Light : Light Reflection
dispersion REFLECTION OF LIGHT : REFLECTION OF LIGHT The process of sending back the light rays which fall on the surface of an object, is called reflection of light.
Laws of reflection of light
The incident ray, the reflected ray and the normal all lie in the same plane.
The angle of reflection is always equal to the angle of incidence. Slide 3: The path of the incident ray
The ray falling parallel to the principal axis on the concave mirror
The ray falling on the concave mirror
through the centre of curvature.
The ray falling on the concave mirror
through the principal focus.
The ray falling obliquely on the pole of the concave mirror. Slide 4: The ray travelling towards the principal focus and incident on the convex mirror…….
he ray passing parallel to the The ray proceeding towards the centre of curvature and falling on the convex mirror…… Centre of curvature : Centre of curvature Slide 6: principal focus Spherical Mirrors : Spherical Mirrors Concave
Mirror Some technical terms relating to spherical mirrors : Some technical terms relating to spherical mirrors Centre of curvature
Centre of curvature is the centre of the sphere of which the spherical mirror is a part.
Pole is the geometric centre of the
reflecting surface Slide 10: Radius of curvature
Radius of curvature is the radius of the sphere of which the spherical mirror is a part.
Principal axis is the straight line passing through the centre of curvature and the pole. Aperture : Aperture Aperture is the diameter of the reflecting surface. When the diameter increases aperture also increases. Relation between focal length and radius of curvature : Relation between focal length and radius of curvature ? : ? What does the length PC indicate?
What about PF?
Which is the angle of incidence?
Which is the angle of reflection?
What is the reason< LMC =<MCF
What is the reason? Slide 14: What is the relation between angles MCF and FMC?
Aren't the sides opposite to the equal angles in a triangle the same?
Therefore, in CMF, we get
CF = FM
In a mirror of small aperture, the point of incidence M and the pole P are close to each other Slide 15: Therefore PF = FM
Thus we can infer that
CF = MF or CF = PF
That is, F is the mid point of CP
If PC is radius of curvature (r) and PF the focal length (f) the relation between them is PC = PF + FC = 2PF
That is r = 2f Image formation by concave mirror : Image formation by concave mirror Slide 18: In the figure which are the light rays starting from M?
What are the peculiarities of the path of the incident ray?
Which are the path of these light rays after reflection and which is the point where they meet in the figure? Position of the object at infinity : Position of the object at infinity Position of the object: beyond Ci) position of the image:ii) Size of the image: iii) Nature of the the image: : Position of the object: beyond Ci) position of the image:ii) Size of the image: iii) Nature of the the image: Position of object: beyond C : Position of object: beyond C Position of the object: at C or Btwn C&F i) position of the image:ii) Size of the image: iii) Nature of the image: : Position of the object: at C or Btwn C&F i) position of the image:ii) Size of the image: iii) Nature of the image: Position of the object: at F i) position of the image:ii) Size of the image: iii) Nature of the image: : Position of the object: at F i) position of the image:ii) Size of the image: iii) Nature of the image: Position of the object: Btwn F & Pi) position of the image:ii) Size of the image: iii) Nature of the image: : Position of the object: Btwn F & Pi) position of the image:ii) Size of the image: iii) Nature of the image: Question: complete the dgrm. : Question: complete the dgrm. Characteristics of image : Characteristics of image P: Image is always behind the mirror
S: Smaller than the object.
N: The image is virtual and erect.
Where ever be the object the image will always be between F and P. The New Cartesian Sign Convention : The New Cartesian Sign Convention Principal axis is conceived as X-axis and pole as origin.
All distances are measured from the pole.
The incident ray is conceived as passing from left to right.
Distances measured in the same direction of the incident ray are taken as positive (+) and the distances measured in the opposite direction are taken as negative (-). Slide 29: Heights upwards positive
Heights downwards negative
Distance against incident light negative
Distance along incident light positive. Give signs to the following: : Give signs to the following: Focal length of a concave mirror
Focal length of a convex mirror
Radius of curvature of a concave mirror
Radius of curvature of a convex mirror Mirror formula : Mirror formula Problems : Problems 1) When a bright object is placed 10 cm away from a concave mirror, its real image is formed at a distance 40cm from the mirror. What is the focal length of the mirror? Slide 33: 2) An object placed 20cm away from a convex mirror is found to give a virtual image 10cm behind the mirror.
3) Find out the focal length of the mirror. An object is placed at a distance 20cm from a concave mirror of focal length 15cm. Find how far away is the image formed? Slide 34: Magnification Sign convention : Sign convention If m is negative (-) the image is inverted and real
If m is positive (+) the image is erect and virtual. PROBLEMS : PROBLEMS 1) An object placed at a distance of 15cm from a concave mirror produces an image at a distance 45cm from the mirror. Find out the magnification.
2) An object 2.5mm in height placed at a distance 5cm from a mirror produces a real image of 1cm in height. Find the position and magnification of the image. Refraction : Refraction Laws of refraction - Snell's law Laws of refraction : Laws of refraction The incident ray, refracted ray and the normal to the point of incidence lie in the same plane.
When light rays travel from one medium to another is a constant as far as these two media are concerned. This is called Snell's law. Refractive Index : Refractive Index Refractive index (n) : Refractive index (n) According to Snell's law is a constant. This is the refractive index of the medium and is indicated by the letter n . Slide 41: Refractive index of a medium =
the velocity of light in vacuum ÷ velocity of light in the medium Slide 43: If the angle of incidence of a ray of light falling on the glass surface is 30° and the angle of refraction is 19°. what is the refractive index of glass? THANK YOU : THANK YOU