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Physics · Waves

Light

CIE 06252 min read

Reflection of light

  • Normal ray — The line perpendicular from the reflecting surface at the point of incidence.
  • Angle of incidence — The angle between the incident ray and the normal.
  • Angle of reflection — The angle between the reflected ray and the normal.

Angle of incidence = angle of reflection.

Reflection of a light ray at a plane mirror: the incident and reflected rays make equal angles with the normal, both measured from the normal rather than from the mirror

Characteristics of an optical image: same size, same distance from mirror, virtual.

Plane mirror image formation: reflected rays appear to come from a virtual image behind the mirror, the same size as the object and the same distance behind it

Refraction of light

When light slows down, the ray bends to the normal. When light moves faster, the ray bends away from the normal.

  • Critical angle — The angle of incidence where the angle of refraction is 90 degrees. This varies from medium to medium
  • Total internal reflection — The complete reflection of a ray of light from the surrounding surfaces of optically less dense medium back into the denser medium.

Two conditions for TIR to take place:

  1. Light rays must travel from an optically denser medium to a less dense medium.
  2. The angle of incidence in optically denser medium > critical angle.

Total internal reflection: below the critical angle light refracts out, at the critical angle the refracted ray travels along the boundary, and above it the ray is totally internally reflected

Applications of TIR:

  • Prism
  • Endoscope
  • Optical fibres (flexible, carries more data, has high transmission quality)
  • Refractive index (n) — The ratio of the speeds of a wave in two different regions.

Thin lenses

  • Focal length — Distance from the middle of the lens to F.
  • Principal axis — The straight line passing the middle of the lens.
  • Principal focus (focal point) — The point where parallel light rays converge/diverge.

Types of light beam: parallel rays staying equally spaced, converging rays meeting at a focus, and diverging rays spreading from a point

An image can be described as:

  1. Inverted or Upright
  2. Diminished or Enlarged/Magnified
  3. Real or Virtual

Convex lens ray diagram with the object beyond 2F: the image forms between F and 2F and is real, inverted and diminished

Convex lens ray diagram with the object between F and 2F: the image forms beyond 2F and is real, inverted and magnified

Convex lens ray diagram with the object inside F: the image is upright, magnified and virtual, on the same side of the lens as the object

A virtual image is formed when diverging rays are extrapolated backwards and does not form a visible projection on a screen.

To correct long-sightedness, use converging lenses. To correct short-sightedness, use diverging lenses.

The human eye: light is refracted by the cornea and the lens, passes the iris and pupil, and forms a real inverted image on the retina which the optic nerve carries to the brain

Dispersion of light

  • Dispersion — process of white light splitting into a spectrum.

Dispersion of Light: When white light is passed through a glass prism it splits into its spectrum (Band of coloured components of light) of colours (in order red, orange, yellow, green, blue, indigo, violet)

  1. White light is made up of many different wavelengths, each wavelength corresponding to a different colour.
  2. Each beam of light is slowed differently by the glass.
  3. Violet light -> shorter wavelength -> slowed more.
  4. When light travels from one medium to another, the speed of its propagation changes -> it ‘bends’ or is ‘refracted’.

Dispersion of white light by a glass prism: each wavelength refracts by a different amount, so the light spreads into red, orange, yellow, green, blue, indigo and violet, with red refracted least and violet most

Visible light of a single frequency is described as monochromatic.

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