Category Science & Technology

Why do stars twinkle?

          We see millions of stars twinkling in the sky during night. They radiate their light in all the directions. Even though they appear very small yet in reality they are very big. Most of the stars are many times bigger than our earth. They look smaller only because of their great distance from the earth. Do you know why the stars twinkle?

          Our earth is surrounded by a thick cover of air known as the atmosphere. There is vacuum beyond the atmosphere. The gases present in the atmosphere are in constant motion. Because of the movement of gases the density of air in the atmosphere is not uniform everywhere. As such the refractive index of air varies from place to place. When the light from a star enters our atmosphere, it gets deviated from its path several times before reaching our eyes because of the changing density and the consequent change in the refractive index of the air.

          The deviation in the path of light in its passage from one medium to another is called ‘refraction’. Because of this refraction, the light reaching our eyes from the star varies. Due to this variation the stars appear to be twinkling.

          Now the question arises: why don’t moon, sun and other planets twinkle like the stars? This is so because compared to the stars, sun, moon and the planets are very near to earth and as such they appear bigger than the stars. Hence the angles subtended by the moon, sun and the planets at our eyes are larger than the angles subtended by the stars. Because of the larger angles, our eyes are not able to detect the deviation in the path of light from the sun, moon and the planets and hence they do not appear to be twinkling. 

How does a microwave oven work?

          Over the years application of modern technology in the domestic sphere has made our lives more comfortable and easier. Washing machines, vacuum cleaners, microwave ovens etc. are a few examples that meet the demands of the modern society of today. The new gifts of technology especially for household use, always generate a curiosity in us about their working principles. Now let us discuss how does a microwave oven work?

          A microwave oven does not have burning flames or red-hot plates like gas and electric cookers. Its operational mechanism is simple. The user puts the food into a metal box and presses a switch. Inside the box, invisible energy rays bombard the food. 

          The oven gets its name from the rays that cook the food – the rays are called microwaves. Microwaves are part of the spectrum of electromagnetic waves which includes light waves and X-rays. They have wavelengths from about 30cms to one millimetre which places them between the shortest radio waves and infra-red waves. One of their properties is to excite molecules, especially in liquids, and make them vibrate and heat up. For this reason, food with a large proportion of water gets cooked very quickly.

          The source of microwaves in the oven is a magnetron. It is a two-electrode valve for generating high-frequency oscillations. Microwaves generated by the magnetron travel down a metal-duct and are scattered around by a metal fan for a more even cooking. The molecules rub against each other and the friction produces heat. The water then gets heated up and consequently the food gets cooked quickly.

          Microwave ovens are a potentially dangerous source of radiation. For this reason the ovens are metal lined and have strong doors. The oven will not function until the doors are closed thus ensuring safety. Most modern ovens contain small computers that automatically cook the food at the appropriate temperature for the right length of time. Microwave ovens are preferred and widely used in restaurants, hotels etc. where quick service is important.