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.

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.

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

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

• Size of image = Size of object

• Magnification = Unity

• Distance of image = Distance of object

• A plane mirror may form a virtual as well as real image.

• A man may see his full image in a mirror of half height of man.

• When two plane mirror are held at an angle θ, the number of images of an object placed between them is given as below

(a) n = [(360°/θ) – 1 ], where 360°/θ is an integer.

(b) n = integral part of 360°/θ, when 360°/ θ is not an integer.

• An image formed by a plane mirror is virtual, erect, laterally inverted, of same size as that of object and at the same distance as the object from the mirror.

[A plane mirror may form a real image, when the pencil of light incident on the mirror is convergent. Children, during their play form an image of sun as wall by a strip of plane mirror.]

• Kaleidoscope and periscope employ the principle of image formation by plane mirror.

• If keeping an object fixed, a plane mirror is rotated in its plane by an angle θ, then the reflected ray rotates in the same direction by an angle 2θ.

• Focal length as well as radius of curvature of a plane mirror is infinity. Power of a plane mirror is zero.

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.

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

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.

(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

Sign Convention for Spherical Mirrors

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.

(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.

(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.

(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.

POSITION, SIZE AND NATURE OF IMAGE

By a concave mirror

 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

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) = $\frac{\mathrm{h}\mathrm{’}}{\mathrm{h}}$

In triangles A′B′P and ABP, we have,

With the sign convention, this becomes

Uses of concave mirrors

• Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light.

• They are used as shaving mirrors to see a larger image of the face.

• The dentists use concave mirrors to see large images of the teeth of patients.

• Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

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.

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,

n21 = $\frac{\mathrm{sin}\mathrm{i}}{\mathrm{sin}\mathrm{r}}$ = constant

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 = $\frac{\mathrm{c}}{\mathrm{v}}$

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 = $\frac{4}{3}$ = 1.33;

Refractive index of glass = $\frac{3}{2}$ = 1.50

Refractive index of second medium with respect to first medium,

Refractive index of first medium with respect to second medium,

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

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.

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

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.

(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.

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

Nature, position and relative size of the image formed by a convex 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 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

Lens Formula

$\frac{1}{\mathrm{f}}=\frac{1}{\mathrm{v}}–\frac{1}{\mathrm{u}}$

where,

f = focal length of the lens,

u = distance of object,

v = distance of image.

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.

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

$\mathbf{}\mathrm{or P =}\frac{1}{\mathrm{f}}$

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

1 D = 1m–1.

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

Power of the combination P = P1 + P2

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

m = m1 m2 m3 ...