Optics subjective question 1

Question
A narrow monochromatic beam of light of intensity I is incident on a glass plate A as shown below in the figure

Another identical glass plate B is kept close to A  and parallel to it. Each glass plate reflects 25% of the light intensity incident on it and transmits the remaining.Find the ratio of maximum and minimum intensities in the interference pattern formed by the two beams obtained after reflection from each plate.

Solution:

Hi First try to solve the question on your own it's an easy one . The answer is ratio of intensities is 49. I will provide the solution to this question.

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Optics questions for PMT - pre medical exams

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What is diffraction

It is a common observation with the waves of all kinds that they bend round the edge of an obstacle.

Light like other waves also bends round the corner but in comparison to sound waves small bending of light is due to very short wavelength of light which is of the order of 10^-5 cm.

This effect of bending of light beams round the corner was first discovered by Grimaldi (1618-1663).

We now define diffraction of light as phenomenon of bending of light waves around the corners and their spreading into geometrical shadow.

Fresnel first explained that the diffraction phenomenon was the result of mutual interference between the secondary wavelets from the same wavefront.

Thus we can explain diffraction phenomenon using Huygen's principle.

The diffraction phenomenon is usually divided into two classes
1. Fresnel class of diffraction phenomenon where the source of light and scteen are in general at finite distance from the diffracting aperture
2. Fraunhofer class of diffraction phenomenon where source and the screen are at the infinite distance from the aperture , this is easily achieved by placing the source of light on the focal plane of a convex lens and placing screen on focal plane of another convex lens. This class of diffraction is simple to treat and easy to observe in practice.

Coherent Sources of light

Coherent sources are those sources of light which emit continous light waves of same wavelength , same frequency and are in same phase or have constant phase difference.

For observing interference phenomenon coherence of light waves is a must.

For light waves emitted by two sources of light , to remain coherent the initial phase difference between waves should remain constant in time. If the phase difference changes continously or randomly with time then the sources are incoherent.

Two independent sources of light are not coherent and hence can not produce interference because light beam is emitted by millions of atoms radiating independently so the phase difference between waves from such sources fluctuates randomly many times per second.

The coherent sources can be obtained either by the source and obtaining its virtual image or by obtaining two virtual images of the same source. This is because any change of phase in real source will cause a simultaneous and equal change in its image.

Generally coherence in interference is obtained by two methods
(1) Division of wave front where wavefront is divided into two parts by reflection, refraction or diffraction and those two parts reunite at a small angle to produce interference as done in case of Young's double slit experiment and Fresnel's biprism experiment.
(2) Division of amplitude where amplitude of a section of wavefront is divided into two parts and reunited later to produce interference such as in case of thin films.

Laser light is almost monochromatic light with little spreading and two independent sources of laser light can produce observable interference pattern.

Properties of light

1. Speed of light is greatest in the vacuum and it approximate value is 3 x 10⁸. This value of speed of light is same for all wavelengths.
2. light has got dual nature i.e., sometimes it acts as particles and sometimes a s waves depending on the situation for example Einstein photoelectric effect experiment establishes particle properties of waves and experiments like interference and diffraction of waves establishes wave nature of light.
3. Light waves does not require any medium for its propagation.
4. When light waves travels from one medium to another it's frequency remains unchanged however its wavelength and speed do changes with the change of medium.
5. Frequency of light determines its frequency.
6. Light follows the path along which it takes less time to travel from one place to another i.e., it travels in accordance to Fermat's principle.
7. All bodies which emits light are known as sources of light and light sources can be of any shape or size.
8. When a source of light possess its own it is termed as luminious for example sun and if it does not have its own light but it visible because of reflected light from it then it is known as non luminous source of light for example human body etc..

Physics for IITJEE:Optics Short Notes-1

Agenda :Summary of the basics of Optics

Reflection of light
-When light travelling in one medium is incident on a second medium some part of it is sent back to the first medium according to some definite laws and is said to be reflected.
-In practice mirrors are used to reflect light.
-The angle which the incident ray and the reflected ray make with normal to the surface are termed as Angles of incidence and of reflection respectively.

Laws of reflection:-
Law 1- The incident ray, the reflected ray and the normal to the surface , all lie in the same plane.
Law 2- The angle of incidence i is equal to the angle of reflection r.

Spherical Mirrors :-
Assuming that all the reflecting surfaces are portions of a sphere, if the inside surface reflects, the mirror is concave and if the outside surface which reflects, it is convex as shown below in the figure.
-Relation between focal length f and radius of curvature R of the mirror is
f=R/2
and both f and R are taken as positive for concave mirror and negative for convex mirror.
-Mirror formula is the relation between distance of the object (u), distance of the image (v) and the focal length (f) of the mirror and is,
(1/u)+(1/v)=(1/f)
-Mirror formula holds for a spherical mirror of small aperture only.
-The ratio of size of the image formed (by spherical mirror) to the size of the object is called magnification. Mathematically,
m=I/O
=-v/u
-Spherical abberation is the inability of a spherical mirror of large aperture to bring all the rays of wide beam of light falling

on it to focus at a single point.











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