NCERT Solutions Class 10th Science Chapter – 9 Light Reflection and Refraction Notes

NCERT Solutions Class 10th Science Chapter – 9 Light Reflection and Refraction

NCERT Solutions Class 10th Science Chapter – 9 Light Reflection and Refraction

Chapter – 9

Light Reflection and Refraction

Notes

Laws of Reflection (1) Angle of incidence is equal to the angle of reflection. (2) The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Spherical Mirrors : Mirrors whose reflecting surface is curved. • Convex Mirror • Concave Mirror • Reflecting surface is curved outwards. • Reflecting surface is curved inwards. • Diverging mirror • Converging mirror Principal axis – The line joining the pole and center of curvature. 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. Focus (F) – The point on principal axis where all the parallel light rays actually meet or appear to meet after reflection. Focal length (f) – The distance between the pole and the focus.

Rules for making ray diagrams by concave mirror (i) A ray parallel to the principal axis will pass through the principal focus, after reflection. (ii) A ray passing through the principal focus of concave mirror will emerge parallel to principal axis after reflection. (iii) A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path as it is a normally incident ray. (iv) A ray incident obliquely to the principal axis of a concave mirror is reflected obliquely making equal angle.

Ray diagrams for images formed by concave mirror (i) When object is at infinity – Parallel rays from Image object at infinity Image Position – At ‘F’ Nature – Real, inverted Size – Point sized or highly diminished (ii) When object is beyond ‘C’ Image Position – Between ‘F’ and ‘C’ Nature – Real, inverted Size – Diminished (iii) When object is at ‘C’ Image Position – At ‘C’ Nature – Real, inverted Size – Same size as that of object (iv) When object is placed between ‘F’ and ‘C’ Image Position – Beyond ‘C’ Nature – Real, inverted Size – Enlarged (v) When object is placed at ‘F’ Image Position – At Infinity Nature – Real, inverted Size – Highly enlarged (vi) When object is between ‘P’ and ‘F’ Image Position – Behind the mirror Nature – Virtual, erect Size – Enlarged

Uses of Concave Mirror (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.

Rule for image formation by Convex Mirror (i) A ray of light parallel to the principal axis of a convex mirror appear to diverge from the principal focus. (ii) A ray which is directed towards the focus of the convex mirror will emerge parallel to the principal axis after reflection. (iii) A ray directed towards the center of curvature of a convex mirror is reflected back along the same. (iv) A ray incident obliquely to the principal axis is reflected obliquely.

Ray diagrams of images formed by convex mirror (i) When object is placed at infinity Image Position – At ‘F’ Nature – Virtual, erect Size – Point sized (ii) When object is placed between pole and infinity Image Position – Between ‘P’ and ‘F’ Nature -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 Or New Cartesian Sign Convention (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

REFRACTION Bending of light when it enters obliquely from one transparent medium to another.Speed of light is maximum in vaccum. It is 3 x 108 m/s. Cause of refraction – Change in speed of light. Some examples of refraction (i) The bottom of swimming pool appears higher. (ii) A pencil partially immersed in water appears to be bent at the interface of water and air. (iii) Lemons placed in a glass tumbler appear bigger. (iv) Letters of a book appear to be raised when seen through a glass slab.

Refraction through glass slab The extent of bending of ray of light at the opposite parallel faces of rectangular glass slab is equal and opposite, so the ray emerges parallel to incident ray. Lateral displacement depends on – (a) Refractive index of glass slab (b) Thickness of the glass slab

Optically rarer medium – Out of two given media, the medium with lower value of refractive index. When light enters obliquely from a rarer to a denser medium, it bends towards the normal. When light enters obliquely from denser to a rarer medium, it bends away from the normal. Refractive index of a medium does not depend on physical density.

Rules for image formation by convex lens (i) A ray of light parallel to principal axis of a convex lens always pass through the focus on the other side of the lens. (ii) A ray of light passing through the principal focus will emerge parallel to principal axis after refraction. (iii) A ray of light passing through the optical center will emerge without any deviation.

Ray Diagrams of Imaged formed by Convex Lens (i) When object is at infinity Image Position – At ‘F2’ Nature – Real, inverted Size – Point sized or highly diminished (ii) When object is beyond ‘2F,” Image Position – Between ‘F2’ and ‘2F2’ Nature – Real, inverted Size – Diminished (iii) When object is at ‘2F,’ Image Position – At ‘2F2’ Nature – Real, inverted Size – Same size (iv) When object is between ‘F,’ and ‘2F,’ Image Position – Beyond ‘2F₂’ Nature – Real, inverted Size – Enlarged (v) When object is at ‘F’ Image Position – At Infinity Nature – Real, inverted Size – Highly enlarged (vi) When object is between ‘F,’ and optical centre Image Position – On the same side of the lens as object Nature – Virtual and erect Size – Enlarged On the basis of this position, this lens is also known as magnifying lens.

Rules for Image Formation by Concave Lens
S.No.	Light ray from object is	Ray diagram	How it appears after refraction
1.	Parallel to the principal axis		After refraction from a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.
2.	Passing through a principal focus		After refraction from a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.
3.	Passing through the optical center of a lens		After refraction from a concave lens will emerge without any deviation.

Ray Diagrams of Images Formed by a Concave Lens (i) When object is placed at infinity Image Position – At ‘F’ Nature – Virtual, erect Size – Point sized or highly diminished (ii) When object is placed between infinity and optical centre Position – Between ‘F’ and ‘O’ Nature – Virtual, erect Size – Diminished

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