Vidyarthi Academy

Home NCERT Solutions Chapter Notes Test Papers Contact Us

CBSE NOTES CLASS 10 SCIENCE CHAPTER 10

LIGHT, REFLECTION AND REFRACTION

Characteristics of Light

Reflection Of Light:

Laws of Reflection

Plane Mirror

Spherical Mirror

Concave Mirror

Convex Mirror

Centre of Curvature

Radius of Curvature

Pole of a Mirror

Principal Axis

Focus

Focal Length

Sign Convention for Spherical Mirrors

Types of rays for mirrors

Position, Size And Nature Of Image By A Concave Mirror

Position, Size And Nature Of Image By convex mirror

Linear Magnification due to Mirros

Uses of concave mirrors

Uses of convex mirrors

Refraction Of Light

Laws of Refraction

Snell’s law

Refractive Index

Optical density

Lens

Convex Lens

Concave Lens

Important rays through a lens

Nature, position and relative size of the image formed by a convex lens for various positions of the object

Nature, position and relative size of the image formed by a concave lens for various positions of the object

Lens Formula

Magnification (m) due to lens

Power of a Lens

Focal Length of a Lens Combination

CBSE NOTES CLASS 10 SCIENCE CHATER 10

LIGHT, REFLECTION AND REFRACTION

Characteristics of Light

Light waves are electromagnetic waves, whose nature is transverse. The speed of light is different in different media. In vacuum it is 3×108 m.

The speed and wavelength of light change when it travels from one medium to another; but its frequency remains unchanged.

Important Terms

Luminous Objects -The objects which emits its own light, are called luminous objects, e.g., sun, other stars, an oil lamp etc.

Non-Luminous Objects: The objects which do not emit its own light but become visible due to the reflection of light falling on them, are called non-luminous objects, e.g., moon, table, chair, trees etc.

Ray of Light: A straight line drawn in the direction of propagation of light is called a ray of light.

Beam of Light: A bundle of the adjacent light rays is called a beam of light.

Vidyarthi Academy

Image: If light ray coming from an object meets or appear to meet at a point after reflection or refraction, then this point is called image of the object

Real Image: The image obtained by the real meeting of light rays, is called a real mage. Real image can be obtained on a screen. Real image is inverted.

Virtual Image: The image obtained when light rays are not really meeting but appears to meet only, is called a virtual image.

Reflection Of Light:

The bouncing back of light rays into the same medium on striking a highly polished surface such as a mirror is called reflection of light.

Vidyarthi Academy

Laws of Reflection

(i) The incident ray, the reflected ray and the normal at the point of incidence all three lie in the same plane.

(ii) The angle of incidence (i) is always equal to the angle of reflection (r).

TYPES OF REFLECTION

Regular Reflection

When a parallel beam of reflected light rays is obtained for a parallel beam of incident light rays after reflection from a plane reflecting reflection is called regular reflection.

Irregular or Diffused Reflection

When a non-parallel beam of reflected light rays is obtained for a parallel beam of incident light rays after reflection from a surface, then such type of reflection is called irregular or diffused reflection

Vidyarthi Academy

Vidyarthi Academy

Mirror: A smooth and highly polished reflecting surface is called a mirror.

Plane Mirror: A highly polished plane surface is called a plane mirror.

Properties of image by plane mirror

Spherical Mirror

A highly polished curved surface whose reflecting surface is a cut part of a hollow glass sphere is called a spherical mirror. Spherical mirrors are of two types

(a) Concave Mirror: A spherical mirror whose bent in surface is reflecting surface, is called a concave mirror.

Image result for spherical mirrors

(b) Convex Mirror: A spherical mirror whose bulging out surface is reflecting surface, is called a convex mirror.

Image result for spherical mirrors

Some Terms Related to Spherical Mirrors

(i) Centre of Curvature (C): It is the centre of the sphere of which the mirror or lens is a part (C). The line joining any point on the mirror to C is normal to the mirror.

(ii) Radius of Curvature (R): The radius of the hollow sphere of which the mirror is a part, is called radius of curvature.

(iii) Pole (P): The central point of the spherical mirror is called its pole (P).

(iv) Principal Axis: The straight line passing through the pole and the centre of curvature of a spherical mirror is called the principal axis.

(v) Focus (F):

A parallel beam of light rays incident on a concave mirror after reflection converges at a point on the principal axis. This point is called principal focus of the concave mirror.

A parallel beam of light rays incident on a convex mirror after reflection appears to diverge from a point on the principal axis. This point is called principal focus of the convex mirror.

Vidyarthi Academy

(vi) Focal Length: The distance between the pole and focus is called focal length (f). Relation between focal length and radius of curvature is given by

f =R2

Sign Convention for Spherical Mirrors

Vidyarthi Academy

  1. All distances are measured from the pole of the mirror.

  2. Distances measured in the direction of incident light rays are taken as positive.

  3. Distances measured in opposite direction to the incident light rays are taken as negative.

  4. Distances measured above the principal axis are positive.

  5. Distances measured below the principal axis are negative.

The focal length of concave mirror is taken negative and for a convex mirror taken as positive

Types of rays for mirrors

(i) 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.

Vidyarthi Academy

(ii) 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.

Vidyarthi Academy

(iii) A ray 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.

Vidyarthi Academy

(iv) A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis.

Vidyarthi Academy

POSITION, SIZE AND NATURE OF IMAGE

By a concave mirror

Vidyarthi Academy

Vidyarthi Academy

Vidyarthi Academy

Position of the object

Position of the image

Size of the image

Nature of the image

At infinity

At the focus F

Highly diminished, point-sized

Real and inverted

Beyond C

Between F and C

Diminished

Real and inverted

At C

At C

Same size

Real and inverted

Between C and F

Beyond C

Enlarged

Real and inverted

At F

At infinity

Highly enlarged

Real and inverted

Between P and F

Behind the mirror

Enlarged

Virtual and erect

By convex mirror

Position of the Object

Position of the image

Size of the image

Nature of the image

At infinity

At the focus F, behind the mirror

Highly diminished, point-sized

Virtual and erect

Between infinity and the pole P

Between P and F, behind the mirror

Diminished

Virtual and erect

Vidyarthi Academy

Vidyarthi Academy

Linear Magnification

The ratio of height of image (h’) formed by a mirror to the height of the object (h) is called linear magnification (m).

Linear magnification (m) = hh

In triangles A′B′P and ABP, we have,

BABA= BPBP

With the sign convention, this becomes

-h'h =  v-u       m=h'h= -vu

Uses of concave mirrors

Uses of convex mirrors

Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving. Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

Refraction Of Light

The deviation or bending of a light ray from its path when it travels from one transparent medium to another transparent medium is called refraction of light.

Vidyarthi Academy

Cause of Refraction: The speed of light is different in different media, but the frequency remains same, hence the wavelength also changes.

Laws of Refraction

(i) The incident ray, the refracted ray and the normal at the point of incidence, all three lies in the same plane.

(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is constant for a pair of two media,

where n21 is called refractive index of second medium with respect to first medium.

This law is also called Snell’s law.

Refractive Index:

The ratio of speed of light in vacuum (c) to the speed of light in any medium (v) is called refractive index of the medium.

Refractive index of a medium

n = cv

The refractive index is maximum for violet colour of light and minimum for red colour of light. i.e., nv > nR.

Refractive index of water = 43 = 1.33;

Refractive index of glass = 32 = 1.50

Refractive index of second medium with respect to first medium,

n21 = Speed of light in medium 1Speed of light in medium 2 =v1v2  

Refractive index of first medium with respect to second medium,

n12 = Speed of light in medium 2Speed of light in medium 1 =v2v1

Obviously, n21 =1n12 

Optical Density

A medium with larger refractive index is called optically denser and a medium with smaller refractive index is called optically rarer medium.

If n21 > 1, r < i , i.e., the refracted ray bends towards the normal. In such a case medium 2 is said to be optically denser than medium 1.

If n21 <1, r > i, the refracted ray bends away from the normal. This is the case when incident ray in a denser medium refracts into a rarer medium.

Optical density and mass density are different. It is possible that mass density of an optically denser medium may be less than that of an optically rarer medium For example, turpentine and water. Mass density of turpentine is less than that of water but its optical density is higher.

Refraction through a Glass Slab - Lateral Shift

Vidyarthi Academy

For a rectangular slab, refraction takes place at two interfaces (air-glass and glass-air) and r2 = i1, i.e., the emergent ray is parallel to the incident ray - there is no deviation, but it does suffer lateral displacement/shift with respect to the incident ray.

Lens

A lens is a uniform transparent medium bounded between two spherical or one spherical and one plane surface.

Convex Lens: A lens which is thinner at edges and thicker at middle is called a convex or converging lens.

Vidyarthi Academy

Concave Lens: A lens which is thicker at edges and thinner at middle, is called a concave or diverging lens.

Vidyarthi Academy

The centre of the sphere is caslled centre of curvature of the lens.

Since there are two centres of curvature, we may represent them as C1 and C2.

An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.

A parallel beam of light rays incident on a convex lens after refraction converges a point on the principal axis. This point is called principal focus of the convex lens.

A parallel beam of light rays incident on a concave lens after refraction appears to diverge from a point on the principal axis. This point is called principal focus of the concave lens.

The central point of a lens is called its optical centre. It is represented by the letter O.

The effective diameter of the circular outline of a spherical lens is called its aperture.

We shall assume in our discussion that the aperture of lenses is much less than its radius of curvature. Such lenses are called thin lenses with small apertures.

Important rays through a lens

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

Vidyarthi Academy

(ii) A ray of light passing through the first principal focus (for a convex lens) or appearing to meet at it (for a concave lens) emerges parallel to the principal axis after refraction.

Vidyarthi Academy

(iii) A ray of light, passing through the optical centre of the lens, emerges without any deviation after refraction

Vidyarthi Academy

Nature, position and relative size of the image formed by a convex lens for various positions of the object

Vidyarthi Academy

Vidyarthi Academy

Vidyarthi Academy

Position of the Object

Position of the image

Size of the image

Nature of the image

At infinity

At focus F2

Highly diminished, point-sized

Real and inverted

Beyond 2F1

Between F2 and 2F2

Diminished

Real and inverted

At 2F1

At 2F2

Same size

Real and inverted

Between F1 and 2F1

Beyond 2F2

Enlarged

Real and inverted

At focus F1

At infinity

Infinitely large or highly enlarged

Real and inverted

Between focus F1 and optical centre O

On the same side of the lens as the object

Enlarged

Virtual and erect

Nature, position and relative size of the image formed by a concave lens for various positions of the object

Position of the Object

Position of

the image

Size of the image

Nature of the image

At infinity

At focus F1

Highly diminished, point-sized

Virtual and erect

Between infinity and optical centre O

Between focus F1 and optical centre O

Diminished

Virtual and erect

Vidyarthi Academy

Vidyarthi Academy

Lens Formula

1f=1v1u

For convex lens, f is +ve, u is –ve, v = for five cases +ve and for the last one it is –ve.

For concave lens f, u and v, all are –ve.

Magnification (m) produced by a lens is defined as the ratio of the size of the image to that of the object.

m = hh=vu

Power of a Lens

The power P of a lens is defined as the the reciprocal of the focal length of a lens, when it is measured in metre

or P = 1f

The SI unit for power of a lens is dioptre (D).

The power of a lens of focal length of 1 metre is one dioptre.

Power is positive for a converging lens and negative for a diverging lens.

Focal Length of a Lens Combination

1f= 1f1+1f2

 Linear magnification = hh=vu

Total magnification m of the combination is a product of magnification (m1, m2, m3,...) of individual lenses

m = m1 m2 m3 ...