Chapter 12 - Light

1.

State the properties of the image formed by a plane mirror

  • Upright
  • Image size equals object size
  • Virtual
  • Image distance equals object distance
  • Laterally inverted.

2.

State one advantage of using a plane mirror in optical testing.

  • When a plane mirror is used, the image of the optical test card will be at a distance equal to the object distance. In this way, it can appear further than it actually is.
  • The larger distance between the viewer and the test card can be obtained. This distance can even be longer than the room itself.

3.   

Explain why refraction takes place when light travel from air to glass.

  • Light travel at different speeds in different optical media.
  • Its speed in air is 3.0 x 10^8 m/s while its speed in glass is lower at 2.0 x 10^8 m/s.
  • At the boundary of the two media, the sudden change in speed causes the path of light to bend.

4.   

Explain why TIR can only take place when light is traveling from denser to less dense medium

  • Only when light travels from denser to less dense medium, then will it bend away from the normal.
  • This is essential for the refracted angle in the less dense medium to be 90 degrees for a given incident angle in the denser medium. This incident angle is called the critical angle.
  • When the incident angle is more than the critical angle, TIR will then take place.

5.   

*Why do diamonds have such brilliant sparkle?

  • Having high refractive indexes, diamonds have small critical angles.
  • A large proportion of light at the diamond-air boundary will have incident angle greater than that of the critical angle, hence undergoing TIR.
  • The diamond is also cut in such a way that when light enters, it will undergo TIR a few times before most of them emerge from the top surface.

6.   

State some uses of optical fibres.

  • Telecommunication – transmission of signal.
  • Endoscope – to see organs in the body by directly light through a thin plastic tube into the body.
  • Binoculars – prisms allow for TIR and reduce length of the instrument.
  • Periscope – prisms are positioned to reflect light and give clear images.

7.   

State the advantages of using optical fibres over copper wires in telecommunications.

  • Optical fibres can carry much more information than copper wires.
  • Optical fibres experience less signal loss
  • Optical fibres are lighter and hence easier to install.
  • Optical fibres are cheaper to manufacturer. (many physics questions will state to avoid this economical reason)