LIGHT, CBSE CLASS X, PHYSICS NOTES PART II

 

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

According to the CBSE Syllabus 2025-26

CBSE Class 10 Science Chapter 10  Light Notes

Remaining Topics of this Chapter  

3. Reflection.

  -Laws of reflection. 

  -Virtual and real image.                                                                               

4. Image formed by a plane mirror.

 -Characteristics of the image formed by a plane mirror. 

   -Lateral inversion and its application. 

5. Spherical mirrors.  

   -Properties of a concave mirror. 

   -Properties of a convex mirror.

    -A common term for spherical mirrors.                                                 

6. Rules for making ray diagrams by spherical mirrors.

7. Ray diagrams for images formed by a concave mirror.

  -When an object is at infinity. 

  -When an object is beyond C.

  -When an object is at C. 

  -When an object is placed between F and C.

  -When an object is placed at F.

   -When an object is between P and F.

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.

REFLECTION

Bouncing back of light when it strikes a polished surface like a mirror.

LAWS OF REFLECTION

(i) The angle of incidence is equal to the angle of reflection.
(ii) The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

 VIRTUAL AND REAL IMAGE

An image is a point where at least two light rays actually meet or appear to meet.

 REAL IMAGE

 a. Formed when light rays actually meet.

b. Can be obtained on screen.

c. Inverted.

Example: image formed on cinema screen and formed by a concave mirror.

VIRTUAL IMAGE

a. Formed when light rays appear to meet.
b. Can’t be obtained on screen.
c. Erect
Example: an image formed by a plane mirror or a convex mirror.

 IMAGE FORMED BY A PLANE MIRROR
CHARACTERISTICS OF IMAGE FORMED BY PLANE MIRROR

(i)  Virtual and erect.
(ii) The size of the image is equal to the size of the object.
(iii) The image is formed as far behind the mirror as the object is in front of it.
(iv) Laterally inverted.
Lateral Inversion: The right side of the object appears left side of the image and vice-versa.

APPLICATION OF LATERAL INVERSION

The word AMBULANCE is written in reverse direction so that it can be read correctly in the rear view mirror of vehicles going in front of it.

SPHERICAL MIRRORS

Mirrors whose reflecting surface is curved. There are two types of spherical mirrors:
(i) Convex Mirror
(ii) Concave Mirror

PROPERTIES OF A CONCAVE MIRROR

a. The reflecting surface is curved inwards.  
b. Converging mirror 

PROPERTIES OF A CONVEX MIRROR

 a. The reflecting surface is curved outwards.
b. Diverging mirror
 

LIGHT INCIDENT ON THE SURFACE SEPARATING TWO MEDIA

When light travels from one medium to another medium, it either:
a. gets absorbed (absorption)
b. bounces back (reflection)
c. passes through or bends (refraction)
When light is incident on a plane mirror, most of it gets reflected, and some of it gets absorbed in the medium.

CHARACTERISTICS OF LIGHT

a. The speed of light is given as c=λμ, where λ is its wavelength and μ is its frequency.
b. The speed of light is a constant, which is 2.998×108m/s or approximately 3.0×108m/s.

REFLECTION OF LIGHT BY OTHER MEDIA

A medium that is polished well without any irregularities on its surface will cause regular reflection of light. For example, a plane mirror. But even then, some light gets absorbed by the surface.

LAWS OF REFLECTION

a. The incident ray, reflected ray, and the normal all lie in the same plane. 
b.  Angle of incidence = Angle of reflection. [∠i=∠r]

PROPAGATION OF LIGHT

Rectilinear propagation of light: Light travels in a straight line between any two points.

FERMAT’S PRINCIPLE

a. The principle of least time: Light always takes the quickest path between any two points (which may not be the shortest path).
b. Rectilinear propagation of light and the law of reflection  [∠i =∠r] can be validated by Fermat’s principle of least time.

APPLICATIONS OF FERMAT’S PRINCIPLE

We can make several observations as a result of Fermat’s Principle, which will prove useful as we explore the realm of geometric optics:
a. In a homogeneous medium, light rays are rectilinear. That is, in any medium where the index of refraction is constant, light travels in a straight line.
b. The angle of reflection of a surface is equal to the angle of incidence. This is the Law of Reflection.

 EXAMPLE OF FERMAT’S PRINCIPLE

Mirage is an example of this phenomenon. Sometimes, we feel like we are seeing water on the road, but when we get there, the road is dry. What we really witness is the light of the sky, which is reflected on the road. Since the air is very hot just above the road, it is cooler up higher. Hot air expands more than cool air and is thinner,  which leads to a smaller decrease in the speed of light.

PLANE MIRROR

Any flat and polished surface that has almost no irregularities on its surface that reflect light is called a plane mirror.

IMAGE FORMATION BY A PLANE MIRROR

a. The image formed by a plane mirror is always virtual and erect.
b. The object and image are equidistant from the mirror.

CHARACTERISTICS OF IMAGES

a. Images can be real or virtual, erect or inverted, magnified or diminished. A real image is formed by the actual convergence of light rays. A virtual image is an apparent convergence of diverging light rays.
b. If an image formed is upside down, then it is called inverted or otherwise, it is an erect image. If the image formed is bigger than the object, then it is called magnified. If the image formed is smaller than the object, then it is diminished.

PRINCIPLE OF REVERSIBILITY OF LIGHT

If the direction of a ray of light is reversed due to reflection off a surface, then it will retrace its path.

SPHERICAL MIRROR

Consider a hollow sphere with a very smooth and polished inside surface and an outer surface with a coating of mercury so that no light can come out. Then, if we cut a thin slice out of the shell, we get a curved mirror, which is called a spherical mirror.

IMPORTANT TERMS RELATED TO SPHERICAL MIRROR

Principal axis: The line passing through the pole and the centre of curvature is the main or principal axis.                                                                                    

Pole (P): The centre of the spherical mirror.                                    

Aperture (MN):  It is the effective diameter of the spherical mirror.          

Center of Curvature (C): The centre of the hollow glass sphere of which the mirror was a part.                                  

Radius of Curvature (R): The distance between the pole and the centre of curvature. This radius will intersect the mirror at the pole (P).                   

Focus (F): The point on the principal axis where all the parallel light rays actually meet or appear to meet after reflection. Take a concave mirror. All rays parallel to the principal axis converge at a point between the pole and the centre of curvature. This point is called the focal point or focus. 

Focal length (f): The distance between the pole and the focus.   

Relationship between focal length and radius of curvature: f = R/2                     

Concave Mirror: A spherical mirror with a reflecting surface that bulges inwards.    

Convex Mirror: A spherical mirror with a reflecting surface that bulges outwards.

 RULES FOR MAKING RAY DIAGRAMS BY SPHERICAL MIRROR

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

RELATIONSHIP BETWEEN FOCUS AND RADIUS OF CURVATURE

Focal length is half the distance between the pole and the radius of curvature.

F = R/2

CURVED MIRROR

A mirror (or any polished, reflective surface) with a curvature is known as a curved mirror.

RULES OF RAY DIAGRAM FOR REPRESENTATION OF IMAGES FORMED

a. A ray passing through the centre of curvature hits the concave spherical mirror and retraces its path.
b. Rays parallel to the principal axis pass through the focal point or focus.

 RAY DIAGRAMS FOR IMAGES FORMED BY A CONCAVE MIRROR

(i) When object is at infinity

Image Position − At ‘F’
Nature of image – Real, inverted
Size – Point-sized or highly diminished

 (ii) When the object is beyond ‘C’

Image Position – Between ‘F’ and ‘C’
Nature of image – Real, inverted
Size – Diminished

 (iii) When the object is at ‘C’

Image Position – At ‘C’
Nature of image – Real, inverted
Size – Same size as that of the object

(iv) When object is placed between ‘F’ and ‘C’

Image Position – Beyond ‘C’
Nature of image– Real, inverted
Size – Enlarged

(v) When object is placed at ‘F’

Image Position – At Infinity
Nature of image – Real, inverted
Size – Highly enlarged

 (vi) When the object is between ‘P’ and ‘F’

Image Position – Behind the mirror
Nature of image – Virtual, erect
Size – Enlarged

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