Optics

A ray o light s the straight line path followed by light in going from one point to another. Ray optics is also called Geometrical Optics.

Any body at a temperature above 0 K emits electromagnetic radiation.

For the same power per unit area, yellow light produces the strongest sensation of sight.

Photometry is a branch of physics which deals with the measurement of light.

All body radiate energy by virtue of their temperature. This energy is called radiant energy. It depends on nature of surface of the body and its temperature.

Luminous flux of a source of light as the Luminous energy (i.e. visible energy) emitted per second from the source.

Luminous intensity of a light source in any direction is the luminous flux emitted by that source in a unit solid angle in that direction. It’s unity is Candela.

Photometers are the instruments which can be used for comparing the illuminating power of two sources is called a photometer.

Fluorescent tube considered better than an electric bulb because efficiency of fluorescent tube is about 50 lumen/watt, whereas efficiency of electric bulb is about 12 lumen/watt. Thus for the same amount of electric energy consumed,  the tube gives nearly 4 times more light than the filament bulb.

Luminance of a surface refers to brightness of the surface. It is the luminous flux reflected from unit area of the surface.

Illuminance of a surface is the intensity of illumination of the surface. It is the luminous flux incident on unit are of the surface.

Human eye is most sensitive to the wavelength, lambda = 555nm.

In a plane mirror, image formed is virtual, erect and at the same distance behind the mirror as the object is in front of the mirror. Also, image in a plane mirror is always laterally invested.

n =(360 degre/theta)-1, if 6=360/ theta is even integer.

For a given incident of ray, when a plane mirror is turned clockwise or anticlockwise through an angle theta; the reflected ray turns in the same direction, clockwise or anticlockwise, through an angle 2 theta.

A convex mirror is used as a reflector in street lamps. as a result, the light from the lamp diverges over  a large area.

A concave mirror is used as a reflector in search light, head lights of motor vehicles, telescopes, solar cookers etc.

A concave mirror is also used as a shaving mirror/make up mirror as it can form erect and magnified image.

we cannot obtain the image formed by a convex mirror on a screen because the image formed by a convex mirror is virtual. Convex mirror is divergent. Convex mirror cannot form a magnified image.

A virtual image can be photographed.

Focal length of plane mirror is infinity. focal length of every mirror is only depend upon the radius of curvature.

Size mirror of does not affect the nature of image.

A concave mirror of small aperture forms a sharper image because a concave mirror of small aperture is free from the defect of spherical aberration.

Refraction of light is the phenomenon of change in the path of light, when it goes form one medium to another. The basic cause of refraction is change in the velocity of light in going from one medium to the other.

Snell’s law = MU2/MU1 = Sin i/ Sin r

Refractive index is a characteristics property of medium, whose value depends only on nature of material of the medium and the color or wavelength of light.

Critical Angle for a pair of media in contact as the angle of incidence in the denser medium corresponding to which angle of refraction in the rare medium is 90 degree.

MU b/ MU a= I/Sin C; C is critical angle

In TIR, 100% of incident light is reflected back into the denser medium and there is no loss of intensity, while in reflection from mirrors and refraction from lenses, there is always some loss of intensity. That is why images formed by TIR are much brighter than those formed by mirrors or lenses.

SOFAR channel in the ocean is a particular layer of the ocean that acts like an optical fibre for sound. It allows sound to travel tens of thousands of kilometer without any loss of intensity. This channel is used by ships in distress for sending signals. It has military applications as well.

An air bubble inside a water behaves as a concave lens because water is denser than air.

When lens of glass is immersed in water the power of lens get decreased.

Refractive index is minimum for vacuum and maximum for diamond.

Critical Angle depends upon the colour of light.

The absolute refractive index of a material can never be less than one.

Frequency of light remain same when it goes from air into water.

The deviation through a prism is maximum when i1= 90 degree.

Cause of dispersion: each color has its own characteristic wavelength (lambda).

Cauchy’s formula

MU= A+B/lambda^2+C/Lambda^4+…….

Angular dispersion: it produced by a prism for white light is the difference in the angle of deviation of two extreme color i.e. violet and red color.

Dispersive power: the dispersive power of a prism is defined as the ratio of angular dispersion to the mean deviation produced by the prism.

In the dispersed beam, yellow color is taken as the mean color. Deviation for yellow color is called the mean deviation. Similarly, refractive index of the material of the prism for yellow color is called mean refractive index.

The sun is visible to us before actual sun rise and after actual sunset. This is because of atmospheric refraction of light. Red hues of sunrise and sunset is due to scattering of light.

Rayleigh law: Is directly proportional to 1/lambda^4

i.e. intensity of scattered light various inversely as the fourth power of the wavelength of incident light. Rayleigh further said that the ray does not undergo any change in wavelength on scattering.

Rainbows are popularly known as ‘A smile in the sky’.

Dispersion of light cannot occur in a hollow prism.

Wavelength region of visible spectrum is 3800 angstrom to 7600 angstrom.

Refractive index of material of a prism is maximum for violet light and minimum for red light. Violet colour suffers maximum deviation and red color suffers the least deviation.

Angular dispersion of a prism depends upon angle of prism and nature of material of the prism.

Dispersive power of prism depends upon nature of material.

A pure spectrum is that in which there is no overlapping of color.

Speed of light n glass increase on increasing the wavelength.

There is no effect of changing intensity on speed of light in glass.

The function of iris is to adjust and allow suitable quantity of light into the eye. the front transparent part of the hard outer surface of the eye ball is called cornea. It protects the inner delicate parts of the eye.

The least sensitive spot on the retina is called the blind spot. There is another spot at about the center of the retina, which is most sensitive to light. This is called the yellow spot.

The retina contains rods and cones, which sense light intensity and color respectively.

Advantages of a reflecting type telescope, especially for high resolution astronomy

  1. There is no chromatic aberration as the objective is a  mirror.
  2. Spherical aberration is reduced using mirror objective in the form of a paraboloid.
  3. Image is brighter compared to that in a refracting type telescope.
  4. Mirror requires grinding and polishing only on one side.
  5. High resolution is achieved by using a mirror of large aperture.
  6. A mirror weigh much less than a lens of equivalent optical quality.

The lenses which have been duly corrected for most of the optical defects of images formed by them are called anastigmatic lenses.

We prefer a magnifying glass of smaller focal length because  the magnifying power will be more when focal length is small.

In normal adjustment of telescope the final object is formed at infinity.

 

 

 

 

 

 

 

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