does not mean that only three rays emanate from the point A An infinite number of rays emanate from any source, in all directions Thus, point A¢ is image point of A if every ray originating at point A and falling on the concave mirror after reflection passes through the point A¢ FIGURE 9
9
108-111
An infinite number of rays emanate from any source, in all directions Thus, point A¢ is image point of A if every ray originating at point A and falling on the concave mirror after reflection passes through the point A¢ FIGURE 9 4 Geometry of reflection of an incident ray on (a) concave spherical mirror, and (b) convex spherical mirror
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109-112
Thus, point A¢ is image point of A if every ray originating at point A and falling on the concave mirror after reflection passes through the point A¢ FIGURE 9 4 Geometry of reflection of an incident ray on (a) concave spherical mirror, and (b) convex spherical mirror FIGURE 9
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110-113
FIGURE 9 4 Geometry of reflection of an incident ray on (a) concave spherical mirror, and (b) convex spherical mirror FIGURE 9 5 Ray diagram for image formation by a concave mirror
9
111-114
4 Geometry of reflection of an incident ray on (a) concave spherical mirror, and (b) convex spherical mirror FIGURE 9 5 Ray diagram for image formation by a concave mirror Rationalised 2023-24 Ray Optics and Optical Instruments 225 We now derive the mirror equation or the relation between the object distance (u), image distance (v) and the focal length ( f )
9
112-115
FIGURE 9 5 Ray diagram for image formation by a concave mirror Rationalised 2023-24 Ray Optics and Optical Instruments 225 We now derive the mirror equation or the relation between the object distance (u), image distance (v) and the focal length ( f ) From Fig
9
113-116
5 Ray diagram for image formation by a concave mirror Rationalised 2023-24 Ray Optics and Optical Instruments 225 We now derive the mirror equation or the relation between the object distance (u), image distance (v) and the focal length ( f ) From Fig 9
9
114-117
Rationalised 2023-24 Ray Optics and Optical Instruments 225 We now derive the mirror equation or the relation between the object distance (u), image distance (v) and the focal length ( f ) From Fig 9 5, the two right-angled triangles A¢B¢F and MPF are similar
9
115-118
From Fig 9 5, the two right-angled triangles A¢B¢F and MPF are similar (For paraxial rays, MP can be considered to be a straight line perpendicular to CP
9
116-119
9 5, the two right-angled triangles A¢B¢F and MPF are similar (For paraxial rays, MP can be considered to be a straight line perpendicular to CP ) Therefore, B A B F PM FP ′ ′ ′ = or B A B F BA FP ′ ′ ′ = (∵PM = AB) (9
9
117-120
5, the two right-angled triangles A¢B¢F and MPF are similar (For paraxial rays, MP can be considered to be a straight line perpendicular to CP ) Therefore, B A B F PM FP ′ ′ ′ = or B A B F BA FP ′ ′ ′ = (∵PM = AB) (9 4) Since Ð APB = Ð A¢PB¢, the right angled triangles A¢B¢P and ABP are also similar
9
118-121
(For paraxial rays, MP can be considered to be a straight line perpendicular to CP ) Therefore, B A B F PM FP ′ ′ ′ = or B A B F BA FP ′ ′ ′ = (∵PM = AB) (9 4) Since Ð APB = Ð A¢PB¢, the right angled triangles A¢B¢P and ABP are also similar Therefore, B A B P B A B P ′ ′ ′ = (9
9
119-122
) Therefore, B A B F PM FP ′ ′ ′ = or B A B F BA FP ′ ′ ′ = (∵PM = AB) (9 4) Since Ð APB = Ð A¢PB¢, the right angled triangles A¢B¢P and ABP are also similar Therefore, B A B P B A B P ′ ′ ′ = (9 5) Comparing Eqs
9
120-123
4) Since Ð APB = Ð A¢PB¢, the right angled triangles A¢B¢P and ABP are also similar Therefore, B A B P B A B P ′ ′ ′ = (9 5) Comparing Eqs (9
9
121-124
Therefore, B A B P B A B P ′ ′ ′ = (9 5) Comparing Eqs (9 4) and (9
9
122-125
5) Comparing Eqs (9 4) and (9 5), we get B P – FP B F B P FP FP BP ′ ′ ′ = = (9
9
123-126
(9 4) and (9 5), we get B P – FP B F B P FP FP BP ′ ′ ′ = = (9 6) Equation (9
9
124-127
4) and (9 5), we get B P – FP B F B P FP FP BP ′ ′ ′ = = (9 6) Equation (9 6) is a relation involving magnitude of distances
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125-128
5), we get B P – FP B F B P FP FP BP ′ ′ ′ = = (9 6) Equation (9 6) is a relation involving magnitude of distances We now apply the sign convention
9
126-129
6) Equation (9 6) is a relation involving magnitude of distances We now apply the sign convention We note that light travels from the object to the mirror MPN
9
127-130
6) is a relation involving magnitude of distances We now apply the sign convention We note that light travels from the object to the mirror MPN Hence this is taken as the positive direction
9
128-131
We now apply the sign convention We note that light travels from the object to the mirror MPN Hence this is taken as the positive direction To reach the object AB, image A¢B¢ as well as the focus F from the pole P, we have to travel opposite to the direction of incident light
9
129-132
We note that light travels from the object to the mirror MPN Hence this is taken as the positive direction To reach the object AB, image A¢B¢ as well as the focus F from the pole P, we have to travel opposite to the direction of incident light Hence, all the three will have negative signs
9
130-133
Hence this is taken as the positive direction To reach the object AB, image A¢B¢ as well as the focus F from the pole P, we have to travel opposite to the direction of incident light Hence, all the three will have negative signs Thus, B¢ P = –v, FP = –f, BP = –u Using these in Eq
9
131-134
To reach the object AB, image A¢B¢ as well as the focus F from the pole P, we have to travel opposite to the direction of incident light Hence, all the three will have negative signs Thus, B¢ P = –v, FP = –f, BP = –u Using these in Eq (9
9
132-135
Hence, all the three will have negative signs Thus, B¢ P = –v, FP = –f, BP = –u Using these in Eq (9 6), we get – – v– f v f u + = – or v– f v f u = v f uv = + 1 Dividing it by v, we get 1 1 1 v u f + = (9
9
133-136
Thus, B¢ P = –v, FP = –f, BP = –u Using these in Eq (9 6), we get – – v– f v f u + = – or v– f v f u = v f uv = + 1 Dividing it by v, we get 1 1 1 v u f + = (9 7) This relation is known as the mirror equation
9
134-137
(9 6), we get – – v– f v f u + = – or v– f v f u = v f uv = + 1 Dividing it by v, we get 1 1 1 v u f + = (9 7) This relation is known as the mirror equation The size of the image relative to the size of the object is another important quantity to consider
9
135-138
6), we get – – v– f v f u + = – or v– f v f u = v f uv = + 1 Dividing it by v, we get 1 1 1 v u f + = (9 7) This relation is known as the mirror equation The size of the image relative to the size of the object is another important quantity to consider We define linear magnification (m) as the ratio of the height of the image (h¢) to the height of the object (h): m = h h ′ (9
9
136-139
7) This relation is known as the mirror equation The size of the image relative to the size of the object is another important quantity to consider We define linear magnification (m) as the ratio of the height of the image (h¢) to the height of the object (h): m = h h ′ (9 8) h and h¢ will be taken positive or negative in accordance with the accepted sign convention
9
137-140
The size of the image relative to the size of the object is another important quantity to consider We define linear magnification (m) as the ratio of the height of the image (h¢) to the height of the object (h): m = h h ′ (9 8) h and h¢ will be taken positive or negative in accordance with the accepted sign convention In triangles A¢B¢P and ABP, we have, B A B P BA BP ′ ′ ′ = With the sign convention, this becomes Rationalised 2023-24 Physics 226 – – h v h u ′ = – so that m = – h v h u ′ = (9
9
138-141
We define linear magnification (m) as the ratio of the height of the image (h¢) to the height of the object (h): m = h h ′ (9 8) h and h¢ will be taken positive or negative in accordance with the accepted sign convention In triangles A¢B¢P and ABP, we have, B A B P BA BP ′ ′ ′ = With the sign convention, this becomes Rationalised 2023-24 Physics 226 – – h v h u ′ = – so that m = – h v h u ′ = (9 9) We have derived here the mirror equation, Eq