What makes a mushroom sprout out of soil soon after monsoon but not after sprinkling water?

A majority of mushrooms that sprout directly from soil during the monsoons belong to the Tricholomataceae family. Calocybe indica, a beautiful and white coloured umbrella shaped mushroom is the most prominent among them. This genus originated from the forests of West Bengal. Today, its variants are cultivated commercially under the popular name 'Milky mushroom'.

Mushroom spores are hyaline, broadly ellipsoidal, thin-walled with prominent apiculis. Under favourable conditions such as low temperature, high relative humidity and faded light, the spores germinate and the mycelial spread resumes. It takes nearly 50 days for the completion of the mycelial spread and subsequent emergence of the sporophores. During the entire period of mycelial spread, very high relative humidity, 25-30 degree Celsius temperature and faint light are necessary. Unlike sprinkling water, monsoon changes the entire microclimate. Sprinkling cannot reduce the atmospheric temperature, control sunshine or maintain high relative humidity for long time whereas the monsoon's prominent effects will be the high or stable relative humidity coupled with decrease in the intensity of sunshine, which are necessary for spawn spread. Mushroom beds, after inoculation with mushroom spawn, should be kept for 30 days under darkness with high atmospheric humidity. After the mycelial spread the plastic cover of the bed is removed, sterilised casing mixture composed of equal quantities of soil, sand and cowdung is loosely spread over the bed. Only faded light is permitted but high humidity is maintained. After about two weeks mushroom start sprouting.

Source: The Hindu

Does the permanent tooth form only after the milk tooth falls down or is it formed already?

The permanent tooth is not formed after the loss of primary tooth. Normally there are tooth buds in the developing foetus itself. These tooth buds, sometimes called the tooth germs, are an aggregation of cells, which eventually form the tooth. A whole lot of processes happen inside the tooth bud to form a tooth. This phenomenon is generally referred to as calcification and this calcification of primary teeth takes place between 13 and 16 months of postnatal life. There are ten sets of primary tooth buds in both the jaws. After complete calcification the primary dentition takes two to three years to emerge into the oral cavity.

The transition from primary to permanent teeth begins at about 6 years of age. The permanent tooth buds stay beneath the primary teeth. The calcification process starts in the permanent tooth too. For the permanent teeth there are 16 sets of tooth buds, each forming at different years of age. The crown portion of the permanent teeth starts to form and this crown portion is formed by enamel. There are four centres for formation of each tooth. All these centres coalesce to form a lobe, which forms the crown of the tooth. After the crown is formed, root formation begins. At the cervix of the crown, cemetum accumulates for the root to be formed. Once root development takes place in the permanent teeth underneath the primary teeth, this initiates resorption or eroding of the roots of the primary teeth by cells called osteoclasts. This resorption of primary teeth roots causes exfoliation of the primary teeth, which is normally referred to as tooth shake. Once the primary tooth falls off, the permanent tooth starts to emerge through the gingival or the gums. The first permanent tooth that erupts is the lower first molar at the age of 6 years. Gradually all primary teeth begin to exfoliate and permanent teeth emerge into the oral cavity.

Source: The Hindu

Why do raindrops falling on a windowpane not always run down straight?

A water drop on a vertical windowpane is pulled down by gravity and should run down straight if there is no other force. Adhesive forces between the glass and water keep a small drop stuck to the glass surface. There is also the surface tension acting at the free surface between the drop and air. Due to surface tension the free surface behaves like a stretched membrane and this gives shape to the drop. Similar surface energies are present between the windowpane and water and also between windowpane and air. These three surface energies fix the contact angle of the drop surface at the contact line. Contact angle, surface tension and gravity decide the shape of the drop. As a drop rolls down there is a hairpin shaped contact line on the windowpane. Most of the water is concentrated at the head with a tail of thin film between the two boundaries of the contact line. When the drop rolls down further there is an increase in the surface area. Creating more surface area needs additional work. If there is already a water-film or drop, this additional work is not needed and water takes this path of least resistance. If such a film downstream does not exist, the complex shape of the drop's head affects the direction of progress. This direction need not always be straight. Surface waviness, dust particles acting as barriers, impurities present, which locally change the contact angle and surface tension, thus modify the direction in which the water stream progresses. Path of raindrops on a windowpane is also affected by the wind.

Source: The Hindu

Why does a coin go in a zigzag fashion and not straight down when dropped in a bucket of water?

When a coin is dropped in a bucket of water, the free fall is resisted by friction or drag due to the water. This force consists of two parts: 
  1. Due to pressure acting perpendicular to the coin surface
  2. Due to viscous forces acting along the coin surface.
The first part is the pressure drag and the second part is called the skin friction drag. Note that the pressure acting on the coin is not just hydrostatic (which generates the Archimedes buoyancy force) but pressure modified by the motion of the coin itself. Skin friction also depends on the motion of the coin. It is usually smaller than the pressure drag unless the body under consideration is highly streamlined like an aircraft wing. Vortices are formed behind (lee side) the falling coin and they grow in size and get detached periodically. This formation and shedding of the vortices drastically influences the pressure distribution on the coin, especially on the leeward side. This may lead to a sudden change in the motion of the coin including its rotation and hence a zigzag motion. 

The skin friction drag is small on the coin but it affects the vortex shedding and hence the pressure distribution on the coin. It is easy to imagine how complicated this water motion can be. A coin dropped in air, say from a high-rise building, is subjected to similar motion but the frictional forces are smaller due to low air density. We observe similar motion when the leaves are falling from a tree, especially in the absence of wind.Even a coin released carefully in a bucket of water starting from a vertical position soon turns inclined and then tumbles. The laws governing the motion of the coin and the fluid (Newton's laws for a fluid are known as the Navier-Stokes equations) are known but it is difficult to solve them accurately even using a supercomputer to predict the exact motion of this coin. But it is fun to release a coin gently in a bucket of water and try to hit a particular spot at the bottom.

Source: The Hindu

Why does a candle flame take a teardrop shape?

The flame in a candle is caused by the burning of the wax, a process that liberates a large amount of energy in the form of heat. This heat, in turn, excites the molecules and atoms in the air and the carbonaceous combustion products of the burnt wax. These excited atoms and molecules get de-excited and emit the light we see. The flame is, thus, a collection of highly heated gas atoms and molecules, which having lower density than the surrounding air, lifts itself up. During this, it goes farther from the source of heat and gets cooled by transferring heat to the surrounding air, which rushes from the neighbourhood. As the height increases, the cool air exerts a transverse pressure on the flame from all sides making it teardrop-shaped. 

The progressively increased effective cooling of the flame at higher levels from the tip of the candle can be demonstrated by a simple experiment. Take a flat-bottomed plate with some ordinary water and hold it for a short while at about half the length of the original flame. Take care not to hold the plate for too long. You observe that the central portion has no mark but the outer regions have the black mark of the carbon soot indicating the cooler gas being at the periphery of the flame compared with the inner region. The hotter gas in the inner region converts all the carbon into CO{-2} but not at the periphery. Hence black soot is formed. Next, hold the plate above the flame and you will collect a smaller soot mark without the clear middle region. This is because the hot gas has got cooled by the time it has reached the top and the carbon in the gas escapes conversion into CO{-2}. The teardrop shape of the flame is mainly because of the transverse air movement around the flame.On Earth, gravity-driven buoyant convection causes the teardrop shape. In microgravity, where convective flows are absent, the flame is spherical.

Source: The Hindu

The glass becomes hot when an electric bulb is kept switched on. It is not so in the case of a tube light. Why?

When a current is passed through the filament of an electric bulb, the filament gets heated to very high temperature (about 3000 degrees centigrade) when it emits light according to the laws of black body radiation. Note that, in the electric bulb the power dissipated gets converted to both heat and light. In fact, only a small fraction of the power is available to us as light and the rest appears as heat, which heats up the glass bulb.

However, in a tube light, the electrons emitted by the filaments at the ends of the tube excite the gas atoms/molecules inside the tube through an electric discharge process. These excited molecules and atoms radiate UV radiation which cause fluorescence of the white phosphor coated on the inside wall of the glass tube. The light emitted is not due to black body radiation. Actually, the colour of light obtained from a tube light would correspond to a colour temperature of about 6,500 degrees Kelvin. This only represents the emission colour. It may be pointed out here that this process of fluorescent light emission does not require heat as input; therefore there is no heating of the surrounding glass tube.

Source: The Hindu

Why does fresh tamarind become dark in colour after some months, but not when stored in air-conditioned godowns?

The tamarind fruit pericarp (pulp) contains several phenolic compounds most of which are proantho cyanidins. In addition, tamarind pulp also has ascorbic acid and tartaric acid in high amounts. All these compounds are easily prone to oxidation (which is why they act as `antioxidants' protecting cellular components from damage by oxygen radicals). Oxidation of phenolic compounds leads to darkening of the pulp. This is the reason why many fruits and vegetables including tamarind turn dark when exposed to air.

Darkening due to aging and exposure to air also occurs in some kinds of wine, mustards and ketchup preparations. The speed at which they turn dark depends on the chemical composition of phenolic compounds present in the fruit/ingredients. Additionally, the presence of oxidative enzymes may also contribute to the process. In the case of tamarind darkening due to age, oxidation occurs primarily as a non-enzymatic chemical reaction. 

As a general rule of thumb, there is a 50 per cent reduction in the rate of chemical reactions for every 10{+0}C reduction in temperature. So, refrigeration would significantly reduce the rate of oxidation and thus darkening of tamarind pulp. Enzyme mediated oxidations are also reduced under refrigerated conditions due to optimal temperature requirements of most enzymes.

Source: thehindu.com

Why do new clothes not absorb water as easily as old ones?

Fabrics before being made into garments pass through several process sequences like bleaching, dyeing, finishing, pre-shrink and the like. To improve the aesthetic appearances, fabrics are normally finished with chemical softeners to improve feel, handle, suppleness and bounce of the garments. These chemical softeners are also added to improve the ease of sewability, otherwise needle holes may appear along the stitches. These chemical softeners are of two types (hydrophobic and hydrophilic). Commonly all dyers used to prefer hydrophobic softeners because it was cheaper. These hydrophobic softeners, while applying, are superficially deposited on the surface of the fabrics and will not allow the garments to absorb water. But these softeners are not permanent and could be washed away during subsequent laundering of the garments. The older garments may not have these softeners after five to six laundering cycles and hence will absorb water quickly compared to new ones.

Source: thehindu.com

Why is LCD (Liquid Crystal Display) device so named?

The LCD device is built around a material, which is a liquid as far as its flow properties are concerned. However, the large, rod like organic molecules constituting the material can be aligned in an orderly manner, more like a crystal. Hence the name Liquid Crystal Display (LCD). Such alignment is achieved in the devices by application of a localized electric field between an opaque and another transparent electrode. The aligned molecules form a pattern in the shape and size of the transparent electrode and the pattern is observed in polarised light. The device consists of a pair of polariser films on either side of the assembly of the two electrodes sandwiching the liquid crystal layer, oriented at perpendicular directions. Further the special design of the electrodes twists the crystal to different extents, which controls the opacity or visibility of the desired pattern. Different innovative combinations of liquid crystal materials, pattern and strengths of the electric field and orientations of the polarising filters produce many special visual effects of the liquid crystal displays.

Source: thehindu.com

How does the human body react to a severe electric shock?

The human body contains body fluid mainly consisting of water, which gets electrolysed on passage of electric current in the event of an electric shock. This leads to other more complex electric effects in the body as follows:When an electric current passes through a living tissue the nerves respond in a typical way that makes the muscular tissues contract and a twitching is noticed as was observed by the famous Italian scientist Luigi Galvani in the 1780s.In fact, the symptom of such spasms has been recorded in more controlled experiments where a definite quantity of electric charge was deposited on the spinal cord of frogs and the frogs exhibited the same type of spasms in their muscles; more charge resulted in greater twitching of muscles. In the human body also, the same effect takes place. In severe cases of such electric shock, the spasms can be so strong that the muscles of some critical organs like the heart can be thrown out of their characteristic rhythmic motion. And the electric current can also disrupt the functioning of the brain. The combined effect can be fatal.

Source: thehindu.com