Why are camera lenses always round in shape?

The lens in a camera focuses the light constituting the image onto the recording screen like the photographic film in a conventional camera or the CCD array device in a modern digital camera. The distance between the lens and the screen is a few millimetres or at best a few centimetres. And so, the focal length of the lens has to be of that order. This is achieved by combination lens systems. 

The combination lens also corrects for two known sources of image distortion, the spherical aberration and the chromatic aberration. As a consequence of spherical aberration, the outer parts of an image appear unnaturally bent and the bending decreases as you approach the centre of the image. This is quite noticeable in pictures taken using a wide angle lens. And the chromatic aberration, as the term suggests, causes the light of different colours to focus at different distances from the lens. Consequently, the image gets blurred unless the lens is corrected for the aberrations. The region of the lens farther from the centre contributes more to these aberrations. Thus, the correction needed also has the circular symmetry. The aberrations are corrected by appropriately shaping the curvature of the lens. Therefore, it is convenient to correct a lens of circular shape. 

In fact, you see a few new model cameras with a rectangular shape of the lens, but actually the lenses of these cameras are also circular but a rectangular mask is fitted in front which presents a rectangular shape. This mask is designed to restrict the light in different directions of the field of view, so as to capture natural brightness all over the image. If the lens is made in a rectangular shape, the aberration compensations also have to be done in the same symmetry. In the case of combination lenses it would be practically impossible to provide the necessary aberration corrections differently in different directions; instead the job at hand is conveniently and accurately accomplished by choosing lenses of circular shape.

Source: thehindu.com

How is vibration produced when mobiles are put on the vibrate mode?

A small motor that has an uneven or unbalanced weight on it's output shaft is commonly found inside mobile phones that offer vibration mode. When this motor rotates, the rotating uneven weight on the output shaft causes the mobile phone to vibrate. Similar mechanisms are also found in gaming devices such as Play station, Dual Shock, XBOX and Wii controllers.

Source: thehindu.com

Unlike other muscles that get tired and sore after working out for sometime, how come the muscles of the heart never get tired even after beating continuously?

Even though it seems as if the heart is beating without a break, the heart does rest for a very short time in between two beats. The human heart has four chambers — the left and right atria on top, which drain, into two corresponding ventricles below, through valves. The atria receive blood from the entire body through veins. This blood flows through the connecting valves into the ventricles, and the ventricles contract and push blood out into arteries, which branch and supply to the whole body. 

The heart beats in 'cycles' and each cycle consist of a systole and a diastole. Taking the average heart rate to be 75 beats per minute each cycle lasts just 0.8 seconds. In the systole, which lasts about 0.27 seconds, the ventricles contract pumping blood out into the arteries and in the diastole, which lasts around 0.53 seconds, the ventricles receive blood from the atria. It is during diastole that the heart muscle rests. Also, if we were to plot the contraction of a single heart muscle fibre on a graph it can be seen that there is a refractory period for the fibre during which it will not contract on applying a second stimulus. This prevents the heart muscles from developing tetanus.

Source: thehindu.com

How are we able to clearly see dust particles in a sunbeam?

We see an object when light reflected from it enters our eyes. There is a threshold level of intensity required for the eye to detect the object. We do not see an object in the dark or in dim light and also a candle kept far away, say at 10 km distance. Extremely small objects like bacteria will not be visible in normal light since the area reflecting the light and hence the intensity is very small. 

Dust particles are much bigger than bacteria but are still quite small for good visibility in normal light. They may be 10-100 micrometres in size. When exposed to light they scatter it in all directions like every other object. A spherical particle scatters light symmetrically around the direction of light propagation (but not spherically symmetrical). It generally scatters more light in the (forward) direction of light propagation. Angular distribution of light intensity depends on the size and shape of the particle, its material properties (for example, refractive index) and wave length of light (0.38-0.78 micrometre for visible light). But the important thing here is that light is scattered in the transverse direction also. It is this scattered light that makes the particles visible. 

A particle may not be visible in the open sunlight but visible in a sunbeam or a projector light beam in a room. In the open sunlight, bright light being in all directions, it swamps the scattered light and our eye cannot distinguish it. In a room the scattered light we are considering is transverse to the directed bright light and the eye can detect it. The eye is also adjusted to the low intensity light. A particle scattering the light, if in motion, also shifts the frequency (colour) of the light. This shift is due to the well-known Doppler effect but is very small compared to the frequency of the original light itself. Hence to measure the velocity of a dust particle by measuring the frequency shift, a very good quality coherent light source like a laser is required. This principle is used in a laser Doppler velocimeter to measure fluid velocity.

Source: thehindu.com

How does the telephone wire not lose its spiral nature?

The telephone wire is made up of a flexible plastic (usually PVC along with some colouring additives) material moulded along with the conductors inside This is the cable. This cable is again given the helical shape by raising its temperature to approximately 120 degrees Celsius, the point where the cable does not melt but the polymer configuration easily gets altered This changes the physical form. It is then cooled to ambient temperature where the shape is retained, however, with the inherent elastic flexibility of the material. Thus the cable can be stretched easily to a helix of larger pitch and when released it resumes the original helical shape instead of getting untwisted. Of course, if the cable is held stretched for a while in an ambience of raised temperature, it would lose its shape.

Source: thehindu.com

Why does paper curl on burning?

The upper or flame side of the paper loses water faster compared with the lower side. This is because paper is not a good conductor of heat and hence both sides are not heated to the same extent by the flame. That only the upper side curls can be further tested by the turning the burning paper by 180 degrees. The curling reverses and does not continue to be in the direction of the side that is now the lower side.Another interesting way to check the direction of curling is by wetting only one side of the paper. The side of the paper that is dry will tend to curl.Paper tends to curl basically because the expansion of the fibers on both sides of the paper is different when it either brunt or wetted on one side only. In the case of the burning paper, the upper side loses water faster and hence the curling is seen on that side. In the case where the paper is wetted on one side only, the side that is dry tends to curl. Curling is not seen when both sides of the paper gets wet at the same time as when it is immersed in water. One can control the way direction of curling in a paper that is kept with neither sides being the upper or lower side by gently curling it in one direction. The curling is seen in the direction that is concave.The nature and extend of curling when the paper is burnt can be checked using papers of different thickness. The thinner the paper, the lesser the curling seen in one direction.

Source: thehindu.com

If increase of viscosity increases the stability of an emulsion, why is milk more stable and show less settling even though the viscosity is less?

An emulsion is a dispersion of one liquid in another immiscible one (for example, oil and water). One phase is called continuous and other is called disperse phase. Oil droplets are dispersed in water by fragmenting it in presence of a surface-active species such as surfactants or polymers, which provide stability. The diameter of the disperse phase varies from a few nanometres to several microns. Milk is a natural emulsion where each fat globule is stabilized by a membrane of phospholipids and proteins. Milk contains 88 per cent water, 3.3 per cent protein (casein 82 per cent ), 3.3 per cent fat , 4.7 per cent carbohydrate, 0.7 per cent ash. The percentage of disperse phase in milk is very low compared to some of the industrial products. Now coming to the answer to the question, what happens when the viscosity of the continuous phase (water) goes up? The settling velocity decreases due to viscous resistance and hence the settling time increases. Therefore, we may be right to say that the stability of an emulsion goes up with viscosity. However, the answer is not fully correct. The density mismatch between the two phases, the diameter of the dispersed droplets and the electrical charges of the surfactant molecules also play a crucial role in the stability of emulsions.

Source: thehindu.com

At times raindrops are bigger when it just begins to rain and sometimes smaller? What is the reason?

Rain happens when warm, moist air rapidly cools at high altitudes in the presence of surrounding colder air. At any given temperature, dry air can accommodate only a certain amount of water vapour. As the moist air temperature drops, due to cooling, it becomes super-saturated with water vapour. When air can accommodate no more water vapour under equilibrium conditions, it is saturated. Under super-saturated conditions, air holds more moisture than it possibly can under equilibrium conditions. Hence, there is the tendency of air to precipitate out any excess moisture present and it rains. Rain drops form due to condensation of water vapour around preferential sites such as microscopic dust particles, pollen grains etc. that are present in air. As the drop-wise condensation proceeds, microscopic droplets of water initially formed grow in size and the water droplet begins to fall under the action of gravity. While falling, the droplets can collide with other droplets and coalesce to form bigger droplets. Alternatively, droplets can collide with other droplets and break apart into smaller droplets due to the impact. There is another reason for the break-up of water drops. Due to the relative velocity between the falling water drop and the air around it, viscous shearing forces develop on the surface of the drop. These drag forces cause the drop to distort in shape. During its fall the waterdrop accelerates, and the magnitude of the drag forces increases roughly in proportion to the square of the fall speed of the drop. Due to shape distortions and the rapidly increasing effects of the shear stresses on the air-water interface of the drop, the drop may break apart. Large drops of water have a tendency to break apart more than the smaller ones. Due to these reasons growth of drop size due to coalescence, and break up due to collisions and viscous shear rain drops come in varying sizes, from the smallest to the largest, by the time they hit the ground.

Source: thehindu.com

Why is the nib of a fountain pen split?

A fountain pen allows a controlled flow of ink because of its specially designed flow system for ink and air. The flow system consists of an ink reservoir, a feed bar (also called feed) and a metal nib. 

The nib is split starting from a breather hole till the tip to form two tines (branches). The feed has a longitudinal flow channel on its top surface starting from the inside end almost up to the forward end and it is covered by the nib. The channel is a broad groove and along its floor are cut two or more narrower grooves (called fissures). The broad channel brings ink up to the breather hole and the split of the nib. The split carries the ink to the tip and paper. The channel allows an occasional flow of air in the opposite direction into the reservoir so that pressure there does not decrease too much due to emptying of the ink. When such an air-flow takes place the fissures still contain ink and enable immediate filling of the channel. The nib tip has a special shape consisting of two hemispherical ends at the bottom of the tines. Ink forms a meniscus between these two rounded parts and spreads on paper as soon as a contact is established. The split width will change if pressure is applied and this will change the flow. The feed often has slots, combs or other design elements so that the capillary action can draw ink and hold it to have a steady flow or to avoid an overflow. Even when a pen is not used for a while, capillarity replenishes through the split, the evaporated ink and the pen is kept ready. The capillary force that maintains a continuous ink flow also helps to support ink from dripping due to gravity while not writing.

Source: thehindu.com

Why do bats hang upside down from tree brances?

Bats hang upside down primarily to help them get in to flight ("take off") quickly. Unlike birds, they do not have strong wings that provide enough lift for taking off directly from the ground. Their hind legs are also not strong enough to run fast to get the necessary take off speed. When they hang upside down they can get in to flight right away just by releasing their grip. If a bat accidentally falls to the ground, it has to climb up to a tall spot such as a tree branch to take off. 

Bats also obtain additional advantages due to hanging upside down such as safe guarding themselves from predators. The unique ability to hang upside down from tree branches and on the inner roof of caves makes it difficult for predators to reach them. Newborn bat pups can cling on to the mother's body and get transported by her. Usually, they stay in the bat roost where several bats nurse the newborns. The muscle arrangement in their legs is such that when they hang, their claws are kept closed due to their body weight. This makes it easier for them to hang spending minimal energy. They have to use energy only to open the grip and fly. Their grip is so strong that if a bat dies while hanging, it will keep hanging until forcibly knocked off by another bat or something.

Source: thehindu.com