Light: Reflection and Refraction
What is tested
This domain tests laws of reflection, spherical mirrors, sign convention, mirror and lens formulae, magnification, refraction and refractive index. Numerical questions on image position are common.
Core rules
- Angle of incidence equals angle of reflection; the incident ray, reflected ray and normal lie in one plane.
- Mirror formula: 1/v + 1/u = 1/f; magnification m = -v/u. Distances are measured from the pole, with the object distance negative.
- Lens formula: 1/v - 1/u = 1/f; magnification m = v/u. Power P = 1/f in metres, in dioptres.
- Refraction occurs because light changes speed between media. When light enters a denser medium, it bends toward the normal.
- Refractive index n = speed of light in vacuum / speed of light in medium.
Worked example
An object is 10 cm in front of a concave mirror of focal length 15 cm. Use u = -10 cm, f = -15 cm. 1/v = 1/f - 1/u = -1/15 + 1/10 = 1/30, so v = +30 cm. The image is behind the mirror, virtual and erect. Magnification m = -v/u = -30/(-10) = +3, so it is three times larger.
Traps
A real image is inverted and can be caught on a screen; a virtual image is erect. Concave mirrors can form virtual images when the object is within the focus. Keep signs consistent in formulae.