Natural Phenomena
Natural phenomena in this exam focuses on light: reflection at mirrors, refraction through lenses and glass, the human eye, and optical defects. Questions test the laws, ray diagrams, the mirror and lens formulae, power of a lens, and everyday effects such as dispersion and scattering.
Core rules and concepts
- Angle of incidence equals angle of reflection; the incident ray, reflected ray and normal lie in one plane.
- For spherical mirrors, 1/v + 1/u = 1/f; for lenses, 1/v − 1/u = 1/f. Use the sign convention consistently.
- Power P = 1/f with focal length in metres; the unit is the dioptre (D).
- A concave mirror can form real or virtual images; a convex mirror always forms a virtual, diminished image.
- A convex lens converges light; a concave lens diverges it.
- Myopia is corrected with a concave lens, hypermetropia with a convex lens.
- Dispersion splits white light because different colours refract by different amounts.
Worked example
A convex lens has focal length 10 cm. An object is placed 15 cm in front of it. Taking u = −15 cm and f = +10 cm: 1/v − 1/(−15) = 1/10, so 1/v = 1/10 − 1/15 = 1/30. Thus v = +30 cm. The image is real, inverted and formed 30 cm behind the lens. Magnification m = v/u = 30/(−15) = −2, so the image is twice the object size.
Traps that cost marks
- Substituting focal length in centimetres into P = 1/f without converting to metres.
- Mixing the mirror and lens formulae.
- Ignoring the negative sign for a virtual image.
- Saying a concave lens can form a real image on a screen.
- Confusing scattering, which makes the sky blue, with dispersion, which makes a rainbow.