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# Tag Archives: Optics

## Huygens Wave Theory and rectilinear propagation of light

As per 12th Physics textbook, it has been mentioned that according to Huygens wave theory of light, light does not travel in straight line.

But I have heard that it is wrong and rectilinear propagation of light is one of the merits of Huygens theory rather than demerit please clear my doubt

The wave nature of light becomes prominent only when the size of the aperture of the obstacle is comparable to the wavelength.

The light of wavelength λ coming through an aperture of width a can be considered to be in rectilinear motion up to a distance of

# ZF= a2/λ,

where ZF is called the Fresnel distance.

As

Thus, wavelength λ decreases, the limit for rectilinear propagation increases.This is the reason why we say, ray optics is valid when the limiting case wavelength λ tending to zero is applied.

## Numerical on Eye defect

A myopic eye has a far point of 3m and a near point of 150mm . State the type of spectacle lens needed for viewing an object at normal far point and determine its power in dioptres . Find the near point when wearing this lens ?

## Optics

“when we exchange the lenses of compound microscope, then why it does not act as a telescope?”

## What accounts for the acceleration or deceleration of light when it travels from one medium to another?

“When light passes through different mediums speed changes but who provide acceleration to change velocity?”

This question was posted by Jishnu

Answer: Just imagine you are participating in a cross country race. When the road is good you can travel faster; but if the path is a marshy place, your speed will decrease. This is just an analogy. To understand it better, you should know that the way light travels in a medium is different from the way it travels in vacuum.

## Speed of Light

“what is the speed of light?”

Ans:

Speed of Light in Vacuum is 3 x 108 m/s

The various experiments conducted to determine the speed of light are

• [wiki]Foucault[/wiki]’s method
• [wiki]Fizeau[/wiki]’s method
• [wiki]Michelson[/wiki]’s method
• … and many more

## LIGHT IS TRANSVERSE

In transverse wave,the particles vibrate perpendicular to the direction of wave propagation.

“light is a transverse wave.”
but light can be propagated without a medium , that is particles.
so, how can light be called as a transverse wave?

Light is an electromagnetic wave which requires no medium for the propagation. In electromagnetic waves including light, electric and magnetic fields vary with time in mutually perpendicular directions and also perpendicular to the direction of propagation.

## A Numerical from Combination of thin lenses in contact

Two thin lenses of focal lengths f1=20cm and f2=60cm are placed in contact. find the focal length f3 of the combined lenses. find also the focal length f of a third lens placed in contact with these two that would result in overall focal length f11=-40cm. (Asked Young)

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