Understanding Reflection, Refraction, and Spherical Lenses
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Reflection of Light
Reflection is one of the unique properties of light. It is the reflection of light, which enables us to see any object.
Reflection: The bouncing back of rays of light from a polished and shiny surface is called reflection or reflection of light.
Laws of Reflection of light: The angle of incidence and angle of reflection is equal. The incident ray, reflected ray and normal to the point of reflection lie in the same plane.
The angle of incidence is denoted by 'i' and angle of reflection is denoted by 'r'. The law of reflection is applicable to all types of reflecting surface.
Mirror and Reflection of Light: Mirror is a shiny polished object (glass) which reflects most of the rays of light falling upon it. One side of mirror is polished with suitable material to make the other side reflective.
Types of Image formed by mirrors: Image which is formed in front of the mirror and it can be obtained on a screen is called real image. Image which is formed behind the mirror and cannot be obtained on a screen is called virtual image.Types of Mirror:
Plain Mirror: A mirror having a flat surface is called plane mirror.
Formation of image in plane mirror:
Fig: Image formation in plain mirror
- A plane mirror always forms virtual and erect image.
- The distance of image and that of object is equal from the mirror.
- The image formed by a plane mirror is laterally inverted.
Spherical Mirror
Mirrors having curved reflecting surface are called spherical mirrors. A spherical mirror is a part of a sphere.
Types of Spherical Mirror:
Concave Mirror: Spherical mirror with reflecting surface curved inwards is called concave mirror.
Convex Mirror: Spherical mirror with reflecting surface curved outwards is called convex mirror.
Important terms in the case of spherical mirror:
Fig: Concave Mirror
Pole: The centre of reflecting surface of a spherical mirror is known as Pole. Pole lies on the surface of spherical mirror. Pole is generally represented by ‘P’.
Centre of Curvature: The centre of sphere; of which the reflecting surface of a spherical mirror is a part; is called the centre of curvature of the spherical mirror. Centre of curvature is not a part of spherical mirror rather it lies outside the mirror. Centre of curvature is denoted by letter ‘C’.
In the case of concave mirror centre of curvature lies in front of the reflecting surface. On the other hand, centre of curvature lies behind the reflecting surface in the case of convex mirror.
Radius of Curvature: The radius of sphere; of which the reflecting surface of a spherical mirror is a part; is called the Radius of Curvature of the spherical mirror. The radius of curvature of a spherical mirror is denoted by letter ‘R’.
Similar to centre of curvature, radius of curvature lies in front of concave mirror and lies behind the convex mirror and is not a part of the mirror as it lies outside the mirror.
Aperture: The diameter of reflecting surface of a spherical mirror is called aperture.
Principal Axis: Imaginary line passing through the centre of curvature and pole of a spherical mirror is called the Principal Axis.
Focus or Principal Focus: Point on principal axis at which parallel rays; coming from infinity; converge after reflection is called the Focus or Principal Focus of the spherical mirror. Focus is represented by letter ‘F’.
In the case of a concave mirror, parallel rays; coming from infinity; converge after reflection in front of the mirror. Thus, the focus lies in front of a concave mirror. In the case of a convex mirror, parallel rays; coming from infinity; appear to be diverging from behind the mirror. Thus, the focus lies behind the convex mirror.
Focal length: The distance from pole to focus is called focal length. Focal length is denoted by letter ‘f’.
Reflection of Rays parallel to Principal Axis:
In the case of concave mirror: A Ray parallel to principal axis passes through the principal focus after reflection from a concave mirror.
Fig: Rays parallel to principal axis
Similarly, all parallel rays to the principal axis pass through the principal focus after reflection from a concave mirror. Since, a concave mirror converge the parallel rays after reflection, thus a concave mirror is also known as converging mirror.
In the case of convex mirror: A ray parallel to principal axis appears to diverge from the principal focus after reflecting from the surface of a convex mirror.
Fig: Rays parallel to principal axis
Similarly, all rays parallel to the principal axis of a convex mirror appear to diverge or coming from principal focus after reflection from a convex mirror. Since, a convex mirror diverges the parallel rays after reflection, thus it is also known as diverging mirror.
Reflection of ray passing through the Principal Focus:
In the case of concave mirror: Ray passing through the principal focus goes parallel to principal axis after reflection in the case of concave mirror.In the case of convex mirror: A ray directed towards principal focus goes parallel to principal axis after reflecting from the surface of a convex mirror.
Ray passing through the Centre of curvature:
In the case of concave mirror: Ray passing through the centre of curvature returns at the same path after reflecting from the surface of a concave mirror.
In the case of convex mirror: Ray appears to passing through or directed towards the centre of curvature goes parallel to the principal axis after reflecting from the surface of a convex mirror.
Refraction: The change of direction of light because of change of medium is known as Refraction or Refraction of Light. The ray of light changes its direction or phenomenon of refraction takes place because of difference in speed in different media.
The light travels at faster speed in rare medium and at slower speed in denser medium. The nature of media is taken as relative. For example air is a rarer medium than water or glass.
Laws of Refraction:
- The incident ray, refracted ray and normal to the interface of given two transparent media, all lie in same plane.
- The ratio of sine of angle of incidence and sine of angle of refraction is always constant for the light of given colour and for the pair of given media.
The Second Law of Refraction is also known as Snell’s Law of Refraction.
The constant is called refractive index of the second medium in relation to the first medium.
Therefore; refractive index of medium 2 with respect to medium 1 (n21)
Absolute Refractive Index: When one medium is taken as vacuum and speed of light is taken in it, then the refractive index of second medium with respect to vacuum is called Absolute Refractive Index and it is generally denoted by n2.
The speed of light in vacuum is slightly faster than in air. Let speed of light in air is ‘c’ and the speed of light in given medium is ‘v’. Therefore, refractive index of the given medium:
Since, Refractive Index is the relative value of the speed of light of a medium with respect to the speed of light in vacuum, thus light will travel faster in the medium having lower value of refractive index.
Optical Density: Medium having greater value of refractive index is called optically denser medium, this means light will travel at slower speed in optically denser medium compared to in an optically rarer medium.
Spherical Lens
Spherical Lens: Most of the lenses are made by the combination of parts of transparent sphere. Concave and Convex lens are most commonly use spherical lens.
Convex lens is the most commonly used lens in our day to day life.
Convex lens: A lens having two spherical surface bulging outwards is called Convex Lens. It is also known as biconvex lens because of two spherical surface bulging outwards.
Fig: Spherical Lens
Concave lens: A lens having two spherical surface bulging inwards is called Concave Lens. It is also known as biconcave lens because of two spherical surface bulging inwards.
Important terms for spherical lens:Centre of curvature: The centre of sphere of part of which a lens is formed is called the centre of curvature of the lens. Since concave and convex lenses are formed by the combination of two parts of spheres, therefore they have two centres of curvature.
One centre of curvature is usually denoted by C1 and second is denoted by C2.
Similar to centres of curvature; convex and concave lenses have two Foci. These are represented as F1 and F2.
Principal Axis: Imaginary line that passes through the centres of curvature of a lens is called Principal Focus.
Optical centre: The central point of a lens is called its Optical Centre. A ray passes through optical centre of a lens without any deviation.
Radius of curvature: The distance between optical centre and centre of curvature is called the radius of curvature, which is generally denoted by R.
Focal Length: The distance between optical centre and principal focus is called focal length of a lens. Focal length of a lens is half of the radius of curvature.Power of lens: A convex lens with short focal length converges the light rays with greater degree nearer to principal focus and a concave lens with short focal length diverges the light rays with greater degree nearer to principal focus.
The degree of divergence or convergence of ray of light by a lens is expressed in terms of the power of lens. Degree of convergence and divergence depends upon the focal length of a lens. The power of a lens is denoted by ‘P’. The power of a lens is reciprocal of the focal length.
The SI unit of Power of lens is dioptre and it is denoted by ‘D’.
Power of a lens is expressed in dioptre when the focal length is expressed in metre. Thus, a lens having 1 metre of focal length has power equal to 1 dipotre.
Therefore, 1 D = 1 m−1
A convex lens has power in positive and a concave lens has power in negative.