Brief Notes: Chapter 10 - Lights Class 10
1. Light: Light is the radiant energy which produces in us the sensation of sight.
2. Rules of reflection of light:
(i) incident light, reflected light and perpendicular on the plane of reflection are in the same plane.
(ii) Angle of incidence 'i' is equal to the angle of reflection 'r'
3.Image Formed in Plane Mirror:
Image in a plane mirror is virtual and is at the same distance behind the mirror as that of the distance of object in front of mirror.
4. Cartesian sign conventions are used for spherical mirror and lens.
According to this convention -
- The object is always placed towards left.
- All distance are measured from the pole of the mirror. (from optical centre in case of lenses)
- Distances of the left of the mirror or lens are taken as negative and distances towards the right are taken as positive.
- The distances mesured above the principal axis are taken as positive and distances measured below the principal axis are taken as negative
Focal length and radius of curvature of a concave mirror are always negative while they are always positive for convex mirror. Similarly, the focal length of a concave lens is always taken as negative and focal length of convex lens is taken as positive.
5. In convex mirror, the image is always virtual, upright and simaller than the object. For a concave mirror the nature of image depends upon distance of object from the mirror.
6.Mirror Formula:
If object distance for a mirror is $u$, image distance is $\mathrm{v}$ and focal length is $\mathrm{f}$ then the morror formula is given as
$$\frac{1}{v}+ \frac{1}{u}=\frac{1}{f}$$
7.Magnification:
Ratio of height of image to the height of an object is called magnification.
8. Rules of refraction
(i) Incident ray, refracted ray and the perpendicular on the plane of refraction are in the same plane.
(ii) For a refracting medium the ratio of sine of angle of incidence 'i' and sine of angle of refraction ' $r$ ' is constant.
9.. When light rays travel through rarer medium to denser medium then at the plane of separation of both the mediums the rays bend towards the perpendicular after refraction. On the contrary when rays go from denser to rarer medium then after refraction the rays bend away from the perpendicular
10. Refractive Index
Ratio of speed of light in vacuum to the speed of light in a transparent medium is called refractive index of the transparent material.
11 Lense:
A refracting medium of two curved surfaces or one curved surface and one plane surface is called a lens.
12. Types of Lense
Lens are of two types.
(i) Convex lens or converging lens
(ii) Concave lens or diverging lens
13. Image formed in Lenses:
Image from a concave lens is always virtual, upright and smaller than the object. Image in a convex lens depends upon the distance of object from the lens.
14.Lens Formula:
If an object is at a distance $u$ from a lens, image is at distance $\mathrm{v}$ and focal length of lens is $\mathrm{f}$ then lens formula is given as
$$
\frac{1}{v}-\frac{1}{u}=\frac{1}{f}
$$
15 Power of the Lens.
Reciprocal of the focal length of a lens is known as the power of the lens. Its unit is diopter.