Ray Optics — Part 1: Mirrors, refraction and total internal reflection: visual lesson, CBSE Class 12 Physics

Lesson

Ray Optics and Optical Instruments

Part 1 — light as rays: images in curved mirrors, bending at boundaries, and light trapped inside glass by total internal reflection.

In this lesson you will

Apply the mirror formula and magnification with the New Cartesian sign convention
Use Snell’s law and the refractive index
Explain total internal reflection, the critical angle and optical fibres

Diagram

Mirror formula and magnification

With the New Cartesian sign convention (distances from the pole; along the incident light positive; heights up positive): $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f},\qquad f = \dfrac{R}{2},\qquad m = \dfrac{h'}{h} = -\dfrac{v}{u}$ Concave mirror: $f < 0$ . Convex mirror: $f > 0$ . Real images: $v < 0$ and $m < 0$ .

Worked example

Worked example 1: a concave mirror

An object is 25 cm in front of a concave mirror of focal length 15 cm. Find the position, nature and magnification of the image.

Step 1 — signs: $u = -25$ cm, $f = -15$ cm.
Step 2 — $\dfrac{1}{v} = \dfrac{1}{f} - \dfrac{1}{u} = -\dfrac{1}{15} + \dfrac{1}{25} = -\dfrac{2}{75}$ , so $v = -37.5$ cm.
Step 3 — $m = -\dfrac{v}{u} = -\dfrac{-37.5}{-25} = -1.5$ : real, inverted, 1.5 times enlarged, 37.5 cm in front of the mirror.

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