Optics
Optics on ORT Physics tests ray behaviour at boundaries, image formation by lenses, and the wave evidence for interference and diffraction. Many marks are lost through sign conventions and unit slips rather than through difficult physics.
Core rules
- Reflection: angle of incidence equals angle of reflection, both measured from the normal.
- Refraction: n₁ sinθ₁ = n₂ sinθ₂, with n = c/v. Frequency is unchanged across a boundary; wavelength and speed change.
- Total internal reflection occurs only when light travels from a denser to a less dense medium and the angle exceeds the critical angle, where sin C = n₂/n₁.
- Thin lens equation: 1/f = 1/v + 1/u in a real-is-positive convention; magnification m = −v/u = image height/object height.
- A converging lens forms a real, inverted image when the object is beyond f; inside f the image is virtual, upright and magnified.
- Two-source interference needs coherent sources. Young's slits: fringe spacing w = λD/s. A diffraction grating: d sinθ = nλ.
Worked example
An object is 30 cm from a converging lens of focal length 10 cm. Using 1/f = 1/v + 1/u: 1/v = 1/10 − 1/30 = 2/30, so v = 15 cm. The image is real and forms on the opposite side of the lens. Magnification m = −v/u = −15/30 = −0.50: the image is inverted and half the object height. If the object were moved to 5 cm from the lens, the calculation would give a negative v, signalling a virtual image on the same side, upright and magnified.
Traps
- Measuring angles from the surface instead of the normal.
- Forgetting that refractive index varies with wavelength, so colours focus differently.
- Expecting total internal reflection when light enters a denser medium.
- Mixing sign conventions midway through a lens calculation.
- Drawing ray diagrams with rays that do not obey the reflection or refraction rules.