What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

Write the mirror equation and magnification.

\(\dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f}\) and \(m=-\dfrac{v}{u}\), using sign conventions.

State Snell's law of refraction.

\(n_1\sin\theta_1=n_2\sin\theta_2\); the ratio of sines equals the relative refractive index.

What is the condition for total internal reflection?

Light must travel from a denser to a rarer medium and strike at an angle greater than the critical angle.

How do powers of lenses in contact combine?

They add algebraically: \(P=P_1+P_2+\cdots\).

What is the refractive index in terms of minimum deviation?

\(n=\dfrac{\sin\frac{A+D_m}{2}}{\sin\frac{A}{2}}\) for a prism of angle \(A\).

What causes dispersion of light by a prism?

Refractive index varies with wavelength, so different colours deviate by different amounts.

How is myopia corrected?

With a concave (diverging) lens of suitable power to shift the image onto the retina.

Give the magnifying power of a simple microscope.

\(m=1+\dfrac{D}{f}\) when the image forms at the near point \(D\).

Give the magnifying power of a telescope in normal adjustment.

\(m=\dfrac{f_o}{f_e}\), the ratio of objective to eyepiece focal lengths.

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:

Flashcards & interactive practice