1. LIGHT - REFLECTION Contents: How do we see objects?
Common phenomena of light
Straight line path of light
Particle and wave nature of light
Spherical mirrors
How to draw ray diagram?
Image formation by a concave mirror
Image formation by a convex mirror
Uses of concave and convex mirror
Sign convention for reflection by spherical mirror
Mirror formula and magnification formula
Plane Mirror
Created by C. Mani, Deputy Commissioner, KVS RO Gurgaon

2. What makes things visible?
We are unable to see anything in a dark room.
On lighting up the room, things become visible.
Eye
What makes things visible?
An object reflects light that falls on it. The reflected light, when received by our eyes, enables us to see things.

3. - image formation by mirrors - image formation by lenses
Common phenomena of light are
- image formation by mirrors
- image formation by lenses
- twinkling of stars
- beautiful colours of a rainbow
- bending of light by a medium
- sparkling of diamonds
- colours formed on thin oil films
- colours formed in soap bubbles
- reddish appearance of sun while rising and setting
- blue colour of the sky
and many more

4. • • A small source of light casts a sharp shadow of an opaque object.X Y X Y S •
Shadow S •
Opaque Slit
Opaque object
Shadow
Screen
Screen
This shows that light travels in a straight line path.

5. Phenomenon like interference of light exhibits wave nature of light.
Bright Band
Dark Band S1 •
Bright Band S2 •
Dark Band
Bright Band

6. Phenomenon like diffraction of light also exhibits wave nature of light.
Shadow X Y X Y S •
Shadow S •
Opaque Slit
Opaque object
Shadow
Screen
Screen
Diffraction at an obstacle
Diffraction at a slit
X & Y – Region of diffraction

7. Phenomenon like polarisation of light also exhibits wave nature of light.
Optic Axis •
Unpolarised light
Plane Polarised light
Plane Polarised light
Polariser Tourmaline Crystal
Analyser Tourmaline Crystal •
90°
No light
Plane Polarised light
Unpolarised light

8. Phenomenon like photo electric effect exhibits particle nature of light.UV
Visible light
Photoelectrons
Photoelectrons
Metals
Alkali Metals
Visible light
No photoelectrons
Metals other than Alkali Metals

9. REFLECTION OF LIGHT Laws of Reflection of Light: Normal Incident Ray
Reflected Ray
Plane containing incident ray, reflected ray and normal i r
Plane mirror
Laws of Reflection of Light:
The angle of incidence is equal to the angle of reflection.
The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
Note:
These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces.

10. Formation of image by a plane mirror
Normal
Normal
Object
Plane Mirror
Image
Properties of image formed by a plane mirror
Image is always virtual and erect.
Size of the image is equal to the size of the object.
Image is as far behind the mirror as the object is in front of it.
Image is laterally inverted.

11. SPHERICAL MIRRORS
Mirrors whose reflecting surfaces are spherical are called ‘spherical mirrors’.
A spherical mirror whose reflecting surface is curved inwards, i.e. faces towards the centre of the mirror, is called a concave mirror.
A spherical mirror whose reflecting surface is curved outwards, i.e. faces away from the centre of the mirror, is called a convex mirror.
Concave Mirror
Convex Mirror
Reflecting surface
Painted surface

12. Concave Mirror
Convex Mirror M N M N P X C F X P F C f R f R
Pole (P) is the centre of reflecting surface lying on the surface.
Centre of curvature (C) is the centre of the imaginary sphere from which spherical mirror is cut out.
Radius of curvature (R) is the distance between the pole and the centre of curvature.
Principal axis (PCX or CPX) is the line passing through the pole and the centre of curvature and extending to ∞. It is the normal to the mirror at the pole.
Principal Focus (F) is the point on the principal axis at which the incident rays of light parallel to principal axis either really pass through or appear to pass through after getting reflected from the mirror.
Focal length (f) is the distance between the pole and the principal focus.

13. Rays to be considered for drawing Ray Diagram
Radius of curvature is approximately twice the focal length. R ≈ 2f
Aperture (MN) is the diameter of the reflecting surface. Note that it is not the diameter of the sphere from which the mirror is cut out.
Rays to be considered for drawing Ray Diagram
The intersection of at least two reflected rays give the position of image of the point object.
Any two of the following rays can be considered for locating the image.
1. 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. r i i r P C F P F C

14. 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, after reflection, will emerge parallel to the principal axis. i C P F F C P r i r
3. 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. i.e. retraces the path. F C P F C P

15. In all the above cases the laws of reflection are followed.
4. A ray incident obliquely to the principal axis, towards the pole is reflected obliquely. C P F F C P i i r r
Note:
In all the above cases the laws of reflection are followed.
At the point of incidence, the incident ray is reflected in such a way that the angle of reflection equals the angle of incidence.

16. Image formation by a concave mirror
1) When object is placed at infinity:
Parallel rays from ∞ C P F i
Position of image: At F
Nature of image : Real & inverted
iii) Size of image : Very small
(Highly Diminished) r I
2) When object (AO) is placed beyond C (2F): A O C F P
Position of image: Between C & F
Nature of image : Real & inverted
iii) Size of image : Smaller than object
(Diminished) i r B I

17. 3) When object (CO) is placed at C:F P O
Position of image: At C
Nature of image : Real & inverted
iii) Size of image : Same size as that
of the object i r I
4) When object (AO) is placed between C & F: C F P
Position of image: Beyond C
Nature of image : Real & inverted
iii) Size of image : Larger than object
(Enlarged) A O B I

18. 5) When object (FO) is placed at F:
Position of image: At ∞
Nature of image : Real & inverted
iii) Size of image : Very large
(Highly enlarged) O
Parallel rays meet at ∞
6) When object (AO) is placed between F & O: B I
Position of image: Behind the mirror
Nature of image : Virtual & erect
iii) Size of image : Larger than the object C F P O A
Eye
Rays diverge

19. Image formation by a concave mirror for different positions of the object
Position of the object
Position of the image
Size of the image
Nature of the image
At infinity
At F
Highly diminished
Real and inverted
Beyond C
Between F and C
Diminished
Real and inverted
At C
Same size
Real and inverted
Between F and C
Beyond C
Enlarged
Real and inverted
At F
At infinity
Highly enlarged
Real and inverted
Between P and F
Behind the mirror
Enlarged
Virtual and erect

20. Image formation by a convex mirror
Position of image: Behind the mirror
Nature of image : Virtual & erect
iii) Size of image : Smaller than the object r A O i C P F I B
Image formation by a convex mirror for any position of the object
Position of the object
Position of the image
Size of the image
Nature of the image
At infinity
At F behind the mirror
Highly diminished
Virtual and erect
Between infinity and the pole P
Between P and F behind the mirror
Diminished

21. Uses of concave mirror Uses of convex mirror
Concave mirrors are used in torches, search-lights and headlights of vehicles.
They are used as shaving mirrors.
They are used by dentists to see large images of the teeth.
Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Uses of convex mirror
Convex mirrors are used as rear view mirrors in vehicles as they form diminished images of the large objects on the road.
They are used as magic mirrors and to form funny images.

22. Sign Conventions for Reflection by Spherical Mirrors (New Cartesian Sign Convention)
The object is always placed to the left of the mirror. i.e. the incident rays from the object always move from left to right.
All distances parallel to the principal axis are measured from the pole (P) of the mirror.
All the distances measured to the right of the Pole (along +ve x-axis) are taken +ve while those measured to the left of the Pole (along - ve x-axis) are taken –ve.
Distances measured perpendicular to and above the principal axis (along +ve y-axis) are taken +ve while those measured below the principal axis (along –ve y-axis) are taken –ve.
Note:
While solving numerical problems, new Cartesian sign convention must be used for substituting the known values of u, v, f, h and R.

23. Y Y’ P - ve + ve + ve X X’ - ve P - ve + ve X X’ Y Y’
Direction of incident light P
- ve
+ ve
+ ve X X’
- ve P
Direction of incident light
- ve
+ ve X X’ Y Y’

24. Mirror Formula Magnification u – object distance v – image distance
f – focal length of the mirror 1 v f + = u
Magnification
Magnification produced by a mirror is defined as the ratio of the size of the image to the size of the object. m = h’ h
Magnification produced by a mirror is also defined as the ratio of the image distance to object distance. m = h’ h
= - v u
More of Reflection in Higher Class…