Optics Notes

Edexcel GCSE Physics: Revision notes

Key facts

  • Law of reflection: angle of incidence = angle of reflection, both measured from the normal.
  • Refraction is caused by a change in speed; light bends towards the normal entering glass and away leaving it.
  • Total internal reflection happens in a denser material beyond the critical angle.
  • A coloured object reflects its own colour and absorbs the rest; a filter transmits its own colour.
  • A converging lens focuses parallel rays; a diverging lens spreads them and gives only virtual images.

Reflection and refraction

Reflection bounces light off a surface; refraction bends it as its speed changes; total internal reflection traps it in a denser material.

The normal is a line at right angles to the surface.

Refraction is the change in direction caused by a change in speed.

Total internal reflection (TIR): light in a denser substance hits a boundary with a less dense one at an angle greater than the critical angle, and all of it reflects.

Smooth surfaces give specular reflection; rough ones give diffuse.

mirrorincident rayreflected raynormal40°40°AOBN
Law of reflection: both angles are measured from the normal, and here both are 40°.
  • Angle of incidenceangle of reflection

Specular

  • Smooth surface
  • Light reflects in one direction

Diffuse

  • Rough surface
  • Light scatters in many directions

A ray hits a mirror with an angle of incidence of 25°. What is the angle of reflection?

Core practical: refraction

Trace a ray through a block, then measure the angles of incidence and refraction from the normal.

Shine a ray at a known angle of incidence, trace its path with pins or marks, remove the block, draw the normals and measure the angles. Repeat for a range of angles.

The ray bends towards the normal on entering the denser glass and away from it on leaving.

incident ray40°25°AOPQNN2T1T2B1B2
A ray entering a glass block at 40° bends towards the normal (about 25°) and leaves parallel to its original path.
  1. 1

    Ray at a known angle

    Shine it at the block

  2. 2

    Trace the ray

    Pins or marks on paper

  3. 3

    Draw the normals

    After removing the block

  4. 4

    Measure angles

    Incidence and refraction

  5. 5

    Repeat

    For a range of angles

Refraction in a glass block

How does the ray bend on entering the glass block?

Colour

An opaque object reflects the colours it appears and absorbs the rest; a filter transmits its own colour.

The colour of an opaque object depends on which wavelengths it absorbs and which it reflects.

A blue shirt looks blue in white light because it reflects blue and absorbs the others.

Filters work by transmission: a red filter transmits red light and absorbs the other colours.

Red opaque object

Red light:
Reflected
Other colours:
Absorbed

Red filter

Red light:
Transmitted
Other colours:
Absorbed

White light passes through a red filter. What emerges?

Lenses

A converging lens brings rays together; a diverging lens spreads them apart.

A lens refracts light to form an image. Its power depends on its focal length and shape: a more curved lens has a shorter focal length and higher power.

A converging lens can form a real image (it can be projected) or, if the object is closer than the focal length, a magnified virtual image.

A diverging lens always forms a virtual, upright, diminished image.

lensL1L2R1R1mR2R2mF
A converging lens brings parallel rays to a focus at F.

Converging (convex)

Effect on parallel rays:
Brought together to a focus
Images formed:
Real, or magnified virtual if the object is closer than the focal length

Diverging (concave)

Effect on parallel rays:
Spread apart
Images formed:
Always virtual, upright, diminished

Which lens can form an image that can be projected on a screen?

Try an exam question

Describe how to investigate refraction of light by a rectangular glass block.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.