Light Chapter Class 10

Reflection of Light: 

The process of bouncing back of the light rays which fall on the polished surface of an object is called reflection of light.

► Laws of Reflection: The laws of reflection of light are:

 1. The angle of reflection is equal to the angle of incidence i.e., Zr = Zi.

2. The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane. 

Object: Anything which gives out light rays is called an object i.e, a bulb, a candle etc. 

▸ Images: The point at which the reflected rays converge or the point from where the reflected rays appear to diverge is known as 'Image'. 

Images are of two types-real image and virtual image. 

Real Image 

The image which can be obtained on a screen is called real image

 while 

Virtual Image:-

The image which cannot be obtained on a screen is called a virtual image.

▸ Mirror: The mirror is a shiny polished surface which can reflect the rays of light. Mirrors can be plane (reflecting surface is plane) or spherical (reflecting surface is a part of hollow sphere).

Types of Spherical Mirrors:- Spherical mirrors can be of two types:

 (1) concave mirror (reflecting surface is curved inwards) and (2) convex mirror (reflecting surface is curved outwards).

► Characteristics of Image Formed by a Plane Mirror: Image formed by a plane mirror is:

 (i) virtual and erect.

(ii) laterally inverted.

(iii) of the same size as the object.

(iv) as far behind the mirror as the object is in front of it. 

▸ Special Rays for Formation of Image: 

The intersection of at least two reflected rays gives the position of image of the point object. Any two of the following rays can be considered for locating the image.

(1) A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.

(i) A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis. (i) Aray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path.

(iv) A ray incident obliquely to the principal axis, towards the pole of the mirror, on the concave mirror or a convex mirror is reflected obliquely.

▷ Uses of Concave Mirrors:

 (i) Used in torches, search-lights and vehicles headlights to get powerful parallel beams of light.

 (ii) Used as shaving mirrors to see a larger image of the face. (iii) Used by dentists to see large images of the teeth of patients.

(iv) Used to concentrate sunlight to produce heat in solar furnaces.

 Use of Convex Mirrors:

 Used as rear-view (wing) mirrors in vehicles because they always give an erect and diminished image. Also, they have a wider field of view as they are curved outwards.

 ▷ Mirror Formula:

It is a relation between distance of object, distance of image from the pole of the mirror and its focal length i.e., relation between 'u', 'v' and 'f' is given by

Mirror Formula Image

▸ Magnification or Linear Magnification: 

It is defined as the ratio of height of image to the height of the object. It is represented by 'm'. It is also related to the distance of object (u) and distance of image (v) i.e.,

Magnification (m)= Height of image (I),

Height of object (O)

Image distance(v)

Object distance (u)

Refraction: Bending of light ray when it passes from one medium to another is called refraction. When light travels from a rarer medium to a denser medium, it bends towards the normal (i>r) and when it travels from a denser medium to a rarer medium, it bends away from the normal (i<r),

where, i Angle of incidence, and r= Angle of refraction.

Refraction through a Rectangular Glass Slab: 

When a light ray is obliquely incident on a glass slab, then the emergent ray is parallel to the direction of incident ray, but it is shifted slightly sideward. Here, refraction takes place twice, first, when the ray enters the glass slab from air and second, when it exits from the glass slab to air. Both the cases of refraction have been shown in the given figure. The extent of bending of the ray of light at opposite parallel faces AB and CD of rectangular glass slab is equal and opposite. Hence, the ray emerging from face CD becomes parallel to the incident ray, but shifts slightly sideward.

Diagrams of Rectangular Glass slab

Refraction through a Rectangular Glass slab displacement

where, 

i Angle of incidence, 

r=Angle of refraction,

e=Angle of emergence.

Laws of Refraction: Refraction of light occurs according to the following laws:

1. The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane. 

2. The ratio of sine of angle of incidence to the sine of angle of refraction for light of a given colour is constant for a given pair of media (Snell's law). It is expressed as,

sini

sinr = Constant (u or n)

This constant is known as refractive index (u). It can also be

expressed as:

Speed of light in vacuum or air

Speed of light in medium Lens: The transparent refracting medium bounded by two surfaces in which at least one surface is curved, is called lens. Lenses are of two types: 

1. Convex lens is thicker at the centre and thinner at the edges.

2. Concave lens is thinner at the centre and thicker at the edges. Concave lens is a diverging lens (diverges the light) and convex Xens is a converging lens (converges the light).

Special Rays for Formation of Image: 

For drawing ray diagrams in following rays:

appears to diverge from the principal focus located on the same side of the lens.

lenses, alike of spherical mirrors, we consider any two of the 

(i) A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens. In case of a concave lens, the ray

(ii) A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis.

(iii) A ray of light passing through the optical centre of a lens will emerge without any deviation.