Ray Optics and Optical Instruments
Class 12 Physics · Physics Part II · Chapter 1
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Summary
Treating light as rays, reflection and refraction explain image formation by mirrors and lenses. Spherical mirrors obey the mirror equation \(\dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f}\) with magnification \(m=-v/u\). Refraction at a plane surface follows Snell's law \(n_1\sin\theta_1=n_2\sin\theta_2\); when light passes to a rarer medium beyond the critical angle, total internal reflection occurs, the basis of optical fibres and brilliant gems. Refraction at spherical surfaces leads to the lens maker's formula \(\dfrac{1}{f}=(n-1)\left(\dfrac{1}{R_1}-\dfrac{1}{R_2}\right)\) and the thin-lens equation \(\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}\); lens powers add for combinations. A prism deviates light, the minimum deviation giving the refractive index, and dispersion separates white light into colours. The eye focuses by changing lens curvature, and defects such as myopia and hypermetropia are corrected with lenses. Optical instruments—the simple and compound microscope and the astronomical telescope—extend vision, their magnifying powers determined by focal lengths and the near point, allowing us to observe the very small and the very distant.
Reflection by spherical mirrorsRefraction and total internal reflectionLenses and lens formulaePrism, deviation and dispersionThe human eye and its defectsMicroscope and telescope
Key terms
Mirror equation
\(\dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f}\) relating object, image and focal distances.
Snell's law
Refraction law \(n_1\sin\theta_1=n_2\sin\theta_2\) at an interface.
Total internal reflection
Complete reflection beyond the critical angle into a denser medium.
Lens maker's formula
\(\dfrac{1}{f}=(n-1)\left(\dfrac{1}{R_1}-\dfrac{1}{R_2}\right)\) for a thin lens.
Power of a lens
Reciprocal of focal length in metres, \(P=1/f\); unit dioptre.
Magnifying power
Ratio of angles subtended with and without an instrument.
Extra questions & answers
Try one from this chapter
The mirror equation is: