Light Reflection and Refraction

REFLECTION

Reflection of Light: The phenomenon of bouncing back of light into the same medium by the smooth surface is called reflection.

Incident light: Light which falls on the surface is called incident light.

Reflected light: Light which goes back after reflection is called reflected light.

The angle of incidence: The angle between the incident ray and the normal.

An angle of reflection: The angle between the reflected ray and the normal.

Mirror: The surface which can reflect the light is a mirror.

Plane Mirror: If the reflecting surface is a plane then the mirror is plane.

Spherical Mirror: If the reflecting surface is part of the hollow sphere then the mirror is a spherical mirror.
The spherical mirror is of two types:

· Convex mirror: In this mirror reflecting surface is convex. It diverges the light so it is also called a diverging mirror.

· Concave mirror: In this mirror reflecting surface is concave. It converges the light so it is also called converging mirror.

Parameters of Mirror:

· Center of Curvature: The centre of hollow sphere of which mirror is a part.

· The radius of curvature: The radius of hollow sphere of which mirror is a part.

· Pole: The centre of mirror (middle point) is pole.

· Principal axis: The line joining the pole and center of curvature is called principal axis.

· Aperture: Size of mirror is called aperture of mirror.

· Principal Focus: The point on the principal axis, where all the incident rays parallel to principal axis converge or diverge after reflection through mirror.

· Focal Length: The distance between pole and focus point is focal length.

Special Rays for Formation of Image:

· A ray of light which is parallel to the principal axis of a spherical mirror, after reflection converges or diverges from focus.

· A ray of light passing through or appearing from the center of curvature of spherical mirror is reflected back along the same path.

· A ray of light passing through or appearing from the focus of spherical mirror becomes parallel to the principal axis.

· A ray of light which is incident at the pole of a spherical mirror is reflected back making same angle with principal axis.

Use of Concave Mirror: It is used as a makeup mirror, the reflector in torches, in headlights of cars and searchlights, doctor’s head-mirrors, solar furnace, etc.

Sign Conventions of Spherical Mirror

· All the distances are measured from the pole of the mirror as the origin.

· Distances measured in the direction of incident rays are taken as positive.

· Distances measured opposite to the direction of incident rays are taken as negative.

· Distances measured upward and perpendicular to the principal axis are taken as positive.

· Distances measured downward and perpendicular to the principal axis are taken as negative.
$\frac { 1 }{ f } =\frac { 1 }{ v } +\frac { 1 }{ u }$ …where f, v and u are focal length, image distance, object distance

Linear Magnification: This is the ratio of the height of the image to the height of the object.
$m=\frac { { h }^{ ' } }{ h }$ …where m = magnification, h = height of image, h’ = height of object

Use of Convex Mirror: Convex mirror used as rear view mirror in vehicles, as shop security mirrors, etc.

REFRACTION

Refraction of Light: The bending of light at the interface of two different mediums is called Refraction of light.

· If the velocity of light in medium is more, then medium is called optical rarer.
Example, air or vacuum is more optical rarer.

· If the velocity of light in medium is less, then medium is called optical denser.
Example, glass is denser than air.

Refractive Index: It represents the amount or extent of bending of light when it passes from one medium to another.
There are two types of refractive index

· Relative refractive index and

· Absolute refractive index.

Refractive index of medium with respect to other medium is called Relative Refractive Index.
Refractive index of medium 1 with respect to medium 2 = $\frac { Speed\quad of\quad light\quad in\quad medium\quad 2(V2) }{ Speed\quad of\quad light\quad in\quad medium\quad 1(V1) }$

Refractive index of medium with respect to air or vacuum is called Absolute Refractive Index.
Absolute refractive index of medium (m) = $\frac { Speed of light in air(c) }{ Speed of light in medium (Vm) }$

Incident ray: It is incoming ray on the refracting surface.

Refracted ray: It is an outgoing ray from the refracting surface.

An angle of incidence (i): It is the angle between incident rays and perpendicular line (normal) at the point of incidence.

An angle of refraction (r): It is the angle between refracted rays and perpendicular line (normal) at the point of incidence.

Law of Refraction: According to this law

· “The incident ray, refracted ray and normal at the point of incidence all lie in the same plane.”

· “The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.”
$\frac { sin\quad i }{ sin\quad r }$ = constant (µ)

Lens: The transparent refracting medium bounded by two surfaces in which at least one surface is curved is called lens.
Lenses are mainly two type

· Convex lens and

· Concave lens.

Center of Curvature: The centres of two spheres, of which lens is part is called the centre of curvature.

Radii of Curvature: The radii of spheres, of which lens is part is called radius of curvature.

Principal Axis: The line joining the centres of curvature of two surfaces of lens is called principal axis.

Optical Center: It is a special point on the principal axis. Light incident on the optical centre passes through the lens without deviation.

Principal Focus: The point on the principal axis at which all incident rays parallel to the principal axis converge or appear to diverge after refraction through the lens.

Special Rays for Image Formation by Lens:

· An incident ray, parallel to the principal axis, after refraction passes through (or appears to come from), second focus of the lens.

· An incident ray, passing through the optical center of the lens, goes undeviated from the lens.

· An incident ray, passing through the (first) principal focus of the lens, or directed toward it, becomes parallel to the principal axis after refraction through lens.

Use of Lens: In photographic cameras, magnifying glass, microscope, telescope, the human eye.