Physics · Heat, electricity and light. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Ray optics treats light as straight lines. At a surface the ray is reflected at an equal angle, and it is refracted because its speed changes: it bends towards the normal on entering a denser medium. Going from dense to rare beyond the critical angle, all the light is reflected. Mirrors and thin lenses obey one formula linking object distance, image distance and focal length.
Wave optics explains what rays cannot. Interference needs coherent sources, which have a constant phase difference; two separate lamps do not give fringes. Diffraction is the spreading of light at a slit or edge. Polarisation shows that light is a transverse wave.
Forgetting the sign of a virtual image. If 1/v comes out negative, the image is virtual and upright (for a lens, on the same side as the object; for a mirror, behind it); do not drop the minus sign.
Answer: A virtual, upright image 30 cm from the lens, magnified 3 times.
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