Notes of Light: Reflection and Refraction..... part 2 .......... get ready for boards

Light

Light is a source of energy that produces sensation of vision.

Properties of light

1. It travels in straight line
2. It does not need a medium for propagation
3. Speed of light in vacuum is 3 x 10 ⁸ m/s
4. It is an electromagnetic wave
5. It is a transverse wave

Different phenomenon exhibited by light are :

1. Reflection
2. Refraction
3. Dispersion
4. Scattering

Types of images

1. Real image
2. Virtual image

Real Image

• It can be obtained on a screen.
• It is formed due to actual meeting of two reflected light rays.
• eg: Images formed by concave mirror when object beyond focus

Virtual Image

• It can not be obtained on a screen.
• It is formed when two reflected light rays appear to meet at a point.
• eg: Image formed by plane mirror.

Properties of image formed by Plane Mirror

1. Virtual image
2. Erect image
3. Same size as that of the object.
4. Laterally inverted.

Reflection

When light is incident on a smooth surface it returns back following certain rules. This is called reflection. These rules are called Laws of Reflection.

Laws of Reflection.

1. The incident ray, reflected ray and the normal at the point of incidence lie in the same plain
2. The angle of incidence is equal to the angle of reflection

Spherical Mirrors

A mirror which can be imagined to the part of a sphere is called spherical mirrors

Types of spherical mirrors

• Concave Mirror
• ConvexMirror

  Pole (P)

  The geometrical centre of a spherical mirror is called pole.

  Centre of curvature (C)

  The centre of the sphere of which spherical mirror is a part is called centre of curvature.

  Principal Axis

  A line passing through centre of curvature and pole is called principal axis.

  Radius of curvature(R)

  The radius of the sphere of which spherical mirror is a part is called radius of curvature.

  Principal Focus(F)

  Light rays parallel to principal axis incident on a concave mirror after reflection converges to a point on the principal axis. This point is called principal focus.

  Focal Length(F)

  The distance between pole(P) and focus(F) is called focal length.

  Relation between Radius of curvature and Focal length

  Radius(R) of curvature is equal to twice the focal length(f)

  R = 2f
  

  Rules for drawing ray diagram

  1. A ray incident parallel to principal axis after reflection passes through the focus
  2. a ray incident along the focus is reflected parallel to the principal axis
  3. A ray incident along the centre of curvature will be reflected back along the same direction
  4. A ray incident obliquely at the pole will be reflected according to the laws of reflection

  New Cartesian sign convention

• All distances are measured from the pole of the spherical mirror
• Distances measured in the direction of incident ray are taken as positive
• Distances measured opposite to the direction of incident ray are taken as negative
• Distances measured perpendicular to principal axis and upward is take as positive
• Distances measured perpendicular to principal axis and downward is take as negative

  Mirror Formula

  It is a relation between object distance(u) image distance(v) and focal length(f) of a spherical mirror.

  Magnification

  It is the ratio of image height(h_i) to the object height(h_o).

• 
  

  Uses of concave mirrors

  • They are used as reflectors in vehicle head light.
  • They are used by dentists for seeing large image of tooth.
  • They are used as solar concentrators

  Use of convex mirrors

  They are used as rear view mirrors in vehicles.Because

  • they give virtual and erect image and
  • they give wider field of view.

  Refraction

  The change in direction of light when it travels from one medium to another is called refraction.

  Laws of refraction

1. The incident ray , refracted ray and the normal at the point of incidence lie in the same plain.
2. The ratio of sine of angle of incidence to the sine of angle of refraction is a constant.

   Lenses

   There are two type of lenses. They are

   1. Convex Lens
   2. Concave Lens

   Rules for drawing ray diagram

   1. A ray incident parallel to principal axis after refraction passes through the focus.
   2. a ray incident along the focus after refraction pass parallel to the principal axis.
   3. A ray incident along the optical centre will continue to move along the same direction.

   New Cartesian sign convention

   1. All distances are measured from the optical centre of the lens.
   2. Distances measured in the direction of incident ray are taken as positive
   3. Distances measured opposite to the direction of incident ray are taken as negative
   4. Distances measured perpendicular to principal axis and upward is take as positive
   5. Distances measured perpendicular to principal axis and downward is take as negative

   Image formation by convex lens

Define one dioptre(D)

One dioptre is the power of a lens of focal length one metre.

Power of convex lens is taken as positive.

Power of concave lens is taken as negative.