UNIT 14: REFLECTION AND REFRACTION OF LIGHT

• Beam: It is a bundle of rays.

• Shadows: They are the dark areas that are formed when light is blocked.

• We can see

  • luminous objects because they produce light

  • non-luminous objects when they reflect light

• Regular and irregular reflections

  • Smooth, shiny and polished surfaces (e.g., mirror) give a regular reflection.

  • Rough, dull and unpolished surfaces (e.g., paper) give an irregular reflection.

• Second Law of Reflection: The angle of incidence is equal to the angle of reflection.

i = r

• Plane mirrors

  • Characteristics of image in a plane mirror

    • Same size as the object.

    • Same distance behind the mirror as the object in front of the mirror.

    • Upright

    • Virtual

    • Laterally inverted ( i.e., left appears right and vice versa.)

  • Applications of plane mirrors

    • Eye-sight testing in a small room

    • Removing parallax error in instrument scales

• Real Image: It is an image that can be formed on a screen because rays converge on image.

  • Examples: Projector / Photocopier / Camera

• Virtual Image: It is an image that cannot be formed on a screen because rays only appear to come from image.

  • Examples: Magnifying glass / Reflection in a mirror

• Refraction: It is the bending of light when it enters or exits a medium.

• When light slows down it bends towards the normal………….when light speeds up it bends away from the normal.

• Second Law of Refraction: The ratio of the sine of the angle of incidence in one medium to the sine of the corresponding angle of refraction in the other medium is a constant value.

• Refractive index (of a medium): It is the ratio of the sine of the angle of incidence in vacuum (or air) to the sine of angle of refraction in the medium.

  

  • You use the formula n = (sin i / sin r) when light goes from vacuum (or air) to an optically dense medium
  • You use the formula (1/n) = (sin i / sin r) when light goes from an optically dense medium to vacuum (or air)
  • Value of n is always greater than 1.
  • Greater the value of n, greater is the refraction (bending).

• Consequences of refraction

  • Swimming pools appear shallower.

  • Objects appear bent.

  • Mirages

  • Rainbows

  Note: Rainbows are due to refraction, total internal reflection and dispersion of white light.

• Critical angle: It is the angle of incidence in the optically denser medium for which the angle of refraction in the optically less dense medium is 90^o.

• Total internal reflection: All the ray reflects back into the denser medium when the angle of incidence in the optically denser medium is greater than the critical angle.

  • Application in fibre optic cables: Information is passed in the form of (visible) light that shows total internal reflection. Optical fibers are used in telecommunications because they have following advantages over copper wires:

    • • more data per second

      • more secure

      • less attenuation (so boosters are further apart)

      • less interference (e.g., cross-talk)

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