# UPSC Physics Light Light

## Light

Category : UPSC

Light

1.           Concave and Convex Mirror

• Image formed by a plane mirror is always virtual and erect. The size of the image is equal to that of the object. The image formed is as far behind the mirror as the object is in front of it. Further, the image is laterally inverted.
• Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light. They are often used as shaving mirrors to see a larger image of the face.
• The dentists use concave mirrors to see large images of the teeth of patients. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
• We can see a full-length image of a tall building/tree in a small convex mirror.
• Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving. Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

2.           Refraction of Light

• We might have observed that the bottom of a tank or a pond containing water appears to be raised. Similarly, when a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab. Why does it happen?
• We seen a pencil partly immersed in water in a glass tumbler? It appears to be displaced at the interface of air and water.
• We might have observed that a lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides.
• Let us consider the case of the apparent displacement of a pencil, partly immersed in water. The light reaching us from the portion of the pencil inside water seems to come from a different direction, compared to the part above water. This makes the pencil appear to be, .displaced at the interface.
• These observation indicate that light does not travel in the same direction in all media. It appears that when travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes.
• The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

3.           Total internal reflection in nature and its technological applications

• Mirage is common in hot deserts. Some of you might have noticed that while moving in a bus or a car during a hot summer day, a distant patch of road, especially on a highway appears to be wet. But, we do not find any evidence c. wetness when we reach that spot. This is due to mirage.
• Diamonds are known for their spectacular brilliance. Their brilliance is mainly due to the total internal reflection of light inside them. The critical angle for diamond-air interface is very small, therefore once light enters a diamond, it is very likely to undergo total internal reflection inside it.
• Diamonds found in nature rarely exhibit the brilliance for which they are known. It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly. By cutting the diamond suitably, multiple total internal reflections can be made to occur.
• Prisms designed to bend light by or by  make use of total internal reflection.
• Now-a-days optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection. Optical fibres are fabricated with high quality composite glass/quartz fibres. Each fibre consists of a core and cladding. The refractive index of the material of the core is higher than that of the cladding.

4.           Dispersion by a Prism

• It has been known for a long time that when a narrow beam of sunlight, usually called white light, is incident on a glass prism, the emergent light is seen to be consisting of several colours. There is actually a continuous variation of colour, but broadly, the different component colours that appear in sequence are : violet, indigo, blue, green, yellow, orange and red (given by the acronym VIBGYOR). The red light bends the least, while the violet light bends the most.
• The phenomenon of splitting of light into its component colours is known as dispersion. The pattern of colour components of light is called the spectrum of light.
• Colour is associated with wavelength of light. In the visible spectrum, red light is at the long wavelength end (—700 nm) while the violet light is at the short wavelength end (-400 nm). Dispersion takes place because the refractive index of medium for different wavelengths (colours) is different.
• For example, the bending of red component of white light is least while it is most for the violet. Equivalently, red light travels faster than violet light in a glass prism.
• Thick lenses could be assumed as made of many prisms, therefore, thick lenses show chromatic aberration due to dispersion of light.
• The variation of refractive index with wavelength may be more pronounced in some media than the other. In vacuum, of course, the speed of light is independent of wavelength. Thus, vacuum (or air approximately) is a non-dispersive medium in which all colours travel with the same speed. This also follows from the fact that sunlight reaches us in the form of white light and not as its components. On the other hand, glass is a dispersive medium.

Note - approximately (~), Nano meter (nm).

5.           The rainbow

• The rainbow is an example of the dispersion of sunlight by the water drops in the atmosphere. This is a phenomenon due to combined effect of dispersion, refraction and reflection of sunlight by spherical water droplets of rain.
• The conditions for observing a rainbow are that the sun should be shining in one part of the sky (say near western horizon) while it is raining in the opposite part of the sky (say eastern horizon). An observer can therefore see a rainbow only when his back is towards the sun.
• Sunlight is first refracted as it enters a raindrop, which causes the different wavelengths (colours) of white light to separate. Longer wavelength of light (red) are bent the least while the shorter wavelength (violet) are bent the most.
• Next, these component rays strike the inner surface of the water drop and get internally reflected if the angle between the refracted ray and normal to the drop surface is greater than the critical angle (48°, in this case). The reflected light is refracted again as it comes out of the drop as shown in the figure.
• It is found that the violet light emerges at an angle of 40° related to the incoming sunlight and red light emerges at an angle of 42°. For other colours, angles lie in between these two values.
• When light rays undergoes two internal reflections inside a raindrop, instead of one as in the primary rainbow, a secondary rainbow is formed as shown in. It is due to four-step process. The intensity of light is reduced at the second reflection and hence the secondary rainbow is fainter than the primary rainbow. Further, the order of the colours is reversed.

6.           Scattering of light

• As sunlight travels through the earth's atmosphere, it gets scattered (changes its direction) by the atmospheric particles. Light of shorter wavelengths is scattered much more than light of longer wavelengths. The amount of scattering is inversely proportional to the fourth power of the wavelength. This is known as Rayleigh scattering.
• Hence, the bluish colour predominates in a clear sky, since blue has a shorter wavelength than red and is scattered much more strongly. In fact, violet gets scattered even more than blue, having a shorter wavelength. But since our eyes are more sensitive to blue than violet, we see the sky blue.
• At sunset or sunrise, the sun's rays have to pass through a larger distance in the atmosphere. Most of the blue and other shorter wavelengths are removed by scattering. The least scattered light reaching our eyes, therefore, the sun looks reddish. This explains the reddish appearance of the sun and full moon near the horizon.

7.           Does Light Travel in a Straight Line?

• Ray optics is based on rectilinear propagation of light, and deals with mirrors, lenses, reflection, refraction, etc.
• Light travels as a wave that it can bend around objects, it can diffract and interfere, etc.
• In optical region, light has a wavelength of about half a micrometre. If it encounters an obstacle of about this size, it can bend around it and can be seen on the other side. Thus a micrometre size obstacle will not be able to stop a light ray. If the obstacle is much larger, however, light will not be able to bend to that extent, and will not be seen on the other side.
• This is a property of a wave in general, and can be seen in sound waves too. The sound wave of our speech has a wavelength of about 50 cm to 1 m. If it meets an obstacle of the size of a few metres, it bends around it and reaches points behind the obstacle- But when it comes across a larger obstacle of a few hundred metres, such as a hillock most of it is reflected and is heard as an echo.

8.           Braille System

• The most popular resource for visually challenged persons is known as Braille.
• Louis Braille, himself a visually challenged person, developed a system for visually challenged persons and published it in 1821.
• Braille system has 63 dot patterns or characters. Each character represents a letter, a combination of letters, a common word or a grammatical sign. These patterns when embossed on Braille sheets help visually challenged to recognise words by touching. To make them easier to touch, the dots are raised slightly.
• Some visually challenged Indians have great achievements to their credit. Diwakar a child prodigy has given amazing performances as a singer. Mr. Ravindra Jain, born completely visually challenged, obtained his Sangeet Prabhakar degree from Allahabad. He has shown his excellence as a lyricist, singer and music composer. Mr. Lal Advani, himself visually challenged, established an Association for special education and rehabilitation of disabled in India. Besides, he represented India on Braille problems to UNESCO.

9.         Important Facts

• A simple magnifier or microscope is a converging lens of small focal length. For much larger magnifications, one uses two lenses, one compounding the effect of the other. This is known as a compound microscope.
• The telescope is used to provide angular magnification of distant objects. Modern telescopes use a concave mirror rather than a lens for the objective.
• In 1678, the Dutch physicist Christiaan Huygens put forward the wave theory of light.
• Animals have eyes shaped in different ways. Eyes of a crab are quite small but they enable the crab to look all around. So, the crab can sense even if the enemy approaches from behind.
• Butterfly has large eyes that seem to be made up of thousands of little eyes. It can see not only in the front and the sides but the back as well.
• A night bird (owl) can see very well in the night but not during the day. On the other hand, day light birds (kite, eagle) can see well during the day but not in the night. The Owl has a large cornea and a large pupil to allow more light in its eye. Also, it has on its retina a large number of rods and only a few cones. The day birds on the other hand, have more cones and fewer rods.
• Nature has endowed the human eye (retina) with the sensitivity to detect electromagnetic waves within a small range of the electromagnetic spectrum. Electromagnetic radiation belonging to this region of the spectrum (wavelength of about 400 nm to 750 nm) is called light.
• Light travels with enormous speed and second, that it travels in a straight line. Its presently accepted value in vacuum is $c=2.99792458\times {{10}^{8}}$m s-1. For many purposes, it suffices to take $c=\,3\times {{10}^{8}}$ m s~1. The speed of light in vacuum is the highest speed attainable in nature.

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