Class 10 Science
Chapter 10 Light - Reflection and Refraction
Topics in the Chapter
• Introduction
• Properties of Light
• Reflection
→ Laws of Reflection
→ Virtual and Real image
• Image formed by Plane mirror
→ Characteristics of image formed by Plane mirror
→ Lateral inversion and its application
• Spherical Mirrors
→ Properties of Concave mirror
→ Properties of Convex mirror
→ Common terms for Spherical mirrors
• Rules for making ray diagrams by spherical mirrors
• Ray diagrams for images formed by concave mirror
→ When object is at infinity
→ When object is beyond C
→ When object is at C
→ When object is placed between F and C
→ When object is placed at F
→ When object is between P and F
• Uses of Concave mirror
• Ray diagrams of images formed by convex mirror
→ When object is placed at infinity
→ When object is placed between pole and infinity
• Uses of convex mirror
• Sign convention for reflection by spherical mirror
• Mirror Formula
→ Magnification of Spherical mirrors
• Refraction
• Laws of Refraction
→Refractive index
→ Absolute refractive index
→ Optically denser medium
→ Optically rarer medium
→ Spherical lens
• Rules for image formation by convex lens
• Ray diagram of image formed by Convex lens
→ When object is at infinity
→ When object is beyond 2F1
→ When object is at 2F1
→ When object is between F1 and 2F1
→ When object is at F1
→ When object is between F1 and optical centre
• Rules for image formation by concave lens
• Ray diagrams of images formed by a concave lens
→ When object is placed at infinity
→ When object is placed between infinity and optical centre
• Sign convention for spherical lens
→ Lens Formula
→ Magnification
• Power of a lens
Introduction
→ Light is the form of energy that provides sensation of vision.
→ Some common phenomena associated with lights are image formation by mirrors, the twinkling of stars, the beautiful colours of a rainbow, bending of light by a medium and so on.
Properties of Light
→ Electromagnetic wave, so does not require any medium to travel.
→ Light tends to travel in straight line.
→ Light has dual nature i.e. wave as well as particle.
→ Light casts shadow.
→ Speed of light is maximum in vaccum. Its value is 3 × 108 ms-1.
→ When light falls on a surface, following may happen:
(i) Reflection
(ii) Refraction
(iii) Absorption
Reflection→ Bouncing back of light when it strikes on a polished surface like mirror.
Laws of Reflection
(i) 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 imageImage is a point where atleast two light rays actually meet or appear to meet.
Image Formed by Plane MirrorCharacteristics of Image formed by Plane Mirror
(i) Virtual and erect.
(ii) Size of image is equal to the size of object.
(iii) 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 rear view mirror of vehicles going in front of it.
→ Mirrors whose reflecting surface is curved.
→ There are two types of spherical mirrors:
(i) Convex Mirror
(ii) Concave Mirror
Properties of Concave mirror
• Reflecting surface is curved inwards.
• Converging mirror
Properties of Convex mirror
• Reflecting surface is curved outwards.
• Diverging mirror
Common terms for Spherical mirrors→ 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.
→ Relationship between focal length and radius of curvature: f = R/2
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 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.
Ray diagrams for images formed by 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 object is beyond ‘C’ Image Position – Between ‘F’ and ‘C’
Nature of image – Real, inverted
Size – Diminished
(iii) When object is at ‘C’ Image Position – At ‘C’
Nature of image – Real, inverted
Size – Same size as that of 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 object is between ‘P’ and ‘F’ Image Position – Behind the mirror
Nature of image – 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.
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
→ they always give an erect though diminished image.
→ 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 = hi/ho
Also, m = -v/u
→ If ‘m’ is negative, image is real.
→ If ‘m’ is positive, image is virtual.
→ If hi = ho then m = 1, i.e., image is equal to object.
→ If hi > ho then m > 1 i.e., image is enlarged.
→ If hi < ho 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.
Refraction of light
→ Refraction is bending of light when it enters obliquely from one transparent medium to another.
→ Speed of light is maximum in vaccum. It is 3 × 108 m/s.
→ Cause of refraction: Change in speed of light.
• Some examples of refraction
→ The bottom of swimming pool appears higher.
→ A pencil partially immersed in water appears to be bent at the interface of water and air.
→ Lemons placed in a glass tumbler appear bigger.
→ Letters of a book appear to be raised when seen through a glass slab.
Laws of Refraction
(i) 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.
(ii) Snell’s law: The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for a light of given colour and for a given pair of media.
sin i/sin r = constant
• Refractive index (n): The ratio of speed of light in a given pair of media
n = Velocity of light in medium 1/Velocity of light in medium 2
→ n21 means refractive index of second medium with respect to first medium.
n21 = v1/v2
→ n12 means refractive index of second medium with respect to first medium.
n12 = v2/v1
• Absolute Refractive Index: Refractive index of a medium with respect to vaccum or air.
n = (c/v) x c = 3 × 10^8 m/s.
→ Refractive index of one medium is reciprocal of other’s refractive index in a given pair.
n12 = 1/n21
→ If refractive index of medium 1 w.r.t. air is given as 1nair, and
If refractive index of medium 2 w.r.t. air is given as 2n^air.
Then, refractive index of medium 1 w.r.t. medium 2 = (1n^air)/(1n^air)
→ Refractive index of diamond is the highest till date. It is 2.42. It means speed of light is 1/2.42 times less in diamond than in vaccum.
• Optically denser medium: Out of two given media, the medium with higher value of refractive index.
• Optically rarer medium: Out of two given media, the medium with lower value to 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.
• Spherical lens: A transparent medium bound by two surfaces, of which one or both surfaces are curved.
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 Imag formed by Convex Lens
(i) When object is at infinity
Image Position − At ‘F 2 ’
Nature of image – Real, inverted
Size – Point sized or highly diminished
(ii) When object is beyond ‘2F1’Image Position – Between ‘F2’ and ‘2F2’
Nature of image– Real, inverted
Size – Diminished
(iii) When object is at ‘2F1 ’Image Position – At ‘2F2 ’
Nature of image – Real, inverted
Size – Same size
(iv) When object is between ‘F1’ and ‘2F1’Image Position – Beyond ‘2F2’
Nature of image – Real, inverted
Size – Enlarged
(v) When object is at ‘F1’
Image Position – At Infinity
Nature of image – Real, inverted
Size – Highly enlarged
(vi) When object is between ‘F1’ and optical centre Image Position – On the same side of the lens as object
Nature of image – Virtual and erect
Size – Enlarged
Rules for Image Formation by Concave Lens
(i) A ray of light parallel to the principal axis appear to diverge from the principal focus located on the same side of the lens.
(ii) A ray of light appearing to meet at the principal focus of a concave lens will emerge parallel to principal axis.
(iii) A ray of light passing through the optical centre of a 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 ‘F1’
Nature of image – Virtual, erect
Size – Point sized or highly diminished
(ii) When object is placed between infinity and optical centre
Image Position – Between ‘F’ and ‘O’
Nature of image – Virtual, erect
Size – Diminished
Sign convention for spherical lenses
• Sign conventions are similar to the one used for spherical mirrors, except that measurements are taken from optical center of the lens.
• Focal length of convex lens = PositiveFocal length of concave lens = Negative
Lens Formula
1/v - 1/u = 1/f
Magnification
m = hi/ho = v/u
Power of a lens
→ It is defined as the reciprocal of focal length in meter.
→ The degree of convergence or divergence of light rays is expressed in terms of power.
Power (P) = 1/v - 1/u = 1/f
→ SI unit of Power = dioptre = D
→ 1 D = 1 m-1
→ 1 dioptre is the power of lens whose focal length is one meter.
→ Power of convex lens = Positive
→ Power of concave lens = Negative
→ Power ∝ 1/(focal length or thickness)
→ Power of a lens combination (P) = P1 + P2 + P3 .........
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