^ best physics lectures for neet - KP Physics

^Diffraction of light

January 8, 2022/in Back end /by kp-web-admin

^Diffraction of light

When a wave (light or sound) strikes an obstacle it doesn’t go straight, rather it bends round the obstacle.

Also when a light wave passing from a narrow slit of width AB = d reaches screen placed at a distance D (>>d) from the slit, (Fig. A) then a bright spot on screen at a point just opposite to slit is expected & all other points on the screen are expected to be dark (called regions of geometrical shadow), but in actual practice light spreads into the region of geometrical shadow and alternate patterns of bright & dark bands (Fig. B) of varying intensity are formed.

This phenomena of spreading or bending is called diffraction of wave.

a) Diffraction dominates for longer wavelengths.

b) If the wavelength of the wave is smaller than the dimensions of obstacle diffraction is negligible & the wave behaves like a ray & travels along straight line (called, rectilinear propagation).

c) Diffraction in case of radio waves & sound waves is generally observed, because their wave length is not so small & obstacles/ apertures of theses sizes are readily available.

d) Diffraction with light is generally not observed, because light has very small wavelength (»mm) & obstacles of such small size are readily not available.

^Young’s double slit experiment

January 8, 2022/in Back end /by kp-web-admin

^Young’s double slit experiment

A common experiment to study interference of two light waves is YDSE. In this experiment overlapping of wave fronts of light waves coming from two slits S₁ & S₂ is studied by placing a screen at some distance from the slits. Let slits are separated by a distance ‘d’ & screen is situated ‘D >>d’ distance away from slits. Wavefronts reaching O from S₁ & S₂ are of equal path length produce no phase difference & thus we get maximum intensity at O (called central maxima, CM).

If the overlapping of waves is studied at a point P situated ‘y’ distance above or below the central maxima, then the intensity of the resultant wave depends on the phase difference between the waves S₂P & S₁P. If point P is situated ‘y’ distance above point ‘O’, then the path S₂P is longer than S₁P by an amount d sinθ. As here d << D, thus

thus path difference (p or Δx or simply x) can be expressed as

Phase & path difference for a sinusoidal wave are related as

Using above relation for conditions of maximum intensity we can say that maximum intensity is achieved at following positions from central maxima

Using above relation for conditions of minimum intensity we can say that minimum intensity is achieved at following positions from central maxima

For two waves of equal intensities the intensity of the resultant wave varies as square of the cos of Φ/2 i.e.

Here 4a²is the maximum intensity of the resultant wave at central maxima.

Facts

1. Fringe width of any dark or bright fringe is same & is

2. When interference is studied with white light, each of the seven colours produces its own fringe pattern, having different fringe width & due to overlapping blurred fringes are observed. However central fringe is white, on either side the nearest fringe is blue and farthest fringe is red & then uniform illumination.

3. If a transparent sheet of thickness ‘t’ & refractive index ‘m’ is introduced in one of paths of interfering waves, the entire fringe pattern displaces towards the side in which the sheet is inserted by a distance without any change in the fringe.

Also no. of fringes shifted is

^Interference with independent light sources

December 31, 2021/in Back end /by kp-web-admin

^Interference with independent light sources

Two independent light sources can’t produce a sustained interference, rather they produce uniform illumination on the screen, as the phase of the light emitted by them very rapidly (10⁸ times in every one second) in other words two independent light sources are incoherent.

^Coherent sources

December 31, 2021/in Back end /by kp-web-admin

^Coherent sources

If the positions of constructive & destructive interference remain fixed (or sustained) with time if the phase difference between the overlapping waves remains constant with time, such waves are called coherent & sources producing such waves are called coherent. Coherence is essential condition to observe interference pattern.

Coherence: Φ ∝ time⁰ or constant

^Interference of waves

December 31, 2021/in Back end /by kp-web-admin

^Interference of waves

Let two waves of amplitudes a₁ & a₂ having a phase difference ΔΦ (or simply Φ) superpose (overlap) at a point P. Let R be the amplitude of the resultant wave, from principle of superposition (POS) R is

As for any wave intensity is proportional to square of its amplitude, i.e.

Let I be the intensity of the resultant wave, then

When the intensity the resultant wave becomes maximum, the situation is called constructive interference (CI), physically it implies crest of one wave exactly overlaps with crest of other & trough with trough, this happens if

Here n = ±1, ±2, ±3, _ _ _ i.e. only integers, n =+ve means above O &– ve below O. Also

When the intensity the resultant wave becomes minimum, the situation is called destructive interference, physically it implies crest of one wave exactly overlaps with trough of other, this happens if

As the value of cosf varies between – 1 to + 1, thus intensity of the resultant wave varies between I_min = (a₁– a₂)² to I_max = (a₁+ a₂)² so the average intensity is

This phenomenon of non-uniform distribution of intensity is called interference of waves. It is possible for any no. of waves, however for the simplicity we have discussed it for two waves only. Also it is a property of both types of waves, mechanical & EM wave.

^Myopia versus Hypermetropia

December 30, 2021/in Back end /by kp-web-admin

^Myopia versus Hypermetropia

^Persistence of eye

December 30, 2021/in Back end /by kp-web-admin

^Persistence of eye

Image of an object remains or persists for 0.1 s even after the object has vanished. This time is called persistence of eye.

Now if two objects are shown to eye at a time gap of less than 0.1 s, eye fails to distinguish them separately. This effect is called stroboscopic effect. Movies make use of stroboscopic effect.

^Sensitivity of human eye

December 30, 2021/in Back end /by kp-web-admin

^Sensitivity of human eye

The following graph shows the variation of sensitivity of human eye with the wavelength of light. From the above graph it is clear that the sensitivity of human eye is low for both red & violet colour wavelengths. It is maximum for 555 nm wavelength which corresponds to yellow – green light.

^Human eye

December 30, 2021/in Back end /by kp-web-admin

^Human eye

Eyes ball: 1 inch diameter

Cornea: Front bulged transparent, causes most of the refraction.

Eye lens: Crystalline lens hard at the middle but becomes soft towards the corners & has a average refractive index of about 1.4

Pupil: Adjustable aperture that control the amount of light entering the eye. Its size is adjusted by the iris. In ordinary light the diameter of the pupil is about 2 mm; in dim light it is about 8 mm.

Retina: It is a translucent layer at the curved back surface of the eye composed of about 125 milllion sensitive cells (optical fibers) called rods (sensitive to light) & cones (sensitive to colour) covering the curved back surface the eye & send the information of this image to brain for further processing.

Ciliary muscles: Holding eye lens & adjust the convexity of eye lens so that the near and the far objects are distinctly visible to the eye is called power of accommodation.