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Principle of Working of Microwave Oven

Manisha Chowdhury asked:

Why is it called a microwave oven?
Who invented it?
What is the principle of its working?


A microwave oven is used to cook (or heat) food with the help of microwaves produced by magnetron – the device producing microwaves in the oven. Microwave ovens are so quick and efficient because they channel heat energy directly to the molecules (tiny particles) inside food.

Who invented Microwave Oven?

Percy-spencerPercy Spencer is generally credited with inventing the modern microwave oven after World War II from radar technology developed during the war. Named the “Radarange”, it was first sold in 1946. Raytheon later licensed its patents for a home-use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation, and their use has spread into commercial and residential kitchens around the world.

Working of microwave oven

microwave ovenA microwave oven, commonly referred to as a microwave, is a kitchen appliance that heats and cooks food by exposing it to electromagnetic radiation in the microwave spectrum. This induces polar molecules in the food to rotate and produce thermal energy in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer25–38 mm (1–1.5 inches) of a homogenous (high water content) food item; food is more evenly heated throughout (except in heterogeneous, dense objects) than generally occurs in other cooking techniques.

Magnetron2A microwave oven heats food by passing microwave radiation through it. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency higher than ordinary radio waves but lower than infrared light. Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, which are reserved for this use, so they don’t interfere with other vital radio services. Consumer ovens usually use 2.45 gigahertz (GHz)—a wavelength of 12.2 centimetres (4.80 in)—while large industrial/commercial ovens often use 915 megahertz (MHz)—32.8 centimetres (12.9 in). Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e. as both potential energy and kinetic energy of atoms), is heat.

For more details refer to :


FTL – Faster than light

Charan asked: In a recent research,it was found experimentally that nutreinos are faster than light.In that case,Einstein’s Relativistic mechanics don’t work.What tpe of mechanics are applied in that case?


There was a news about  faster than light by neutrinos recently by the scientists of CERN which they corrected later explaining that the news wa a result of mistake in calculation caused by the error in time interval reporting.

The scientists (and others too) are not yet ready to accept a result of faster than light travel by a material particle. So, if such an observation is reported, the scientific community is to be prepared with sufficient theoretical background to explain contrary to the Theory of Relativity.


Charan asked: Suppose that you are a person 1 standing on a planet.You could see a person 2 moving in a space craft.2 has a mirror(on the surface of planet) exactly at is down which is moving exactly with the same speed that space craft is moving(and also,the line of translatory motion of both the craft and mirror are parallel to each other).If 2 has shot a beam of light from the bottom of space ship,as the mirror is moving exactly with the space craft;for 2,the path of light is straight line and gets reflected back along the same path in time t.If you are observing the whole thing from the surface of planet,for you,the path of light would obviously be ‘V’ shaped(let the time taken be t’).As the ‘V’ shaped path is longer than straight path and speed of light is same for observers,the time measured by 2 is obviously not the same as you measure.If you are considered to be reference frame,will the clock of 2 appear to be moving slower than yours?

Applications on Newtons Second Law

“A particle of mass 2m is on a plane inclined at an angle 36.86 to the horizontal. The particle is attached to one end of a light inextensible string. The string runs parallel to a line of greatest slope of the plane, passes over a smooth pulley at the top of the plane and then hangs vertically carrying a particle of mass 3m at its other end. The system is released from rest with a single taut. Find the acceleration of each particle and tension in the string when the particles are moving freely, given that the plane is smooth.”


Asked Gina

How to find the focal length of convex mirror using convex lens?

How to find the focal length of convex mirror using convex lens?

Asked Manisha


We cannot use an optical method to determine the focal length of a convex mirror without using a convex (converging) lens since a convex mirror does not form a real image of an object in front of it or even parallel rays coming from infinity.

Therefore we have to use a convex mirror to determine the focal length of the convex mirror by an indirect method.


First of all we take a convex lens and determine  its rough focal length by forming a real image of a distant object on a screen. The distance between the convex lens and the screen gives the rough focal length, since when the object is at infinity, the image formed by the convex lens is at its focus. This method is called the distant object method to estimate the focal length of a convex lens. The focal length of a concave mirror may also be estimated using this method.

Experiment for focal length of convex lens

Now, as we know the approximate focal length of the convex lens, keep an object- an optical pin or a lit candle in front of the convex lens at a distance around 2 times the rough focal length determined and place a screen on the other side so as to form a real image of the object on it. This adjustment is done so as to make the distances involved quite manageable.

When you get a clear and sharp image on the screen, mark the positions of the convex lens and the screen on the table.

Place the convex mirror in between the screen and the lens without disturbing the position of the candle and the lens.

Place the screen close to the candle. Carefully adjust the position of the convex mirror alone so that you get a sharp image on the screen which is now kept  at the position of and along with the candle. Mark the position of the convex mirror now.

The distance between the position of the convex mirror and the old position of the screen gives the radius of curvature of the convex mirror. (Since r = 2f; we can determine the focal length by dividing the distance with 2)

Why the distance is r (2f)?

We know that when a ray of light is incident normally on the surface of a mirror, it retraces its path.

We also know that a line drawn from the centre of curvature of the mirror to the surface of a mirror is normal to the surface.

Therefore, when we are getting the image at the same position as the candle, it is formed by retracing of the rays, which means that the old position of the screen is at the position of the centre of curvature of the convex lens.

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See the Videos below to understand how the focal length of a convex mirror is determined using a convex lens in an optic bench.

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