- A concave mirror has an inward-curved reflecting surface, while a convex mirror has an outward-curved reflecting surface.
- Important parts of a spherical mirror: Pole (P), Centre of Curvature (C), Principal Axis, and Principal Focus (F).
- For spherical mirrors, the relation is R = 2f, where R is the radius of curvature and f is the focal length.
- In concave mirrors, parallel rays converge at the focus; in convex mirrors, they appear to diverge from the focus behind the mirror.
Definitions [29]
Definition: Principal Axis
A straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.
OR
The straight line joining the pole and the centre of curvature of the mirror and extended on both sides is called the 'principal axis' of the mirror.
Define the term Pole.
Pole is the centre of the reflecting surface, in this case, a spherical mirror.
Define the term Aperture.
Aperture is the distance between the extreme points on the periphery of the mirror.
Define the term Centre of curvature.
Centre of curvature is the centre of the imaginary sphere to which the mirror belongs.
Define the term Principle focus.
Principal focus of a spherical mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror, converge to or appear to diverge from.
Define the following term:
spherical mirror
“A mirror which is made from a part of a hollow sphere is called Spherical Mirror.
Define the following term:
convex mirror
“A mirror made by silvering the inner surface such that reflection takes place from the bulging surface” is called Convex Mirror.
The Centre of curvature is towards the silvered surface.
Define the following term:
concave mirror
“A mirror made by silvering the outer or the bulging surface such that the reflection takes place from the concave surface.” Centre of curvature is towards the reflecting surface.
Define the following term in relation to concave mirror.
Pole
Pole “is the mid-point of the mirror”.
Define the following term in relation to concave mirror.
Center of curvature
The centre of a hollow sphere of which the mirror forms a part is called the centre of curvature.
Define the following term in relation to concave mirror.
Principal axis
An imaginary line passing through the pole and the centre of curvature of a spherical mirror is called principal axis.
Define the following term in relation to concave mirror.
Principal focus
It is a point on the principal axis, where a beam of light, parallel to the principal axis, after reflection actually meet.
Define the following term in relation to concave mirror.
Radius of curvature
The linear distance between the pole and the center of curvature is called the radius of curvature.
Define the following term in relation to concave mirror.
Focal length
The linear distance between the pole and the principal focus is called focal length.
Define the term Focus of a concave mirror.
The focus of a concave mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror converge to that point.
Define the term Normal.
Normal to the surface of a mirror at any point is the straight line at the right angle to the tangent drawn at that point.
Definition: Pole
The centre of the reflecting surface of a spherical mirror is a point called the pole. The pole is usually represented by the letter P.
OR
The central point of the reflecting surface of the mirror is called the 'pole' of the mirror.
Definition: Concave Mirror
A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.
OR
A concave mirror is one whose reflecting surface is towards the centre of the sphere of which the mirror is a part.
Definition: Convex Mirror
A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.
OR
A convex mirror is one whose reflecting surface is away from the centre of the sphere of which the mirror is a part.
Definition: Centre of Curvature
The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is called the centre of curvature of the spherical mirror. It is represented by the letter C.
OR
The centre of the sphere of which the mirror forms a part, is called the ‘centre of curvature' of the mirror.
Definition: Radius of Curvature
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 mirror. It is represented by the letter R.
OR
The radius of the sphere of which the mirror forms a part, is called the 'radius of curvature' of the mirror.
Define focal length.
The distance between the pole and the principal focus is called the focal length (f) of a spherical mirror.
Definition: Spherical Mirrors
Mirrors whose reflecting surfaces are spherical are called spherical mirrors.
OR
A spherical mirror is a part of a hollow sphere, whose one side is silvered and coated with red oxide and the other side is the reflecting surface.
Define linear magnification produced by a mirror.
The ratio of the height of an image (h') to the height of an object (h) is known as linear magnification
That is,
`mh/h`
where, h' = height of image
h = height of object
Definition: Aperture
The diameter of the periphery of the mirror is called the 'aperture' of the mirror.
Definition: Principal Focus
The point on the principal axis at which light rays parallel to the principal axis, afterreflection from the mirror, actually meet or appear to come from, is called the 'principal focus' of the mirror.
Definition: Focal Length
The distance of the principal focus from the pole of the mirror is called the 'focal length' of the mirror.
Definition: Focal Plane
The plane perpendicular to the principal axis and passing through the principal focus of the mirror is called the ‘focal plane' of the mirror.
Definition: Linear (Lateral / Transverse) Magnification
Linear magnification produced by a spherical mirror is the ratio of the size of the image to the size of the object, both measured perpendicular to the principal axis.
Formulae [3]
Formula: Mirror Formula for Convex Mirror
\[\frac {1}{v}\] + \[\frac {1}{u}\] = \[\frac {1}{f}\]
Formula: Magnification
m = -\[\frac {v}{u}\]
OR
m = -\[\frac {v}{u}\] = \[\frac {f - v}{f}\] = \[\frac {f}{f - u}\].
Formula: Mirror Formula for Concave Mirror
\[\frac {1}{v}\] + \[\frac {1}{u}\] = \[\frac {1}{f}\]
Key Points
Key Points: Spherical Mirrors
Key Points: Sign Convention
- Pole (mirror) or optical centre (lens) is the origin; principal axis is the X-axis.
- Distances to the right are positive, to the left are negative; heights above the axis are positive, below are negative.
- Concave mirror: and R are negative; Convex mirror: and R are positive.
- Real images: image distance and magnification are negative; Virtual images: both are positive.
- Lenses are always negative; they are positive for real images and negative for virtual images; they are positive for convex lenses and negative for concave lenses.
Key Points: Relation between Focal Length and Radius of Curvature
- For a spherical mirror of small aperture, rays close to the principal axis (paraxial rays) obey the law of reflection accurately.
- In both concave and convex mirrors, using geometrical construction and the law of reflection, the focus lies midway between the pole and the centre of curvature.
- Hence, for a small-aperture spherical mirror, the focal length is half the radius of curvature:
f = \[\frac {R}{2}\]
Key Points: Conditions of Image Formation
- Many rays start from an object point, but only two or three rays are sufficient to locate the image.
- In mirrors, reflected rays remain on the same side of the mirror as the object; no real rays exist on the other side.
- If reflected rays actually meet, a real image is formed, which is inverted.
- If reflected rays diverge and meet only on backward extension, a virtual image is formed, which is erect.
- For lenses, real images are formed on the opposite side of the lens and are inverted, while virtual images are formed on the same side as the object and are erect.
Key Points: Uses of Spherical Mirrors
- A concave mirror is used for shaving (erect, magnified image).
- Concave (parabolic) mirrors are used in telescopes to observe distant stars.
- Concave mirrors are used in torches, searchlights and headlights to produce a parallel beam.
- Concave mirrors are used by ENT doctors and eye specialists for examination.
- Convex mirrors are used in street lights to illuminate a large area.
- Convex mirrors are used as rear-view mirrors (erect, diminished image, wide view).
- Image identification:
Erect & same size → Plane mirror
Erect & magnified → Concave mirror
Erect & diminished → Convex mirror
Key Points: Image Formation Rules (Spherical Mirrors)
- Parallel ray rule: A ray parallel to the principal axis passes through the focus (concave) or appears to come from the focus (convex) after reflection.
- Focus ray rule: A ray passing through the focus (concave) or directed towards the focus (convex) becomes parallel to the principal axis after reflection.
- Centre of curvature rule: A ray passing through or directed towards the centre of curvature retraces its path after reflection.
- Law of reflection rule: A ray striking the mirror surface reflects according to the laws of reflection.
Concepts [12]
- Spherical Mirrors
- Fundamental Terms Related to Spherical Mirrors
- Relation Between Focal Length and Radius of Curvature of a Spherical Mirror
- Rules to Trace the Image Formed by Spherical Mirrors
- Conditions of Image Formation
- Position and Nature of Image Formed by Spherical Mirrors
- Sign Convention
- Mirror Formula for Concave Mirror
- Mirror Formula for Convex Mirror
- Linear Magnification by Spherical Mirrors
- Uses of Spherical Mirrors
- Overview: Reflection of Light: Spherical Mirrors
