Intermediate · 11 min
Reflection and refraction
Measure ray angles from the normal and connect refractive index to light speed.
Measure from the normal
At a smooth reflecting surface, the angle of reflection equals the angle of incidence. Both are measured from the normal, an imaginary line perpendicular to the surface. Do not measure them from the surface itself. A plane mirror produces an image as far behind the mirror as the object is in front. Ray diagrams describe this using straight lines and extensions behind the mirror.
Worked example
A ray makes 60° with a mirror surface. Angle to the normal = 90° − 60° = 30° Angle of reflection = 30°
Light changes speed
The refractive index of a transparent material is n = c ÷ v, where c is light speed in vacuum and v is its speed in the material. When light enters a material with a higher refractive index at an oblique angle, it bends towards the normal. Its frequency stays the same, while its wavelength changes. A ray along the normal changes speed without changing direction.
Worked example
Using c = 300,000,000 m/s and v = 200,000,000 m/s: Refractive index = 300,000,000 ÷ 200,000,000 = 1.5
Draw a light path
- Draw a plane mirror, its normal, and an incoming ray at 25° to the normal. Add the reflected ray with the correct angle.
- Draw a ray entering glass from air at an oblique angle. Show how it bends towards the normal and label which quantity remains unchanged: speed, frequency, or wavelength.