LIGHT

NATURE OF LIGHT AND SOURCES OF LIGHT

1. What are the different theories which explain the nature of light
Theories that explain the nature of light are:-
1. Newton’s corpuscular theory.
2. Huygens’ wave theory.
3. Maxwell’s electro magnetic theory.
4. Planck’s Quantum theory.
5. Einstein’s Photon theory.

2. Newton’s corpuscular theory of light:-
Newton proposed corpuscular theory of light. According to this theory:-
1. Light consists of a stream of tiny, light and perfectly elastic particles called ‘corpuscles’.

2. These corpuscles are emitted by a luminous source like sun, candle, electric lamp etc.,.

3. The corpuscles travel in all directions in straight lines.

4. The velocity of corpuscles is different for different dispersive media.

5. When these corpuscles fall on the retina of the eye, they cause sensation of vision.

6. The corpuscles can be of different sizes and the various colours of light are supposed to be due to difference in their sizes.

3. How do you explain the reflection of light by corpuscular theory

1. According to Newton, the velocity ‘V’ of corpuscles can be resolved into two components along two mutually perpendicular directions one (Vx) parallel to the reflecting surface and the other (Vy) normal to it.


2. When corpuscles approach a reflecting plane surface PQ the parallel component Vx is unaffected.

3. Close to the plane surface, the corpuscles are repelled such that their normal component of velocity (Vy) decreases to zero gradually and then increases to the original value in the opposite direction.

4. As the parallel component (Vx) remains unaltered, the resultant velocity remains the same as before.

5. From this, it follows that the angle of reflection (r) is equal to the angle of incidence(i).

4. How do you explain the refraction of light by the corpuscular theory

1. According to Newton, when corpuscles of light approach a refracting surface, they are attracted near the surface.

2. When they enter the denser medium (2) from a rarer medium (1) the normal component of the velocity (Vy) increases in the denser medium while the parallel component (Vx) remains unaltered.

3. The increase in the velocity of normal component (Vy) causes the stream of corpuscles to bend towards the normal in the denser medium (2).

4. From this, it follows that the velocity of light in a denser medium is higher than that of a rarer medium.

5. Failures of Newton’s corpuscular theory:-
1. The assumption by Newton that the velocity of light in a denser medium is greater than that in a rarer medium has been proved to be wrong.

2. The assumption that the differences in sizes of corpuscular give rise to different colours of light has no justification.
3. Corpuscular theory could not explain the experimentally observed phenomena of interference, diffraction and polarisation of light.

6. Huygens’ wave theory of light:-
Huygens assumed that the light travels in the form of waves from a source. This is called wave theory of light.

Assumptions:-
1. The propagation of a light wave requires a material medium. According to Huygens, the medium is an all pervading, subtle and elastic hypothetical medium called ‘ether’.

2. A luminous source sends out light energy uniformly in all directions, causing disturbance in the surrounding ether medium.

3. These disturbances travel through ether medium in the form of longitudinal mechanical waves.

6. Huygens’ principle:-
Huygens’ principle states that every point of a wave front behaves as the source of small secondary wavelets which spread out in all directions with a velocity equal to the velocity of light. The new wave front is then found by constructing a surface tangential to the secondary wavelets.

Explanation:-

1. Consider a point source of light (s) .

2. It sends out spherical wave fronts around it, as shown in the figure.

3. Let AB represent a spherical wave front at any instant
.
4. According to Huygens’ principle each point on this wave front AB acts as a source of light called secondary source.
5. The light emerges from these secondary sources again as small spheres called wavelets.

6. The radius of each of these spheres is given by ct, where C is the velocity of light.

7. A surface tangential to all these secondary wavelets gives an envelope CD. This CD gives the new position of wave front AB after a short interval ‘t’.

8. This process of construction of new wave fronts at different positions can be continued. Thus, light advances in the direction perpendicular to wave front.

7. Differences between Newton’s corpuscular theory and wave theory of light

Newton’s corpuscular theory Wave theory of light
1.  It assumes that light consist of a stream of extremely small particles called corpuscles.

 

 

1.  It assumes that light travels in the form of a wave.
2.  The rectilinear propagation of light is explained by the straight line motion of corpuscles. 2.  The rectilinear propagation of light is explained by the advancement of wave front along the direction of normal drawn to it.
3.  The colours of light are due to difference in the sizes of corpuscles. 3.  The colours of light are  due to differences in the wavelengths.
4.  The reflection and refraction of light are explained by the repulsion and attraction of the corpuscles by the medium. 4.  The reflection and refraction are explained by the construction of secondary wave fronts applying Huygens’ principle.
5.  It cannot explain the phenomena of interference, diffraction and polarisation of light. 5.  It can explain the phenomena of interference, diffraction and polarisation of light.
6.  It predicts that the velocity of light in a denser medium is greater than that in a rarer medium. 6.  It correctly proves that the velocity of light in a denser medium is less than that in a rarer medium.

 

8. Describe a ripple tank. How does it help in under standing reflection and refraction of light

Description:-

1. The ripple tank essentially consists of a rectangular shaped trough (tank) made of transparent glass or Perspex material.

2. This tank containing water is held at a height with the support from four legs.

3. Below the tank a white paper is spread on the floor.

4. Above the tank, a partially covered electric bulb is fixed to illuminate the tank.

5. A small needle (N) is fixed at one end of a metallic strip connected to an electric vibrator.

6. The tip of the needle is kept vertically is contact with the water surface in ripple tank.

Reflection of waves:-

7. A circular water wave produced by the vibrating tip of the needle in the ripple tank travels forward. Such circular waves with origin at ‘S’ are shown in the figure.

8. When such circular waves with ‘S’ as the centre move forward and meet an obstacle like the vertical flat wall (AB) of the ripple tank, they change their direction of motion and start moving in the opposite direction as shown in the figure.

9. These reflected waves are also circular in shape.

10. The centre of the reflected waves will be on the other side of the wall at ‘I’ . This point ‘I’ behaves as the image of the point ‘S’.

11. The distance (IO) of this centre ‘I’ of a reflected wave from the wall will be equal to the distance (OS) of centre ‘S’ of the incident wave from the wall.

12. This proves that image distance is equal to the object distance as in the case of reflection of light from a plane mirror.

1. In a ripple tank two regions A and B can be created by introducing a pile of glass plates in region ‘B’.

2. Thus, region ‘B’ is a shallow water region while region A is a deep water region.
3. Thus, the two regions behave as two media for water waves.
4. It is known that deeper the water, higher the velocity of waves.
5. Since the frequency (ν) of the vibrating source remains constant, the velocity (V1) of water waves in the deeper region A will be greater than the velocity (V2) in the region B i.e., V1>V2. Therefore, from the relation V=νλ, it is seen that λ1>λ2.

6. As far as bending of the waves is concerned, the deep water region (A) behaves as a denser medium (say glass) and the shallow water region (B) behaves as a rarer medium (say air ) because the waves in region B bend away from the normal ON.
7. However, if we consider the velocities, A behaves as a rarer medium and B as a denser medium, because V2 in B is less than V1 in A.

8. In any case, the bending of waves (water waves or light waves) or the change in their velocities at the boundary separating two media is due to the phenomenon of refraction of waves.

9. Explain the formation of bright and dark bands on the paper below the ripple tank

Formation of bright and dark bands due to crests and troughs in a ripple tank

1. The crest of the water wave behaves as a convex lens and the trough as a concave lens.

2. The light incident on the tank therefore is transmitted on to the paper on the floor differently by crests and troughs.

3. In the case of a crest, the light converges and forms a bright band as shown in the figure.

4. In the case of a trough the light diverges and forms a dark band.

5. Thus, a light pattern consisting of successive bright and dark bands can be seen on the paper below the ripple tank.

10. State and Explain the principle of super position of waves

Statement:-
The principle of super position of waves states that when two or more waves travel through the same portion of a medium simultaneously, the resultant displacement at any point is the vector sum of the displacements due to individual waves.

Principle of super position of waves:-

1. A vertical vibrating pin in contact with water surface in the ripple tank sends out circular waves.

2. If another pin, adjacent to the first one, is made to vibrate at the same time with the same frequency the waves due to the second pin also travel through the same space independent of other waves.

3. As these wave trains from the two sources travel through the same space simultaneously, the water waves observed on the surface in the ripple tank are the result of super position of the individual wave trains.

4. The resultant wave will have an amplitude which is the sum of the displacements due to the individual waves.

5. If Y1 and Y2 are the displacement at the point due to first and second sources, then the resultant displacement(R) at a point is given by
R=Y1+Y2
For constructive superposition
R=2Y (where Y=Y1=Y2)
For destructive super position
R=Y1-Y2(where Y=Y1=Y2)
R=0

11. Explain how the phenomenon of interference of water waves can be demonstrated in a ripple tank? What are nodal and anti-nodal lines

Interference of water waves in a ripple tank
N-nodal line, AN-Anti nodal line
1. The interference of water waves in a shallow ripple tank can be produced if we take two identical adjacent pins (S1 and S2) vibrating with the same frequency.

2. When the pins vibrate crests and troughs in the form of circular wave from both sources travel forward on the surface of water and interfere.

3. Under the illumination of the ripple tank, a pattern called ‘interference pattern’ can be observed on the plane white paper spread below the tank.

4. The portions of circular waves consisting of crests are shown by solid lines while the portions due to troughs are shown by dotted lines.

5. Thus, at both the points A and B maximum disturbance (amplitude) occurs and the superposition is therefore a constructive interference. The points A and B (Shown by symbol cross) will appear relatively bright on the paper.

6. Now consider the point C. Here, the trough of the wave from S1 is superposed on a crest of a wave from S2. Similarly, at a point D superposition of a crest from S1 and a trough from S2 takes place. These crests and troughs at both the points C and D destroy each other and give rise to a minimum disturbance (amplitude). Thus at both the pints C and D destructive interference occurs and the points like C and D (shown by open-circles) will appear relatively dark on the paper.

7. All the points A,B fall on a line called anti-nodal line (AN) meaning maximum disturbance. All the points like C,D fall on a line called nodal line (N) meaning minimum disturbance.
8. The formation of anti-nodal and nodal lines results in relatively bright and dark regions called interference pattern of waves.

12. Distinguish between interference and diffraction pattern of light

Interference Diffraction
1.  Interference is due to super position of wave lets of different wave fronts. 1.  Diffraction is due to superposition of wavelets of the same wave fronts.
2.  Alternate bright and dark bands of equal width are observed. 2.  Alternate bands of maximum and minimum brightness are observed.  They are of  unequal width.
3.  The intensities of all bright bands (maximum) or dark bands (minimum) are same. 3.  The intensities of bands gradually fall off from the geometric shadow.
4.  No diffraction bands are observed in  interference. 4.  Interference band can be observed in diffraction.

13. Light ray:-
An imaginary line drawn normal to any wave front represents the path along which the light travels. The path is called a light ray.

14. What is diffraction
The bending of light waves around an obstacle whose dimensions are comparable to the wave length of the incident light and hence its spreading into the geometrical shadow is called diffraction.
Or
The bending of a wave or it deviation from its original direction of propagation when it meets a small obstacle is called diffraction.

15. Explain the diffraction of water in ripple tank at
a) an aperture.
b) a straight-edge.

a) An aperture:-

1. Take a ripple tank. Let the tip (S) of the needle connected to the electrical vibrator produce circular wave fronts traveling along (P) , the direction is shown in the figure.

2. Let X,Y be two vertical glass plates immersed in the tank far away from the vibrating source (S), such that there is an opening AB between them.
3. Then, a plane wave front MN would be incident on AB where the edges A and B act as obstacles.

4. Any opening such as AB is called an ‘aperture’.

5. The portion of the wave front incident on AB escapes through the aperture and results in a number of circular wave fronts which spread out in all directions as if they have originated in the aperture AB.

6. We observe that the waves emerging from AB deviate from their original direction of propagation along P and travel along other directions like Q and R.

7. The formation of circular wave fronts and their propagation in a direction different from the original direction is due to bending of waves. This phenomenon is called ‘diffraction’.

b) A straight-edge:-

1. Let an obstacle like a half-meter scale be placed in the tank such that its length is in contact with water surface but its width perpendicular to its surface.

2. Such an obstacle to the water-waves is called a ‘straight-edge’.

3. In the above figure, A is the tip of the vibrating needle. This behaves as the source of circular water waves which spread out and travel towards the scale, BD.

4. The waves spread out equally in all directions until they reach the straight-edge BD.

5. If there were to be no bending of the waves, the lower portion of the circular wave fronts should be completely cut off and only the upper portion above the straight line BC should travel beyond the scale or straight-edge BD.

6. But some rather weak waves spread out into the shadow region below BC as shown in the figure. This bending of water waves is called ‘diffraction’ at straight-edge.

News Reporter