Light

Light is a form of energy which propagated as electromagnetic waves. It lie between between ultra-violet and infra-red region and has wavelength between 3900 A to 7800 A.

• Electromagnetic waves are transverse, hence light is transverse wave.
• All the electromagnetic wave including light don't require medium to propgate.
• Wave nature of light, explains rectilinear propagation, reflection, refraction, interference, diffraction and polarisation of light
• Quantum theory of light (proposed by Einstein), packet or bundle of energy called photon. E=hv
• Light has dual nature, it behaves as wave and particle both.
• Speed of light was first measured by Roemer, it is max in vacuum and air (3 * 10^8)

Refractive Index: Ratio of speed of light in vacuum to the speed of light in medium., c/v

• Light takes 8 min 19 s to reach from sun to earth.
• The light reflected from moon takes 1.28 s to reach earth

Reflection of Light: 

if the surface is smooth then reflection is regular, if rough then reflection is irregular.

Law of reflection:

• The incident ray, reflected ray and normal to the reflecting surface at the incident point all lie in same plane.
• The angle of reflection is equal to he angle of incidence.

Reflection From Mirror:

Planer Mirror: 

• Image is virtual and literally inverted
• The size of image is equal to that of object.
• The distance of image from the mirror is equal to distance of object from the mirror.
•  If the image moves toward a plane mirror with speed v , the image moves away with speed 2v.
• If a plane mirror is rotated by angle 

  θ 

  the reflected ray is rotated by angle 2

  θ 

• We can see full image in plane mirror if it height is half of person height.
• Focal length of plane mirror in infinite.

Reflection from Spherical Mirror: Concave and Convex.

Concave Mirror: 

• When the Object is at Infinity: The image thus formed is highly diminished, point size, real and inverted and at focus.
• When object is between infinity and center: The image formed is diminished, real and inverted and at between Centre and focus.
• When object is at centre: The image formed is of same size, real and inverted, and at centre.
• When object is between centre and pole: The image formed is enlarged, real and inverted, and at between centre and infinity.
• When object is at focus: The image formed is highly enlarged, real and inverted, and at infinity.
• When object is between focus and pole: the image formed is enlarge, virtual and erect, and at behind the mirror.

Use of Concave mirror:

• As a shaving glass
• As a reflector of head light of vehicle, search light
• In solar cooker
• In concentrating light a a given spot
• In opthalmoscope to examine eye, ear, nose by doctor.

Convex Mirror:

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• When object is at infinity: The image formed is highly diminished, virtual and erect, and at focus.
• When object is at front of mirror: The image formed diminished, virtual and erect and between focus and pole.

** It used as a rear view in vehicle and sodium reflector lamp.

Refraction of Light: The phenomena of change in direction of light at the boundry when it passes from one medium to other medium is called refraction of light.

When light enters from rare medium to denser medium it deviates toward normal and vice versa.
Laws of Refraction:

• Incident ray, refracted ray and normal drawn at the incident point always lie in the same plane.
• Snells Law: For a given color of light, the ratio of sine angle of incidence to the sin angle of refraction is a constant.

This constant is called refractive index of second medium compare to first medium.

• The refractive index of medium decreases with the increase in wavelength of light. It also decreases with increase in temperature.
• When ray of light enter from one medium to another, its frequency doesn't change but its wavelength and velocity changes.

Some example of refraction: 

• Bending of linear object when it partially dipped into liquid.
• Twinkling of star
• Oval shape of sun in the morning and evening.
• An object in a denser medium when seen from a rear medium appear to be smallest distance.

Total Internal Reflection:

Critical Angle: Angle of incidence for which angle of refraction is 90 degree.

If the light is propagating from denser medium to rare medium angle of incidence is more than critical angle then light reflected back to the denser medium , This phenomena is called total internal reflection of light.

Illustrations of total internal reflection:

• Sparkling of diamond
• Mirage and looming
• Shinning of air bubble in water
• Increase in sun duration visibility
• Shinning of a smoked ball or a metal ball on which lamp soot is deposited when dipped into water.

Optical Fibre : made by fine quality of glass and quartz having low refractive index (1.5), worked on Concept of total internal reflection.

Applications: 

• Transmitting optical signal and two dimensional picture
• Transmitting electrical signal by first converting into light.
• For visualizing the internal sites of body by doctors in endoscopy.

Refraction of Light through lens:

There are two types of lens:

• Convex
• Concave

Power of lens:

• Its capacity to deviate a ray. 1/f
• SI unit is dioptre.
• Power of convex lens is positive and that of concave is negative.
• If two lenses are placed in contact, then the power of combination is equal to sum of powers of individual lenses. 

Change in the power of lens:

** if lens change dipped into the liquid, its focal length and power both change, this change depend upon the refractive indices of lens and the liquid.

Assume a is RI of lens and B is the RI of water:

If A>B Then focal length of lens increase

A=B Focal length becomes infinitive

A<B Focal length increases and power decrease as well as nature of lens also changes, Convex lens behave as concave lens and vice versa.

Formation of image by lens:

Summary of Image Formation by Concave and Convex Lens

Image formation by Concave Lens
Object Location	Image Location	Image Nature	Image Size
Infinity	At F1	Virtual and Erect	Highly diminished, point-sized
Beyond Infinity and Optical Centre	Between Focus (F1) and Optical center (O)	Virtual and Erect	Diminished

Image formation by Convex Lens
Object location	Image location	Image nature	Image size
Infinity	At F2	Real and Inverted	Diminished, point sized
Beyond 2F1	Between 2F2 and F2	Real and Inverted	Diminished
Between 2F1 and F1	Beyond 2F2	Real and Inverted	Enlarged
At F1	At infinity	Real and Inverted	Infinitely large or highly enlarged
At 2F1	At 2F2	Real and Inverted	Same size
Between F1 and 0	On the same side as the object	Virtual and Erect	Enlarged

Dispersion of Light:

When a ray of white light passes through a prism, it gets splitted into its constituent colors. This phenomena is called dispersion of light.

VIBGYOR: The deviation is maximum for violet and velocity is minimum and minimum deviation for red color and maximum velocity.

The dispersion of light is due to different velocities of light of different color in a medium.

**he velocity of light in a medium is maximum for that colour  for which refractive index is minimum.

Rainbow: It is appear in the sky during or after little drizzle appearing on the opposite side of sun.

Rainbow is formed due to dispersion and refraction od sun light by the suspended water droplets.

Rainbow is of two types:

• Primary Rainbow: It formed due to two refraction and one total internal Reflection. The red color in on convex side and violet on concave side. Angular width 2 degree at an average angle of elevation of 41 degree.
• Secondary rainbow: it formed due to two refraction and two total internal reflection. Order of color is reverse and has an angular width of 3.5 degree at an average elevation of 52.75 degree. It is less intense than primary rainbow.

Theory of Color: 

Blue, red and green are primary color, because all the color produced by mixing these color.

Blue+Red+Green = White

Secondary Color: Color produced by mixing any two primary color.

Green+red= Yellow, Red+Blue=Magenta, Blue+Green= Cyan.

When three secondary color are mixed, white color is produced.

Complementary color: Any two color when added produced white color, are said to be complementary color 

** A secondary color and remaining primary color are complimentary color.

Scattering of Light: Scattering of light is maximum in case of violet color and minimum in case of red color.

** Blue color of sky is due to scattering of light. Red color of rising and setting sun is due to scattering of light.

Human Eye: Human eye basically consist of convex lens. This lens forms real and inverted image of an object on retina.

There are two kinds of vision cells in the retina:

• rods: Decide intensity of light
• Cones: distinguish color of light

Defect of human eye and remedies:

Myopia or short sightedness: can see near object clearly while far object are not clear.

causes: 

• Elongation of eye ball along the axis
• Shortening of focal length of eye lens
• Over stretching of ciliary muscles beyond the elastic limit.

Remedy: Diverging lens is used.

Hyperopia:  Can see the distant objects clearly but not the near objects.

causes:

• Shortening of eye ball along the axis
• Increase in focal length of eye lens
• Stiffening of ciliary muscles.

Remedy: A Converging Lens is used.

Presbyopia: generally found in elderly person, it happen due to stiffening of ciliary muscles. Person can't see distant as well as near by object.

Astigmatism: This defect arises due to difference in the radii of curvature of cornea in the different planes.

Remedy: Cylindrical lens.

Microscope:

Simple Microscope

Convex lens of small focal length

Magnifying power = 1 + D/F

Compound Microscope: Consist of two convex lenses coaxially fitted in a hollow tube. Both lenses have smaller focal length.

Telescope:

Types of Telescope:

Astronomical telescope: Consist of two convex lenses coaxially fitted in a hollow tube. lens facing the object is called objective and lens towards the eye is called eye piece. Final image is inverted.

  Focal length of Object is larger than eye piece.

Terrestrial Telescope: One addition convex lens between object and eye piece. The role of third lens is to invert the image formed by the objective so that final image is erect.

Galilean Telescope: Consist of two lens, objective is convex and eye piece is concave. The final image is erect.