LIGHT, CBSE CLASS X, PHYSICS NOTES PART III

 

CBSE CLASS 10, LIGHT, PHYSICS NOTES-(PART III)
LIGHT

According to the CBSE Syllabus 2025-26

CBSE Class 10 Science Chapter 10  Light Notes

Remaining Topics of this Chapter  

8. Uses of a concave mirror. 

9. Ray diagram of the image formed by the convex mirror.

  -When an object is placed at infinity.

 -When an object is placed between the pole and infinity.

10. Uses of a convex mirror.

11. Sign convention for reflection by a spherical mirror.

12. Mirror formulas. 

   -Magnification of a spherical mirror.

USES OF CONCAVE MIRROR

Concave and Convex mirrors are used for many daily purposes.

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

(ii) Concave mirrors are used by dentists to see large image of teeth of patients. (Teeth have to be placed between pole and focus).

(iii) Concave mirror is used as shaving mirror to see a larger image of the face.

(iv) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnace.

 

Ray diagrams of images formed by convex mirror

(i)  When object is placed at infinity

Image Position − At ‘F’

Nature of image – Virtual, erect

Size – Point sized

 

(ii) When object is placed between pole and infinity

 

Image Position – Between ‘P’ and ‘F’

Nature of image– Virtual, erect

Size – Diminished

A full length image of a tall building/tree can be seen in a small convex mirror.

 

USES OF CONVEX MIRROR

(i) Convex mirrors are used as rear view mirrors in vehicles because
a. they always give an erect though diminished image.
b. they have a wider field of view as they are curved outwards.
(ii) Convex mirrors are used at blind turns and on points of merging traffic to facilitate vision of both side traffic.
(iii) Used in shops as security mirror.

 

SIGN CONVENTION FOR REFLECTION BY SPHERICAL MIRROR

(i) The object is placed to the left of the mirror.
(ii) All distances parallel to the principal axis are measured from the pole of the mirror.
(iii) All distances measured in the direction of incident ray (along + X-axis) are taken as positive and those measured against the direction of incident ray (along – X-axis) are taken as negative.
(iv) Distance measured perpendicular to and above the principal axis are taken as positive.
(v) Distances measured perpendicular to and below the principal axis are taken as negative.
• Object distance = ‘u’ is always negative.

• Focal length of concave mirror = Negative

• Focal length of convex mirror = Positive

 

Mirror Formula

 

1/v + 1/u = 1/f

where, v = Image distance

u = Object distance

f = Focal length

 

MAGNIFICATION OF SPHERICAL MIRRORS

It is the ratio of the height of image to the height of object.

m = Height of image/Height of object

 m = h_i/h_o

Also, m = -v/u

 If ‘m’ is negative, image is real.

 If ‘m’ is positive, image is virtual.

 If h_i = h_o then m = 1, i.e., image is equal to object.

 If h_i > h_o then m > 1 i.e., image is enlarged.

 If h_i < h_o then m < 1 i.e., image is diminished.

• Magnification of plane mirror is always + 1.

‘+’ sign indicates virtual image.

‘1’ indicates that image is equal to object’s size.

• If ‘m’ is ‘+ve’ and less than 1, it is a convex mirror.

• If ‘m’ is ‘+ve’ and more than 1, it is a concave mirror.

• If ‘m’ is ‘-ve’, it is a concave mirror.

MIRROR FORMULA AND MAGNIFICATION

1/v + 1/u = 1/f

 

Where ‘u’ is object distance, ‘v’ is the image distance and ‘f’ is the focal length of the spherical mirror, which is found by the similarity of triangles.

a. The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object. It is usually represented as ‘m’.

SIGN CONVENTION FOR RAY DIAGRAM

Distances measured towards positive x and y axes (coordinate system) are positive, and towards negative, x and y-axes are negative. Keep in mind the origin is the pole (P). Usually, the height of the object is taken as positive as it is above the principal axis, and the height of the image is taken as negative as it is below the principal axis.

POSITION AND SIZE OF IMAGE FORMED

The size of the image can be found using the magnification formula m = h’/h = – (v/u). If m is -ve it is a real image and if it is +ve it is a virtual image.

REFRACTION THROUGH A GLASS SLAB AND REFRACTIVE INDEX

REFRACTION

The shortest path need not be the quickest path. Since light is always in a hurry, it bends when it enters a different medium as it is still following the quickest path. This phenomenon of light bending in a different medium is called refraction.

LAWS OF REFRACTION

a. 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.

b. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

Absolute and Relative Refractive Index

The refractive index of one medium with respect to another medium is called the relative refractive index. When taken with respect to a vacuum, it’s known as an absolute refractive index.

REFRACTION THROUGH A RECTANGULAR GLASS SLAB

When the light is incident on a rectangular glass slab, it emerges out parallel to the incident ray and is laterally displaced. It moves from rarer to a denser medium and then again to a rarer medium.

REFRACTION AT A PLANAR SURFACE

Following Snell’s Law:

a. Light bends towards the normal when moving from rarer to denser medium at the surface of the two media.

b. Light bends away from the normal when moving from denser to rarer medium at the surface of contact of the two media.

REFRACTIVE INDEX

The extent to which light bends when moving from one medium to another is called the refractive index. This depends on the ratio of the speeds in the two media. The greater the ratio, the more the bending. It is also the ratio of the sine of the angle of incidence and the sine of the angle of refraction, which is a constant for any given pair of media. It is denoted by:

n = sin∠i/sin∠r = speed of light in medium 1/speed of light in medium2

The ratio of the speed of light in a vacuum to the speed of monochromatic light in the substance of interest is known as the relative refractive index. Mathematically, it is represented as:

n = c/v

Where n is the refractive index of a medium, c is the velocity of light in a vacuum and v is the velocity of light in that particular medium.

TOTAL INTERNAL REFLECTION

a. When the light goes from a denser to a rarer medium, it bends away from the normal. The angle at which the incident ray causes the refracted ray to go along the surface of the two media parallelly is called the critical angle.

 

b. When the incident angle is greater than the critical angle, it reflects inside the denser medium instead of refracting. This phenomenon is known as Total Internal Reflection.
E.g. mirages, and optical fibres.

SPHERICAL LENS

REFRACTION AT CURVED SURFACES

When light is incident on a curved surface and passes through, the laws of refraction still hold true, for example, lenses.

SPHERICAL LENSES

Spherical lenses are lenses formed by binding two spherical transparent surfaces together. Spherical lenses formed by binding two spherical surfaces bulging outward are known as convex lenses while spherical lenses formed by binding two spherical surfaces such that they are curved inward are known as concave lenses.

IMPORTANT TERMS RELATED TO SPHERICAL LENSES

a. Pole (P): The midpoint or the symmetric centre of a spherical lens is known as its Optical Centre. It is also called the pole.

b. Principal Axis: The line passing through the optical centre and the centre of curvature.

c. Paraxial Ray: A ray close to the principal axis and also parallel to it.

d. Centre of curvature (C): The centres of the spheres that the spherical lens was a part of. A spherical lens has two centres of curvature.

e. Focus (F): It is the point on the axis of a lens to which parallel rays of light converge or from which they appear to diverge after refraction.

f. Focal length: Distance between optical centre and focus.

g. Concave lens: Diverging lens

h. Convex lens: Converging lens

RULES OF RAY DIAGRAM FOR REPRESENTATION OF IMAGES FORMED

a. A ray of light parallel to the principal axis passes/appears to pass through the focus.

b. A ray passing through the optical centre undergoes zero deviation.

IMAGE FORMATION BY SPHERICAL LENSES

The following table shows image formation by a convex lens.

 

 

 

 

 

S. No.	POSITION OF THE OBJECT	POSITION OF THE IMAGE	RELATIVE SIZE OF THE IMAGE	NATURE OF THE IMAGE
1	At Infinity	At Focus f	Highly diminished point sized	Real and inverted
2	Beyond 2f	Between f and 2f	Duminished	Real and inverted
3	At 2f	At 2f	Same Size	Real and inverted
4	Between f and 2f	Beyond 2f	Enlarged	Real and inverted
5	At focus f	At infinity	Infinitely large or highly enlarged	Real and inverted
6	Between focus f and optical centre O	On the same side of the lens as the object	Enlarged	Virtual and erect

 

LENS FORMULA, MAGNIFICATION AND POWER OF LENS

LENS FORMULA AND MAGNIFICATION

Lens formula: 1/v = 1/u = 1/f, gives the relationship between the object distance (u), image distance (v), and the focal length (f) of a spherical lens.

USES OF SPHERICAL LENS

Applications such as visual aids: spectacles, binoculars, magnifying lenses, and telescopes.

POWER OF A LENS

The power of a lens is the reciprocal of its focal length i.e. 1/f (in metre). The SI unit of power of a lens is dioptre (D).