# Huygen’s Principle

When you open your window in a room, the light enters through the window and spreads throughout the room. Do you why does this happen? This is because light has got some wave nature, that spreads in the room in all the directions. To understand this in a better way let us study the Huygen’s Principle.

## Huygen’s Principle

It states that each point of the wavefront is the source of the secondary wavelets which spread out in all direction with the speed of a wave. So if we consider a point source, it will emit its wavefront and nature of the wavefront will be spherical one.

As per the Huygen’s principle, all the points on the wavefront are going to become a secondary source. So the wavefronts will in the forward direction. All the secondary sources emit wavelets. Tangent drawn to all the wavelets is the new position of the waveform.

This means that, suppose you are standing on the mountain and you throw a stone in the water from a height. What do you observe? You see that the stone strikes the surface of the water and waves are seen surrounding that point. Every point on the surface of water starts oscillating.

The waves spread in all the direction. Earlier the water was at rest. But the moment we throw the stone in the water, within a few fractions of seconds the disturbance spreads in all directions. There are ripples formed in the water. The ripples form the concentric circle around the disturbance and spread out.

These ripples are nothing but the wavefront. The wavefronts gradually spread in all the directions. So at every point, we have a wave coming out. The primary wavefront is formed and again from the primary wavefront, a secondary waveform is formed and so on. The disturbance does not last for a long time. It fades gradually because more and more waveforms are formed.

## Solved Questions For You

Q1. Ray optics is valid when characteristic dimensions are:

1. of the same order as the wavelength of light.
2. much smaller than the wavelength of light.
3. much larger than the wavelength of light.
4. of the order 1mm

Answer: C. Ray optics is valid when characteristic dimensions are much larger than the wavelength of light so that the rectilinear property of light can be used.

Q2. According to Huygen’s Principle, the ether medium pervading entire universe is:

1. Less elastic and denser
2. Highly elastic and less dense
3. Not elastic
4. Much heavier

Ans: B. Huygen considered light needs a medium to propagate called either as highly elastic and less dense.

Q3. Huygens’s concept of secondary wave

1. allow us to find the focal length of the thick lens
2. it is a geometrical method to find the wavelength
3. used to determine a velocity of light
4. it is used to explain polarisation

Ans: B. Huygens’s concept of secondary wave is a geometrical method to find the wavelength.

Huygen’s principle states that every point on the wavefront may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light. The new wavefront is the tangential surface to all these secondary wavelets. Thus, it is a geometrical method to find the wavelength.

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