Light and optics

Revision notes

Light can be modelled as a transverse electromagnetic wave and, for many optical problems, by rays showing the direction of travel. At a plane mirror the angle of incidence equals the angle of reflection, with both angles measured from the normal. A plane mirror forms a virtual image: rays appear to come from behind the mirror even though no light actually converges there.

Refraction occurs when light changes speed on entering a different medium. If light enters a medium in which it travels more slowly at an angle to the normal, it bends towards the normal; when it speeds up it bends away from the normal. The frequency is fixed by the source and remains unchanged at the boundary, so a change of speed is accompanied by a change of wavelength. At normal incidence the direction does not change even though speed and wavelength may.

A converging lens brings parallel rays towards a principal focus. A diverging lens makes parallel rays spread as if they came from a focus on the incident side. Ray diagrams can determine image position and whether an image is real or virtual. A real image forms where rays actually meet and can be projected onto a screen; a virtual image cannot be projected because rays only appear to originate from the image position. Magnification compares image size with object size.

Reflection and ray diagrams

For a plane mirror, angle of incidence equals angle of reflection, measured from the normal. A virtual image appears behind the mirror because reflected rays diverge as though they came from that position. A virtual image cannot be projected onto a screen because the light rays do not actually meet there.

Ray diagrams should use straight rays, arrowheads for direction and a normal where angles are measured. Accuracy matters: examiners use the geometry to judge understanding.

Refraction

Refraction occurs when light changes speed at a boundary. Frequency stays constant because it is set by the source, while wavelength changes with speed. When entering a medium where light travels more slowly, a ray usually bends towards the normal; when speeding up, it bends away from the normal.

At normal incidence the speed and wavelength may change but the direction does not. This is a useful reminder that refraction is fundamentally about changing speed, not simply changing direction.

Lenses and images

A converging lens brings parallel rays towards a principal focus. A diverging lens causes parallel rays to spread as if they came from a focal point on the incident side. Image properties depend on object position relative to the focal length.

Real images form where rays actually converge and can be projected on a screen. Virtual images form where rays only appear to originate and cannot be projected. Magnification compares image size with object size: magnification = image height / object height.

Total internal reflection

Total internal reflection occurs when light travels from a higher refractive-index medium towards a lower refractive-index medium and the angle of incidence exceeds the critical angle. It is used in optical fibres and reflecting prisms.

Exam technique

Always measure angles from the normal, not the surface. When asked about a lens image, state whether it is real or virtual, upright or inverted, and enlarged, diminished or same size where appropriate. For refraction explanations, mention speed and wavelength while keeping frequency constant.

Reflection from different surfaces

Specular reflection happens at a smooth surface, such as a mirror: parallel rays are reflected in a single direction, so a clear image forms. Diffuse reflection happens at a rough surface, such as paper: the rays are scattered in many directions, so no image forms.

Colour

White light is a mixture of colours. Each colour has its own narrow band of wavelengths and frequencies. An opaque object's colour depends on which wavelengths it reflects: a red object reflects red light and absorbs the other colours; a white object reflects all colours; a black object absorbs all colours. A colour filter transmits only its own colour and absorbs the rest, so a red filter lets only red light through.