Unit 14
Light and optics.
Refraction and total reflection, prisms and rainbows, lenses, diffraction, polarisers, and the spectrum.
Before this unit
- Waves and sound
- Trigonometry Mathematics
The unit, lesson by lesson
Lessons
6 lessons · 34 practice questions · about 3 hours
Reflection and refraction
Angles are measured from the normal, light entering glass bends towards it, Snell's law, and light that cannot get out of the glass: the critical angle and total internal reflection.
Prisms and rainbows
White light is a mixture of colours, and each colour bends at a slightly different angle in glass and water: dispersion in a prism, a refractive index that depends on wavelength, and a raindrop that sends light back at up to 42° and makes a bow, red on the outside.
Lenses
A converging lens gathers light to a focus and a diverging lens spreads it; the thin-lens equation, magnification, real and virtual images, and what happens when half a lens is covered.
Diffraction and two slits
A wave passing through an opening spreads more the narrower the opening is compared with its wavelength; why you hear round a corner but cannot see, and the fringes of two slits, in water and in light.
Polarisation
Light is a transverse wave, and a polarising filter picks the direction it shakes in: half of unpolarised light, Malus's law, a third filter that brings light back, and polarising sunglasses.
The electromagnetic spectrum
Radio, light and X-rays are the same wave at different lengths: one speed in a vacuum, frequency from wavelength, brightness against colour, and the energy of a single photon.