Reflections

Refractive index

Light speed in a vacuum/light speed in a medium. It is always greater for equal to 1

Refraction

Occurs when a wave changes its speed when it crosses a boundary resulting in a change in direction

Light rays refract towards the normal

Travel more slowly

Light rays refract away from the normal

They travel faster

Total internal reflection

A critical angle above which there is no refraction and only reflection

Critical angle

The angle of incidence that gives an angle of refraction of 90° (beyond TIR)

How to find the critical angle?

Direct a ray of light towards the curve side of a semi circular block using a ray box. Gradually increase the angle of incidence until the angle of refraction is 90. Angle of incidence= critical angle. Above this is total internal reflection

Specular

A shiny or glossy object has a smooth surface resulting in parallel rays refelcting in the same directions

Diffuse

A rough or matte surface results in parallel rays reflecting in many different directions because they are scattered

Dispersion

Different colours have different wavelengths and frequencies. In a vacuum the travel at the same speed but in other materials the different wavelengths travel at different speed. At a boundary they refract a different amount.

Optical fibres

Transmit infrared or visible light signals. Ray travel down the fibre by repeated TIR. used for data communications eg cable tv, endoscopes

Converging lens

Convex lens refract parallels light rays so they cross at a positive focal point.

Diverging lens

Concave lens refract parallels light rays so they seem to have originated from a point behind the lens (a negative focal point as it's behind the lens)

Power of lens D

1/focal length m

Uses of converging lenses

• Projectors to form an enlarged real image on a screen inverted

• Camera to form a diminished real image inverted

• Magnifying glass to form an enlarged virtual but upright image

The law of reflection

Angle of incidence= angle of reflection