LIGHT REFLECTION & REFRACTION

 LIGHT REFLECTION & REFRACTION

Form of energy produces the sensation of vision in eyes. Light (EM waves wave-length 400 nm to 750 nm).
The path of light (always travel in straight line) is ray of light

Characteristics of light

• Rectilinear propagation of light
• Light travels with a speed of 3 × 108 m/s in air/vaccum.
• Speed of light depends on the medium
• Light shows behaviour such as reflection, refraction, interference, diffraction, polarisation etc.

Law of Refraction
Refraction of light: Bending of light ray while passing from one medium to another medium

• A ray of light bends towards the normal, while going from rarer to denser medium
• And bends away from the normal while going from denser to rarer medium
• Refraction of light takes place because the speed of light is different in the two media

Total internal Reflection : Ray totally reflected back to denser medium
Phenomena based on TIR

• Mirage – optical illusion in deserts
• Looming – optical illusion in cold countries
• Optical fibre
• Brilliance of diamond

Necessary conditions for TIR
(i ) Ray of light must travel from denser to rarer medium
(ii) ∠i > ∠c for two media

Critical angle (c) Angle i in denser medium for which angle of refraction in rarer medium is 90° μ = 1sinC

Snell’s law
μ = sinisinr
For two media
1μ2 = μ2μ1=sinisinr

Reflection of light: Turning back of light in the same medium after striking the reflecting surface or mirror

• After reflection, velocity, frequency and wavelength of light remains same but intensity decreases
• If reflection takes place from denser medium then phase change ‘π’

Regular Reflection

Reflection on smooth surface.

Diffuse Reflection
Reflection on rough surface.

Laws of Reflection

The incident ray the normal and the reflected ray all lie in the same plane The angle of incidence (i) is always equal to angle of reflection (r) i.e., ∠i = ∠r

Mirror formula
1f=1u+1v
When two plane mirrors are held at an angle 9 with their reflecting surfaces facing each other and an object is placed between them, images are formed by successive reflections. .
fconcave = negative
fconvex = positive
and fplane = ∞

Relation between focal length (f) and radius of curvature, R
f = R2

Magnification
m = vu= height of image  height of object 
m = ff−u=f−vf

The incident ray, the normal and the refracted ray all lie in the same plane
Refractive index,
μ = cv= real depth  apparent depth 

Plane Mirror

Is a looking glass, highly polished on one surface.

• Forms virtual and erect image
• Distance of object from mirror = distance of image from mirror.
• The size of the image is same as object.
• Image is laterally inverted.
• Used in kaleidoscope periscope, etc.

Concave Mirror
Spherical glass polished on the outside. It is also known as a converging mirror.

• Images produced are always real, inverted, can be enlarged based on the position except when object is placed between pole and focus.
• Uses: Make-up and shaving mirrors, dentist mirror, in floodlight etc.

Image formation by a convex mirror for different positions of the object

‘‘Position of the object”	“Position of the image”	“Size of the image”	“Nature of the image”
Anywhere between Between Pole(P) and infinity (∞)	Between P and F back of the mirror	Small	Virtual and erect
At infinity	At F	Very small in size	Virtual and erect

Convex Mirror
Spherical glass polished inside. It is also known as diverging mirror.

• It forms virtual, upright and small images.
• Uses: for security’ purposes, in vehicles as rear- view mirror and street lighting.

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 the focus F	Highly -diminished, point-sized	Real and inverted
BeyondC	Between F and C’	Diminished	Real and inverted
At C	At C	Same size	Real and inverted
B/W C and F	Beyond C	Enlarged	Real and inverted
At F	At infinity	Highly enlarged	Real and inverted
B/W P and F	Behind the mirror	Enlarged	Virtual and erect

Atmospheric Refraction

Earth’s atmosphere is thin at the top and dense at the bottom, thus leads to refraction of light,
μ = c/v

• Twinkling of stars
• Rainbow
• Advanced sunrise and delayed sunset

Refraction Through a Glass Slab
x = tsin(i−r)cosr
∴ x ∝ μ

Power of a lens

P = 1f( in metre )
Unit of power of lens is diopter (D)
Pconvex → Positive
Pconcave → Negative
and Pplane → Zero

Lens
Piece of transparent material with two refracting surfaces, at least one is curved and refractive index should different as that of the surrounding.

Lens formula
1f=1v−1u
fconvex → negative
fconcave → positive
and fplane → ∞

Concave Lens
Cental portion of lens is thinner than marginal. It as also known as diverging lens.

Convex Lens
Central portion of lens is thicker than marginal. It is also known us converging lens.

Magnification
Ratio of distance of image to the distance of object from the optical centre. Also equal to height of image to the height of object
m = Io=vu=hIho

Nature, position and relative size of the image formed by a concave lens for various position of the object

‘‘Position of the object”	“Position of the image”	Relative Size of the image”	“Nature of the image”
At infinity	At focus F1	Highly-diminished, point-sized	Virtual and erect
Between infinity and Optical centre O of the lens	Between F1 and Optical centre O	Diminished	Virtual and erect

Nature, position and relative size of the image formed by a convex lens for various positions of the object

Position of the object	Position of the image	Relative size of the image	Nature of the image
At infinity	At focus F2	Highly -diminished, point-sized	Real and inverted
Beyond 2F1	Between F2 and 2F2	Diminished	Real and inverted
At 2F1	At 2F2	Same size	Real and inverted
Between F1 and 2F1	Beyond 2F2	Enlarged	Rea! and inverted
At Focus F1	At infinity	Infinitely large or highly enlarged	Real and inverted
Between F1 and Optical centre O	On the same side of the lens as the object	Enlarged	Virtual and erect