LIGHT NOTES

                                                      LIGHT NOTES

 

Light: Definition

Light is a form of energy that enables us to see things. Light starts from a source and bounces off objects which are perceived by our eyes, and our brain processes this signal, which eventually enables us to see. Maxwell predicted that magnetic and electric fields travel in the form of waves, and these waves move at the speed of light. This led Maxwell to predict that light itself was carried by electromagnetic waves, which means that light is a form of electromagnetic radiation.

Nature of Light

Light behaves as a:

• ray, e.g. reflection
• wave, e.g. interference and diffraction
• particle, e.g. photoelectric effect

Light Incident on the Surface Separating Two Media

When light travels from one medium to another medium it either:

• gets absorbed (absorption)
• bounces back (reflection)
• passes through or bends (refraction)

When light is incident on a plane mirror, most of it gets reflected, and some of it gets absorbed in the medium.

 According to the concept of wave-particle duality in quantum mechanics, light exhibits both   particle and wave nature, depending upon the circumstances. A phenomenon like diffraction,  polarisation and interference could be explained by considering light as a wave. A  phenomenon like the photoelectric effect is explained by assuming that light consists of particles called photons.

 Laws of Reflection

 Light Incident on the Surface Separating Two Media

 When light travels from one medium to another medium it either:

• gets absorbed (absorption)
• bounces back (reflection)
• passes through or bends (refraction)

 When light is incident on a plane mirror, most of it gets reflected, and some of it gets absorbed in the medium.

 Propagation of Light

 Rectilinear propagation of light: Light travels in a straight line between any two points.

 Plane Mirror

 Any flat and polished surface that has almost no irregularities on its surface that reflect light is   called a plane mirror. 

 Image Formation by a Plane Mirror

• The image formed by a plane mirror is always virtual and erect.
• Object and image are equidistant from the mirror.

  Characteristics of Images

• Images can be real or virtual, erect or inverted, magnified or diminished. A real image is formed by the actual convergence of light rays. A virtual image is an apparent convergence of diverging light rays.
• If an image formed is upside down, then it is called inverted or else it is an erect image. If the image formed is bigger than the object, then it is called magnified. If the image formed is smaller than the object, then it is diminished.

• Spherical Mirrors

  Spherical Mirror

  Consider a hollow sphere with a very smooth and polished inside surface and an outer surface with a coating of mercury so that no light can come out. Then if we cut a thin slice out of the shell, we get a curved mirror, which is called a spherical mirror.
  

  Relationship between Focus and Radius of Curvature

  Focal length is half the distance between the pole and the radius of curvature.

  F = R/2

  

  Curved Mirror

  A mirror (or any polished, reflective surface) with a curvature is known as a curved mirror.
  

    Important Terms Related to Spherical Mirror 

• Pole (P): The midpoint of a spherical mirror.
• Centre of curvature (C): The centre of the sphere that the spherical mirror was a part of.
• The radius of curvature (r): The distance between the centre of curvature and the spherical mirror. This radius will intersect the mirror at the pole (P).
• Principal Axis: The line passing through the pole and the centre of curvature is the main or principal axis.
• Concave Mirror: A spherical mirror with a reflecting surface that bulges inwards.
• Convex Mirror: A spherical mirror with a reflecting surface that bulges outwards.
• Focus (F): Take a concave mirror. All rays parallel to the principal axis converge at a point between the pole and the centre of curvature. This point is called the focal point or focus.
• Focal length: Distance between pole and focus.

• Rules of Ray Diagram for Representation of Images Formed

  •  A ray passing through the centre of curvature hits the concave spherical mirror and retraces its path.
  • Rays parallel to the principal axis passes through the focal point or focus.

  Image Formation by Spherical Mirrors

  For objects at various positions, the image formed can be found using the ray diagrams for the special two rays. The following table is for a concave mirror.

  

  Uses of SphericalMirror based on the Image Formed

  Concave and Convex mirrors are used for many daily purposes.
  Example: Rear view mirrors in vehicles, lamps, solar cookers. 

• Refraction Through a Glass Slab and Refractive Index

  Refraction

  The shortest path need not be the quickest path. Since light is always in a hurry, it bends when it enters a different medium as it is still following the quickest path. This phenomenon of light bending in a different medium is called refraction.

  

  Laws of Refraction

  • The incident ray, the refracted ray and the normal to the interface of two transparent media 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 a constant for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

  Absolute and Relative Refractive Index

  The refractive index of one medium with respect to another medium is called the relative refractive index. When taken with respect to a vacuum, it’s known as an absolute refractive index.

  Refraction through a Rectangular Glass Slab

  When the light is incident on a rectangular glass slab, it emerges out parallel to the incident ray and is laterally displaced. It moves from rarer to a denser medium and then again to a rarer medium.

  Refractive Index

  The extent to which light bends when moving from one medium to another is called the refractive index. This depends on the ratio of the speeds in the two media. The greater the ratio, the more the bending. It is also the ratio of the sine of the angle of incidence and the sine of the angle of refraction, which is a constant for any given pair of media. It is denoted by:
  n = sin∠i/sin∠r = speed of light in medium 1/speed of light in medium2.

  The ratio of the speed of light in a vacuum to the speed of monochromatic light in the substance of interest is known as the relative refractive index. Mathematically, it is represented as:
  n = c/v
  Where n is the refractive index of a medium, c is the velocity of light in a vacuum and v is the velocity of light in that particular medium.

• • When the light goes from a denser to a rarer medium, it bends away from the normal. The angle at which the incident ray causes the refracted ray to go along the surface of the two media parallelly is called the critical angle.
  • When the incident angle is greater than the critical angle, it reflects inside the denser medium instead of refracting. This phenomenon is known as Total Internal Reflection.
    E.g. mirages, and optical fibres.

  • Refraction at Curved Surfaces

    When light is incident on a curved surface and passes through, the laws of refraction still hold true, for example, lenses.

    Spherical Lenses

    Spherical lenses are lenses formed by binding two spherical transparent surfaces together. Spherical lenses formed by binding two spherical surfaces bulging outward are known as convex lenses while spherical lenses formed by binding two spherical surfaces such that they are curved inward are known as concave lenses.

  • Image Formation by Spherical Lenses

    The following table shows image formation by a convex lens.

    

    Lens Formula, Magnification and Power of Lens

    Lens Formula and Magnification

    Lens formula: 1/v = 1/u = 1/f, gives the relationship between the object distance (u), image distance (v), and the focal length (f) of a spherical lens.