Refraction & Reflection of Plane Waves using Huygens Principle

# Refraction & Reflection of Plane Waves using Huygens Principle | Physics Class 12 - NEET PDF Download

As we know that when light falls on an object, it bends and move through the material, this is what refraction is. Also when the light bounces off the medium it is called a reflection. Let us know study reflection and refraction of waves by Huygen’s principle.

Reflection using Huygens Principle

We can see a ray of light is incident on this surface and another ray which is parallel to this ray is also incident on this surface.  Plane AB is incident at an angle ‘ i ‘ on the reflecting surface MN.  As these rays are incident from the surface, so we call it incident ray. If we draw a perpendicular from point ‘A’ to this ray of light, Point A, and point B will have a line joining them and this is called as wavefront and this wavefront is incident on the surface.

These incident wavefront is carrying two points, point A and point B, so we can say that from point B to point C light is travelling a distance. If ‘ v ‘ represents the speed of the wave in the medium and if ‘ r ‘ represents the time taken by the wavefront from the point B to C then the distance

BC = vr

In order the construct the reflected wavefront we draw a sphere of radius vr from the point A. Let CE represent the tangent plane drawn from the point C to this sphere. So,

AE = BC = vr

If we now consider the triangles EAC and BAC we will find that they are congruent and therefore, the angles ‘ i ‘ and ‘r ‘ would be equal. This is the law of reflection

### Refraction using Huygen’s principle

We know that when a light travels from one transparent medium to another transparent medium its path changes. So the laws of refraction state that the angle of incidence is the angle between the incident ray and the normal and the angle of refraction is the angle between the refracted ray and the normal.
The incident ray, reflected ray and the normal, to the interface of any two given mediums all lie in the same plane.  We also know that the ratio of the sine of the angle of incidence and sine of the angle of refraction is constant.

A plane wave AB is incident at an angle i on the surface PP' separating medium 1 and medium 2. The plane wave undergoes refraction and CE represents the refracted

wavefront. the figure corresponds to v2 < v1 so that the refracted waves bends towards the normal.

We can see a ray of light is incident on this surface and another ray which is parallel to this ray is also incident on this surface. As these rays are incident from the surface, so we call it incident ray.
Let PP’ represent the medium 1 and medium 2. The speed of the light in this medium is represented by v1 and v2. If we draw a perpendicular from point ‘A’ to this ray of light, Point A, and point B will have a line joining them and this is called as wavefront and this wavefront is incident on the surface.
If  ‘ r ‘ represents the time taken by the wavefront from the point B to C then the distance,

BC = v1r

So to determine the shape of the refracted wavefront, we draw a sphere of radius v2r from the point A in the second medium. Let CE represent a tangent plane drawn from the point C on to the sphere. Then, AE = v2r, and CE would represent the refracted wavefront. If we now consider the triangles ABC and AEC, we readily obtain

where’ i ‘ and ‘ r ‘ are the angles of incidence and refraction, respectively. Substituting the values of v1 and v2 in terms of we get the Snell’s Law,

n1 sin i = n2 sin r

The document Refraction & Reflection of Plane Waves using Huygens Principle | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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## Physics Class 12

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## FAQs on Refraction & Reflection of Plane Waves using Huygens Principle - Physics Class 12 - NEET

 1. What is refraction and reflection of plane waves?
Ans. Refraction is the bending of a wave as it passes from one medium to another, while reflection is the bouncing back of a wave after striking a surface. In the context of plane waves, refraction and reflection refer to the behavior of the wavefronts as they encounter different mediums or surfaces.
 2. How does Huygens Principle explain refraction and reflection of plane waves?
Ans. According to Huygens Principle, every point on a wavefront acts as a source of secondary spherical wavelets. These secondary wavelets combine to form the new wavefront. In the case of refraction, the change in speed of the wavefront as it enters a new medium causes the wavefront to change direction. In reflection, the wavefront encounters a surface and the secondary wavelets bounce back, resulting in the reflection of the wave.
 3. Can you explain how refraction and reflection affect the direction of plane waves?
Ans. Refraction causes a change in the direction of plane waves when they pass from one medium to another. This change occurs due to the change in speed of the wavefront in different mediums. The angle at which the wavefront enters the new medium, known as the angle of incidence, determines the angle at which it is refracted, known as the angle of refraction. Reflection, on the other hand, causes the wavefront to bounce back in the opposite direction when it encounters a surface.
 4. What factors determine the extent of refraction and reflection in plane waves?
Ans. The extent of refraction depends on the change in speed and the angle of incidence of the wavefront. The greater the difference in speed between the two mediums, the greater the change in direction of the wavefront. Additionally, the angle of incidence plays a role in determining the angle of refraction. The extent of reflection depends on the nature of the surface encountered by the wavefront, as well as the angle of incidence. Smooth surfaces tend to have more reflection compared to rough surfaces.
 5. How are refraction and reflection of plane waves useful in practical applications?
Ans. Refraction and reflection of plane waves have numerous practical applications. For example, in optics, lenses are designed based on the principles of refraction to focus or diverge light. In telecommunications, the reflection of radio waves off the ionosphere allows for long-distance communication. Refraction is also used in the field of medicine for techniques such as ultrasound imaging and laser eye surgery.

## Physics Class 12

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