Reflection and the law of reflection, refraction at a boundary, the critical angle, total internal reflection, and the use of optical fibres.

A topic 1.6 Explain question. Light travels more slowly in glass than in air because glass is the denser (optically denser) medium (1 mark). When the light meets the boundary at an angle, one side of the beam slows before the other, so the light changes direction, bending towards the normal as it enters the slower medium (1 mark). Its frequency stays the same but its wavelength shortens (1 mark). Markers reward the change in speed, the bending towards the normal entering the denser medium, and the link to direction. A common error is to say the light bends away from the normal on entering glass.

A topic 1.6 Describe and Explain question. Total internal reflection needs the light to be travelling in the denser medium going towards a less dense one (1 mark), and the angle of incidence must be greater than the critical angle (1 mark). When this happens, all the light is reflected back into the denser medium rather than refracting out (1 mark). In an optical fibre, light enters one end and hits the walls at angles greater than the critical angle, so it is totally internally reflected repeatedly and travels along the fibre with almost no loss, carrying signals for communication or images in an endoscope (1 mark). Markers reward the denser medium, the angle condition, total reflection and the fibre application.