CBSE Class 10 Physics Revision Notes for Light - Reflection and Refraction
In CBSE Class 10, Physics is a combination of theoretical and practical knowledge. Physics can be one of the toughest subjects to understand. In the CBSE Class 10 Physics syllabus, there are crucial topics such as electricity, magnetic effects of electric current, refraction of light, etc. Through the Physical Practical Class 10 CBSE sessions, you dive deeper into how things work. TopperLearning presents study materials for CBSE Class 10 Physics which will help you to effectively prepare for your final examination. Our Physics study materials are prepared by subject experts and include video lessons, revision notes, question banks, sample papers and past years' question papers.
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Light – Reflection and Refraction
Reflection of Light
- Reflection is the phenomenon of bouncing back of light into the same medium on striking the surface of any object.
- Laws of Reflection
- First law: The incident ray, the normal to the surface at the point of incidence and the reflected ray, all lie in the same plane. o
- Second law: The angle of reflection (r) is always equal to the angle of incidence (i). ir
- The image formed by a plane mirror is always
- virtual and erect
- of the same size as the object
- as far behind the mirror as the object is in front of it
- laterally inverted
- Spherical mirrors are of two types:
- Convex mirrors or diverging mirrors in which the reflecting surface is curved outwards.
- Concave mirrors or converging mirrors in which the reflecting surface is curved inwards.
- Some terms related to spherical mirrors:
- The centre of curvature (C) of a spherical mirror is the centre of the hollow sphere of glass, of which the spherical mirror is a part.
- The radius of curvature (R) of a spherical mirror is the radius of the hollow sphere of glass, of which the spherical mirror is a part.
- The pole (P) of a spherical mirror is the centre of the mirror.
- The principal axis of a spherical mirror is a straight line passing through the centre of curvature C and pole P of the spherical mirror.
- The principal focus (F) of a concave mirror is a point on the principal axis at which the rays of light incident on the mirror, in a direction parallel to the principal axis, actually meet after reflection from the mirror.
- The principal focus (F) of a convex mirror is a point on the principal axis from which the rays of light incident on the mirror, in a direction parallel to the principal axis, appear to diverge after reflection from the mirror.
- The focal length (f) of a mirror is the distance between its pole (P) and principal focus (F).
- For spherical mirrors of small aperture, R = 2f.
- Sign Conventions for Spherical Mirrors
According to New Cartesian Sign Conventions,
- All distances are measured from the pole of the mirror.
- The distances measured in the direction of incidence of light are taken as positive and vice versa.
- The heights above the principal axis are taken as positive and vice versa.
- Rules for tracing images formed by spherical mirrors
Rule 1: A ray which is parallel to the principal axis after reflection passes through the principal focus in case of a concave mirror or appears to diverge from the principal focus in case of a convex mirror.
Rule 2: 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 emerges parallel to the principal axis after reflection.
Rule 3: A ray passing through the centre of curvature of a concave mirror or directed towards the centre of curvature of a convex mirror is reflected back along the same path.
- Image formation by a concave mirror
- Ray Diagrams
- Characteristics of images formed
| Position of object | Position of image | Size of image | Nature of image |
| At infinity | At focus F | Highly diminished | Real and inverted |
| Beyond C | Between F and C | Diminished | Real and inverted |
| At C | At C | Equal to size of object | Real and inverted |
| Between C and F | Beyond C | Enlarged | Real and inverted |
| At F | At infinity | Highly enlarged | Real and inverted |
| Between F and P | Behind the mirror | Enlarged | Virtual and erect |
- Image formation by a convex mirror
- Ray Diagrams
- Characteristics of images formed
| Position of object | Position of image | Size of image | Nature of image |
| At infinity | At focus F behind the mirror | Highly diminished, point sized | Virtual and erect |
| Anywhere between infinity and the pole of the mirror | Between P and F behind the mirror | Diminished | Virtual and erect |
- Mirror Formula
The object distance (u), image distance (v) and focal length (f) of a spherical mirror are related as
- Linear Magnification (m) produced by a spherical mirror is
m is negative for real images and positive for virtual images.
- The phenomenon of change in the path of a beam of light as it passes from one medium to another is called refraction of light.
- The cause of refraction is the change in the speed of light as it goes from one medium to another.
- Laws of Refraction
- First Law: The incident ray, the refracted ray and the normal to the interface of two media at the point of incidence, all lie in the same plane.
- Second Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media.
This law is also known as Snell’s law.
The constant, written as 1n2 is called the refractive index of the second medium (in which the refracted ray lies) with respect to the first medium (in which the incident ray lies).
- Absolute refractive index (n) of a medium is given as
- When a beam of light passes from medium 1 to medium 2, the refractive index of medium 2 with respect to medium 1 is called the relative refractive index, represented by 1n2 , where
Similarly, the refractive index of medium 1 with respect to medium 2 is
- While going from a rarer to a denser medium, the ray of light bends towards the normal.
- While going from a denser to a rarer medium, the ray of light bends away from the normal.
- Conditions for no refraction
- When light is incident normally on a boundary.
- When the refractive indices of the two media are equal.
- In the case of a rectangular glass slab, a ray of light suffers two refractions, one at the air–glass interface and the other at the glass–air interface. The emergent ray is parallel to the direction of the incident ray.
- Convex lens or converging lens which is thick at the centre and thin at the edges.
- Concave lens or diverging lens which is thin at the centre and thick at the edges.
- Some terms related to spherical lenses:
- The central point of the lens is known as its optical centre (O).
- Each of the two spherical surfaces of a lens forms a part of a sphere. The centres of these spheres are called centres of curvature of the lens. These are represented as C1 and C2.
- The principal axis of a lens is a straight line passing through its two centres of curvature.
- The principal focus of a convex lens is a point on its principal axis to which light rays parallel to the principal axis converge after passing through the lens.
- The principal focus of a concave lens is a point on its principal axis from which light rays, originally parallel to the principal axis appear to diverge after passing through the lens.
- The focal length (f) of a lens is the distance of the principal focus from the optical centre.
- Sign Conventions for Spherical Lenses
According to New Cartesian Sign Conventions, - All distances are measured from the optical centre of the lens.
- The distances measured in the direction of incidence of light are taken as positive and vice versa.
- The heights above the principal axis are taken as positive and vice versa.
- Rules for tracing images formed by spherical lens
Rule 1: A ray which is parallel to the principal axis, after refraction passes through the principal focus on the other side of the lens in case of a convex lens or appears to diverge from the principal focus on the same side of the lens in case of a concave lens.
- Image formation by a convex lens
- Ray Diagrams
- Characteristics of images formed
| Position of object | Position of image | Size of image | Nature of image |
| At infinity | At focus F1 | Highly diminished | Real and inverted |
| Beyond 2F1 | Between F2 and 2F2 | Diminished | Real and inverted |
| At 2F1 | At 2F2 | Equal to size of object | Real and inverted |
| Between F1 and 2F1 | Beyond 2F2 | Enlarged | Real and inverted |
| At focus F1 | At infinity | Highly enlarged | Real and inverted |
| Between F1 and o |
Beyond F1 on the same side as the object |
Enlarged | Virtual and erect |
- Image formation by a concave lens
- Ray Diagrams
- Characteristics of images formed
| Position of object | Position of image | Size of image | Nature of image |
| At infinity | At focus F1 | Highly diminished | Virtual and erect |
| Between infinity and O | Between focus F1 and O | Diminished | Virtual and erect |
- Lens Formula Object distance (u), image distance (v) and focal length (f) of a spherical lens are related as
- Linear Magnification (m) produced by a spherical lens is
m is negative for real images and positive for virtual images.
- Power of a lens
- Power of a lens is the reciprocal of the focal length of the lens. Its S.I. unit is dioptre (D).
P (dioptre) f metre - Power of a convex lens is positive and that of a concave lens is negative.
- When several thin lenses are placed in contact with one another, the power of the combination of lenses is equal to the algebraic sum of the powers of the individual lenses. P = P1 + P2 + P3 + P4 +..........
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