Light (O-Levels)

What these notes cover

These notes cover light for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): the laws of reflection, the image formed by a plane mirror and how to draw its ray diagram, refraction and refractive index, the critical angle and total internal reflection, optical fibres and prism periscopes, and the dispersion of white light into the visible spectrum. They include an experiment to verify the law of reflection and many past-paper questions.

Key formulas and definitions

• Light is a transverse electromagnetic wave. It does not need a medium and travels at 3.0 × 10⁸ m/s in a vacuum (approximately the same in air).
• Laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal all lie in the same plane. Angles are measured from the normal.
• The image in a plane mirror is virtual, upright, the same size as the object, laterally inverted, and as far behind the mirror as the object is in front of it.
• Refraction is the change in direction of light when it changes speed passing from one medium to another. Entering a denser medium, light slows down, its wavelength decreases and it bends towards the normal; its frequency does not change.
• Refractive index: n = sin i / sin r = (speed of light in a vacuum) / (speed of light in the medium).
• The critical angle c is the angle of incidence in the denser medium for which the angle of refraction is 90°: sin c = 1 / n.
• Total internal reflection happens when light travels from a denser to a less dense medium and the angle of incidence is greater than the critical angle. It is used in optical fibres and in prism periscopes (45° prisms).
• A prism disperses white light into the visible spectrum: red, orange, yellow, green, blue, indigo and violet. Violet is refracted the most and red the least. Light of a single frequency is called monochromatic.