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OpticsEdexcel GCSE Physics: Revision notes

Section 1

What are reflection, refraction and total internal reflection?

  • Reflection is when a wave bounces off a surface. The law of reflection states: the angle of incidence equals the angle of reflection, both measured from the normal (a line at right angles to the surface).

  • Refraction is the change in direction and speed of a wave as it passes from one substance into another with a different density, caused by a change in speed.

  • Total internal reflection (TIR) occurs when a wave travelling in a denser substance hits a boundary with a less dense substance at an angle greater than the critical angle, causing all the light to reflect back rather than refract out.

  • Specular reflection: reflection from a smooth surface, where all the light reflects in one direction

  • Diffuse reflection: reflection from a rough surface, where light is scattered in many directions

Key termsreflectionrefractiontotal internal reflectioncritical anglespecular reflectiondiffuse reflection
Exam tip

Always measure angles of incidence, reflection and refraction from the normal, never from the surface itself.

Section 2

Core Practical: investigating refraction in a glass block

Method:

  1. Shine a ray of light at a rectangular glass block at a known angle of incidence
  2. Trace the path of the ray entering and leaving the block using pins or by marking the ray's path on paper
  3. Remove the block and draw the normal at each surface where the ray meets the block
  4. Measure the angles of incidence and refraction at the point where the ray enters the glass
  5. Repeat for a range of angles of incidence

Result: the ray bends towards the normal on entering the (denser) glass, and away from the normal on leaving it, showing refraction caused by a change in the wave's speed as it moves between substances.

Section 3

How does colour relate to absorption and transmission?

The colour of an opaque object depends on which wavelengths of light it absorbs and which it reflects. A red object absorbs all colours except red, which it reflects to our eyes.

Filters work by transmission: a filter only allows light of certain wavelengths (colours) to pass through it and absorbs the rest. A red filter only transmits red light, absorbing other colours.

Key termsfilter
Example

A blue shirt appears blue in white light because it reflects blue light and absorbs the other colours.

Section 4

How do lenses form images?

A lens refracts light to form an image.

  • The power of a lens depends on its focal length and shape: a more strongly curved lens has a shorter focal length and a higher power
  • A converging lens (convex) brings parallel rays of light together to a focus; it can form real images (which can be projected on a screen) or, when the object is closer than the focal length, magnified virtual images
  • A diverging lens (concave) spreads parallel rays of light apart; it always forms virtual, upright, diminished images that cannot be projected on a screen

Ray diagrams are used to show how each type of lens refracts light and where the resulting image forms.

Key termsconverging lensdiverging lensfocal lengthreal imagevirtual image
Common mistake

Don't say a diverging lens can form a real image — it only ever produces virtual images.

Must Know

  • Law of reflection: angle of incidence = angle of reflection
  • Refraction is caused by a change in wave speed between substances of different density
  • TIR occurs above the critical angle, when travelling from a denser to a less dense substance
  • Specular reflection: smooth surface, one direction; diffuse reflection: rough surface, scattered
  • Colour is determined by which wavelengths are absorbed/reflected/transmitted
  • Power of a lens depends on focal length and shape; shorter focal length = higher power
  • Converging lenses can form real or virtual images; diverging lenses only form virtual images

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