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3.2.1Waves

Reflection OF Light — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 35 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

3.2.1 Reflection of Light — IGCSE PHYSICS notes: State the law of reflection and describe the image formed by a plane mirror.Normal — a line drawn at right angles to the boundary at the point where the ray strikesAngle of incidence i — the angle between the incident ray and the n…

  • Angle of reflection r
  • Line joining object and image
  • Virtual image
  • Angle to mirror
  • Seeing objects
  • Angle of incidence i — the angle between the incident ray and the normal

Part 1 of 6Ray diagram terms

Normal
  • Normal a line drawn at right angles to the boundary at the point where the ray strikes
Angle of incidence i
  • Angle of incidence i the angle between the incident ray and the normal
Angle of reflection r
  • Angle of reflection r the angle between the reflected ray and the normal
Arrows
  • Arrows the incident ray has an arrow towards the boundary and the reflected ray has an arrow away from it

Part 2 of 6Law of reflection

Law
  • Law the angle of incidence equals the angle of reflection, i = r
Plane
  • Plane the incident ray, reflected ray and normal lie in the same plane
Measuring
  • Measuring angles are measured from the normal, not from the mirror
Used in
  • Used in mirrors, cameras, optical fibres and periscopes

Part 3 of 6Image in a plane mirror

Image in a plane mirror
Plane mirror
  • Plane mirror a flat, smooth and polished surface
Image properties
  • Same size as the object
  • Same distance behind the mirror as the object is in front
  • Virtual
  • Upright but laterally inverted
Line joining object and image
  • Line joining object and image perpendicular to the mirror

Part 4 of 6Constructing the image

1
Draw two rays from the same point on the object to the mirror.
2
Reflect each ray using i = r.
3
Trace each reflected ray backwards behind the mirror as dashed lines.
4
The image is where the dashed lines cross.
  • Virtual image
    • the rays only appear to come from it, so it cannot be projected onto a screen

Part 5 of 6Using the law in constructions

Angle to mirror
  • Angle to mirror angle between ray and mirror is 90° minus the angle to the normal
Periscope
  • Periscope two mirrors turn the light so an observer sees over obstacles
Seeing objects
  • Seeing objects light reflects from an object into the eye

Part 6 of 6More from the specification

More from the specification
Law of reflection
  • The angle of incidence equals the angle of reflection, both measured from the normal,
  • and the image in a plane mirror is virtual and the same distance behind it.
Refraction
  • Light changes direction when its speed changes at a boundary, bending towards the normal on entering a denser medium such as a glass block.
Refractive index
  • Refractive index n = sin i / sin r,
    • which is also the speed of light in air divided by speed in the medium.
      What the symbols mean: n = refractive index (no unit) · i or θ1 = angle of incidence · r or θ2 = angle of refraction
      • (both measured from the normal)
Critical angle and total internal reflection
  • If light travelling in a denser medium meets the boundary at more than the critical angle, sin c = 1/n, it is totally reflected,
    What the symbols mean: c = critical angle · n = refractive index of the denser material (no unit)
  • as in optical fibres.
Lenses
  • A converging lens brings parallel rays to the principal focus at the focal length and forms real or virtual images,
  • while a diverging lens spreads rays out.
Refractive index
  • Ratio of the speed of light in a vacuum to its speed in a material
Critical angle
  • Incline angle at which sliding just begins.
Principal focus
  • Point where parallel rays meet or appear to come from
Focal length
  • Distance from the lens centre to the principal focus
Optically dense
  • Having a high refractive index
Refraction
  • Change in direction of light when it changes speed between media.

Key terms

TermMeaning
NormalLine at right angles to a surface at the point of incidence
Angle of incidenceAngle between the incident ray and the normal
Law of reflectionAngle of incidence equals angle of reflection
Virtual imageImage formed where rays only appear to meet, not projectable on a screen
Plane mirrorFlat, smooth, polished reflecting surface

Avoid these mistakes

1
Measuring angles from the mirror surface — measure from the normal.
2
Saying a plane mirror image is real — it is virtual and cannot be shown on a screen.
3
Drawing the reflected ray at a different angle from the incident ray — i must equal r.
4
Drawing the virtual rays solid — they are dashed behind the mirror.

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