1. Laws of Reflection of Light:

(i) Angle of incidence is equal to angle of reflection

(ii) Incident ray, reflected ray, normal drawn to the reflecting surface at the point of incidence, lie in a same plane.

2. Laws of Refraction of Light:

(i) For a given pair of medium, and for given colour of light, the ratio between the sine of angle of incidence and sine of angle of refraction is constant and equal to relative refractive index of medium $2$ with respect to medium $1$ .

If $i$ is the angle of incidence and $r$ is the angle of refraction, then $\frac{\sin i}{\sin r} = constant$ .

(ii) Incident ray, refracted ray, normal drawn to the interface, at the point of incidence, lie in a same plane.

3. Spherical Mirrors:

(i) A mirror, which is polished from the outer side of a hollow sphere, such that the reflecting side is towards hollow side, is called a concave mirror.

(ii) A mirror which is polished from the hollow side of the sphere, such that the reflecting surface is towards bulging side, is called a convex mirror.

(iii) The focal length $(f)$ of a spherical mirror is half of the radius of curvature $(R)$ . $f = \frac{R}{2}$

(iv) The mirror formula for spherical mirrors is $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$ and gives the relationship between object distance $u$ , image distance $v$ and the focal length $f$ .

(v) The magnification produced by a spherical mirror, $m = - \frac{v}{u}$ .

4. New Cartesian Sign Conventions:

In this convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis of the coordinate system. The conventions are as follows –

(i) The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.

(ii) All distances parallel to the principal axis are measured from the pole of the mirror.

(iii) All the distances measured to the right of the origin (along $+$ x-axis) are taken as positive while those measured to the left of the origin (along $-$ x-axis) are taken as negative.

(iv) Distances measured perpendicular to and above the principal axis (along $+$ y-axis) are taken as positive.

(v) Distances measured perpendicular to and below the principal axis (along $-$ y-axis) are taken as negative.

5. Refraction of Light:

(i) The phenomenon due to which a ray of light deviates from its path at the

of the separation of two media, when the ray of light is travelling from one optical medium to another optical medium is called refraction of light.

(ii) The rays of light while travelling from rarer to denser medium bend towards the normal drawn at the point of incidence. The rays travelling from a denser to a rarer medium, bend away from the normal.

(iii) Refractive index of a medium, $μ = \frac{Speed of light in vacuum}{Speed of light in the medium}$ .

(iv) Relative refractive index of medium $2$ with respect to medium $1,$ $μ_{2,1} = \frac{Speed of light in medium 1}{Speed of light in medium 2}$ .

(v) In a rectangular glass slab, when light is passing through two opposite faces, angle of incidence is always equal to that angle of the emergence.

6. Refraction by Spherical Lenses:

(i) Lens formula for spherical lens is $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ and gives relationship between object distance $u$ , image distance $v$ and the focal length $f$ .

(ii) Magnification produced by a lens, $m = \frac{v}{u}$ .

7. Human Eyes and Defects of Vision:

(i) The process by which the ciliary muscles alter focal length of crystalline lens of the eye, so as to focus near or far-off objects clearly on the retina is called accommodation of eye.

(ii) The condition of the eye, when it can see clearly the objects at the point of distinct vision but cannot see far-off objects is called myopia or short-sightedness.

(iii) Short-sightedness can be corrected by using a concave lens of appropriate focal length.

(iv) The condition of the eye, when it can see clearly the far-off objects, but cannot see clearly the objects at the point of distinct vision is called hypermetropia or long-sightedness.

(v) Long-sightedness can be corrected by using a convex lens of appropriate focal length.

(vi) The condition of the eye, when it can neither see clearly the objects situated at the point of distinct vision or the far-off objects is called Presbyopia.

(vii) Presbyopia can be corrected by using bifocal lens. Convex lens for long-sightedness and concave lens for short-sightedness.

8. Refraction of Light Through a Prism:

When refraction of light takes place through a glass prism then $∠ i + ∠ e = ∠ A + ∠ δ$

Where,

$∠ i$ is angle of incidence
$∠ e$ is angle of emergence
$∠ A$ is angle of prism
$∠ δ$ is angle of deviation.

9. Dispersion of Light:

(i) The phenomenon due to which white light splits into its component colours is called dispersion of light.

(ii) A band of seven colours obtained on the white screen, when white light splits into its component colours is called spectrum.

(ii) Rainbow is produced just after the rain, due to the dispersion of sunlight by tiny droplets of water suspended in air.

10. Atmospheric Refraction:

The stars twinkling or the early sunrise and late sunset are due to atmospheric refraction.

11. Scattering of Light:

(i) The phenomenon due to which a wavelength of light on striking some particle larger in diameter than its own wavelength, first gets absorbed and then radiated out in all possible directions is called scattering.

(ii) The scattering of light by the atmosphere causes blue colour of sky and reddening of sun during sunrise or sunset.

lens is used in a camera.

Important Questions on Light

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