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1. What is Huygen's Principle?

Christian Huygens (1629-1695) was a Dutch Physicist who proposed the wave theory of light long before Maxwell's incredible discoveries. In 1678, while living in Paris (indeed, many Frenchmen seem to have studied the nature of light quite extensively), Huygens published his "Traité de la lumiere" in which he described this wave theory, and the principle named after him comes from this text.

Huygens's Principle:
Every point on a propagating wavefront serves as the source of spherical secondary wavelets, such that the wavefront at some later time is the envelope of these wavelets. If the propagating wave has a frequency, f, and is transmitted through the medium at a speed, v, then the secondary wavelets will have the same frequency and speed.

This principle is quite useful, for from it can be derived the laws of reflection and refraction [the latter often referred to as Snell's Law].

 

 

2. How is light formed in a light bulb?

There are several types of light bulbs and they emit light through different mechanisms. I'll try to explain how incandescent light bulbs work.

A regular incandescent light bulb relies on the fact that all bodies with a temperature greater than absolute zero emit radiation. This radiation is logically named thermal radiation and the intensity of radiation emitted at a given wavelength is a function of the temperature of the body. At low temperatures a body emits very low intensity radiation, mostly of lower energy than visible light. The visible light that is emitted is of far to low intensity to be seen by human eyes. However at high temperature the intensity of visible light (and other radiation) emitted by the body increases. Depending on its specific physical properties a body may emit more or less visible light at a given temperature. Incandescent light bulbs use a thin wire called a filament (traditionally made of tungsten, perhaps currently made of a different material) as a thermal radiation emitting body. The filament has a very tiny axial diameter (it is skinny) and therefore a fairly high resistance to current. When AC current flows through the filament it is heated very rapidly to a very high temperatures (that is why incandescent light bulbs are hot) and it emits lots of thermal radiation in the visible spectrum.

 

 

3. I live in the countryside and I've noticed that when walking home in the twilight the spring blossom on trees--and even a flock of white sheep--appear to almost glow in the near darkness. Is there something special about the twilight that produces this effect or is it just me? It is certainly very beautiful.

This glowing effect is caused by the re-radiation or fluorescence of light at longer (visible) wavelengths from scattered ultraviolet, violet and blue light in the atmosphere. Remember that the daytime sky is blue because of the preferential scattering of blue light known as Rayleigh scattering.

Many flowers fluoresce, and also reflect in near-ultraviolet light. This is beneficial because pollinating insects can see these wavelengths. It is particularly noticeable on flowers of the Geranium family, both the wild species and the cultivated pelargoniums.

Because there is no other source of light present, the fluorescence shows up proportionately more, giving rise to the extra-luminous effect described. It is not an optical illusion; it is a real phenomenon.

 

 

4. The surfaces of the incandescent light bulbs where I work become progressively greyer over time. Why?
The greying of the inner surfaces of incandescent bulbs is the result of gradual evaporation of tungsten from the filament while the light is on. This evaporation eventually makes the filament so thin that it burns out.

Various methods have been developed to reduce greying. Filaments of the first incandescent lamps burnt in a vacuum, but it was soon found that introducing inert gas to the bulb reduced the rate of greying. A mixture of nitrogen and argon is used today. In addition, "getters"--reactive metals such as tantalum and titanium--can be placed near the filament to attract the tungsten so that it is not deposited on the glass. Alternatively, a small amount of abrasive tungsten powder can be placed in the bulb. Shaking it occasionally will remove the grey coating from the surface of the glass.

Greying can be almost eliminated by introducing a small amount of the halogens iodine and bromine. As tungsten evaporates from the filament, it reacts with the halogens which then redeposit the tungsten on the filament. This keeps the bulb wall clean. To prevent the tungsten halides from condensing on the bulb and breaking the cycle, the temperature of the bulb wall must be at least 500 °C. This is too hot for glass bulbs, which normally operate at about 150 °C, so fused quartz (silicon dioxide) must be used instead.

Compared with ordinary incandescent lamps, quartz-halogen lamps have longer lives and maintain their light output over time. For example, a quartz-halogen lamp with a 2000-hour life will have dimmed by less than 5 per cent by the time it burns out. When an incandescent lamp with a 1000-hour life burns out, it will have dimmed by more than 15 per cent.

 

5. If I place two mirrors facing each other and stand between them, I can see my reflection stretching away into what looks like infinity. Is this really infinity? How small can my reflection get in this manner, and can it be said to end in any particular way?
No mirror reflects 100 per cent of the light falling on it. If your mirror is very good and reflects 99 per cent of the light, after some 70 reflections only 50 per cent of the light is left, after 140 reflections only 25 per cent of the light is left, and so on until none of the light remains to travel between the two mirrors. In fact, most mirrors reflect some colours of light much better than others and some colours are absorbed better by the glass, so the multiple reflections that you see not only get darker but also more colour-distorted as they recede towards infinity.

Even with perfect reflection of all colours you could never see infinite reflections, for geometric reasons.

First, the faces of the two mirrors would need to be perfectly parallel. This is actually impossible to achieve. There will always be a slight disparity in their positions relative to one another. Hence the curving nature of such reflections, until eventually the reflection is lost "around the bend".

Secondly, even if the mirrors are perfectly reflective, perfectly parallel, and really huge, your eyes are in the middle of your head, not on its edge. Therefore, at some point, the receding and hence apparently more distant mirror images would become smaller than, and hidden behind, the first reflected image of your head. Even with a tiny camera and a giant pair of mirrors, the reducing reflection size would eventually be smaller than the first reflected image of the camera apparatus.

 

 

6. You read a news paper because of the light that it reflects. Then why don’t you see your image in the newspaper?

The reflected light from the newspaper is diffused. Where do these diffused rays go, is not sure and certain. But on those positions where the letters are imprinted, the color is generally black, so light is absorbed there. So the brain makes a distinction there - locations from where light is being reflected (in a diffused manner) and locations where light is absorbed and from where it is not coming out.

To see the image of an object in the newspaper requires a regular and strong reflection. They should collect in enough quantity after reflection to be able to give and impression to the brain. This is possible only in an extremely smooth surface, not in a rough surface like newspaper.

 

 

7. Can a plane mirror ever form a real image?

Yes, plane mirror can form a real image when the converging rays of the virtual object strike on the plane mirror, a real image of the object is formed.

 

8. If you are bringing a plane mirror towards your face at right angles to your face with a speed of 10 ms-1 at what rate is the image approaching?

If the plane mirror is approaching towards the face with the speed 10ms-1, the face also moves towards the mirror at the same rate. So, the relative velocity of the image with respect to the face is given by 10ms-1 + (-10ms-1) = 20ms-1. Hence the rate at which the image is approaching is 20ms-1.
  
9. Ground glasses do not produce any images, but plane and polished glasses do, why?

The image of an object is produced only if the surface gives the regular reflection, in order to have a good collection of energy and make an impression on the brain. In the case of the plane and polished glasses, this is possible while the reflection from the ground glasses is irregular or diffused. So energy do not accumulate after reflection. So there is not a good impression reaching the brain. Hence, ground glasses do not produce any images but plane and polished glasses do. 

 

 

10. Why do the distant images get fainter and fainter when an object is placed between plane mirrors?

As the object is kept between the two mirrors, it undergoes multiple reflection to produce the distant object. At each reflection, the intensity of the incident light gets lost due to the absorption phenomena. Each reflected beam has lesser energy than the incident. Hence, the distant images get fainter and fainter.

 

11. Can we photograph a virtual image?
 

 
Yes, we can photograph a virtual image. The light rays coming from the virtual image and reaching the camera are real. Virtual image is produced due to from the reflecting surface after reflection of real incident rays. So, the real reflected rays enter the camera to give effects on the photographic film. Hence, we can photograph a virtual image. 
 
12. What type of mirror is used in a car to enable the driver to see the traffic behind him?

The type of the mirror used in a car to enable the driver to see the traffic behind him is the convex mirror. Convex mirror produces erect and diminished imaged of the object. It also gives a wider range of the field of view of the traffic behind. So, convex mirror is used in a car.

 

13. If a spherical mirror is immersed in water, does its focal length change?

As the focal length of the spherical mirror is independent of the refractive index 'µ' of the medium. The focal length of the spherical mirror is given by

 f = R/2

where, 'f' is the focal length and 'R' is the radius of curvature

So, the focal length does not altered by immersing the mirror into the water.

 

14. If an object far away from a convex mirror moves towards the mirror, the image also moves. Does it move faster, slower or at the same speed as compared to the object?
If an object far away from a convex mirror moves towards the mirror, the image also moves. It moves slower as compared to the object.

Consider the situation - When the object is at infinity, the mirror would form the image at the focus. When the object is at

 
15. Can you tell by looking whether an object is real or virtual? How can the two be distinguished?
No, the image cannot be distinguished as a real or virtual by looking. If the image is formed by the actual intersection of converging rays of the virtual object on the mirror then the image is real while if image is formed by the virtual intersection of the rays, the image is virtual. A real image can be projected on the screen whereas the virtual image cannot be projected on the screen.
 
16. Is it possible to find whether a mirror is plane, concave or convex from the nature of image of an object? Explain.

Yes, it is possible to find whether a mirror is plane, concave or convex from the nature of image of an object. If the image is virtual and the size of the object is equal to the size of the image i.e. the magnification observed is always unity then the mirror is plane.

If the size of the image is larger than the object when the mirror kept closer to a person is slightly displayed away, then the mirror is concave mirror.

If the size of the image is smaller than the object when the mirror kept closer to a person is slightly displayed away, then the mirror is convex mirror.

 
17. A convex and a concave mirror are fitted on a wall. How will you distinguish between them without touching?

A concave and a convex mirror are distinguished easily by viewing the nature of the image produced by it, of the object without touching it. If the image of the object is magnified erect image when the object kept close to the mirror then the mirror is concave but if a diminished erect image is formed then the mirror is convex.
 

18. Does the focal length of a curved mirror depend on medium in which it is placed?

No, the focal length of a curved mirror does not depend on medium in which it is placed.

The focal length of the spherical mirror is given by

 f = R/2

where, f is the focal length and R is the radius of curvature.

So, the focal length does not altered by placing the spherical mirror into any medium as the focal length of the spherical mirror is independent of the refractive index µ of the medium.

 

19. Is it possible that the absolute refractive index of a medium to be less than 1?

Absolute refractive index µ of any medium is given by

 µ = c/v

From the above relation, the velocity of the light in any medium is always less than the velocity of the light in the air or in the vacuum (i.e. c>v). So, the value of µ is always greater than unity. Thus, it is impossible that the absolute refractive index µ of any medium to be less than 1.

20. Which color of light travels fastest in a medium?

As the value of the refractive index µ for the red light is minimum due to its maximum value of the wavelength as compared to the rest of the colors, the red light travels fastest in the medium in accordance to the following relation

 µ = c/v

where, µ is the refracting index of the medium.

 c is the velocity of the light in air or vacuum.

 v is the velocity of the light in the medium and it is given by the relation

 v = f (λ)

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