Image is produced in a plane mirror by reflection. It is a virtual image, that is, it can only be seen in the mirror but cannot be projected on a screen like an image produced by a concave mirror or a lens. Every point on the virtual image corresponds to a point on the object in front of the mirror, but since the object and the image face each other we get the illusion of lateral inversion that is, the right side of the object appears to become left in the image. For the same reason, written or printed alphabets appear reverse when seen reflected in a mirror, except a few symmetrical alphabets such as M, O, W, T and Y.
A phenomenon called polarity prevents oil and water from mixing. All molecules carry electrical charge which is distributed uniformly or non-uniformly over the length of the molecule. In polar compounds, the positive and negative charges are concentrated at the two ends of the molecule. When such substances are mixed together, the positive and negative regions of their molecules attract each other and as a result a clear solution is obtained. Water is a polar substance and mixes freely with other polar substances. Oil molecules, on the other hand, are non-polar. When polar and non-polar substances are mixed together, the mutual attraction of polar molecules separates out the non-polar molecules and the two substances do not mix.
The solubility of a substance depends on the nature of bonding in both the solute and the solvent. Salt is an ionic substance; that is, its atoms are held together by opposite electrical charges. Water molecule also has electrical charges – the hydrogen atoms are positively charged and the oxygen atom negatively charged. When salt is put in water the charges on the water molecule pull the oppositely charged atoms in salt apart, thus dissolving it. Oil molecules, on the other hand, do not carry any charge and so does not have any effect on the ionic bonds in salt when put in it. As a result, salt does not dissolve in oil.
Bubbles are balls of thin soap films. They show rainbow colours because the light falling on them undergoes what is known as interference. Light travels in the form of waves which have troughs and peaks at regular intervals. For instance, one wavelength of a light-wave has one trough and one peak. When light falls on a soap bubble it is reflected from both the upper and lower surfaces of the film. Thus, there are two waves of reflected light emerging out of surfaces of the soap bubble. If the distance between the two surfaces of the film is less than one wavelength of light, the two reflected waves are so close that interference occurs. The peaks of the two waves may add up and become a bigger peak. Or the trough of one wave may fall with the peak of the other thus cancelling its effect. As the seven component colours of white light have different wavelengths, the adding or cancelling effect of interference is not the same on all of them. In other words, some colours become brighter and some duller. Thus a viewer sees a mixture of rainbow colours instead of the white light emerge from the soap bubble.
Unlike the steps in a staircase, a ladder has open rungs and is usually set at a steeper angle than a staircase. It is easier to maintain a firm grip on the narrow rungs of the ladder with one’s toes if one faces it. Also, when climbing down the ladder, a person tends to lean towards it to keep balance, which is easier to do if one faces it. On the other hand, the gentle slope of staircases and wide steps allow people to step down facing forward.
Burning is a chemical process in which the material which burns combines with oxygen with the generation of large amounts of heat. As a result the temperature of the burning material rises to several hundred degrees Celsius and it may burst into flames. Therefore, any substance that readily combines with oxygen at a few hundred degrees and produces a lot of heat will burn if ignited. Such substances such as paper, wood, cloth, plastics, rubber, etc. are usually rich in carbon and hydrogen. Some volatile liquids such as alcohol, ether, petrol, etc. catch fire easily because they produce highly combustible vapours.
Foam or lather is nothing but a large collection of small soap bubbles. A soap bubble is, in turn, a very thin film of soap solution enclosing some in air. Because of the low surface tension of soap solution, the film can stretch and spread and form innumerable bubbles with a very large total surface area. Because of this, whatever slight tint present in the thin film of the coloured soap solution gets subdued. Although a soap film is more or less transparent, the lather or foam looks white because the light striking this large collection of bubbles gets scattered. That is why all kinds of foam look white.
As our body is not a flat surface, whenever any part of the body is hit by a stick or rod it makes contact with only a small part of the skin depending on the diameter of the stick or rod. When hit with a thin cane, it hurts a lot because the flexible cane has a smaller diameter and contacts a thin strip of the skin on the body b wrapping around the surface. Since the force experienced per unit area is large, the cells under the area of contact are crushed causing either a swelling or bruise in that part of the body. On the other hand, a thick stick has a larger diameter and does not flex easily. When it hits the body the total area of contact is smaller and as the contact point is much flatter the force gets distributed over a large surface area thereby causing less pain.
Spectacles are made up of lenses and are used to improve vision. For nearsightedness, concave lenses are used and for farsightedness convex lenses are used to give clear images. A microscope is an instrument which gives enlarged view of small objects. In an optical microscope, the objective makes a real image of the object which is further magnified by the eyepiece for the eye to see a magnified virtual image. The distance at which the magnified virtual image is formed can be adjusted by moving the eyepiece to suit the power of the spectacles one uses. So, one need not use spectacles while using a microscope in order to see the magnified image clearly. Besides, wearing spectacles also draws the eye of the observer away from the eyepiece thus reducing the field of view.
The pitch, loudness, duration and timbre are the four qualities which define the structure of a piece of music. The pitch or the high-low aspect of the sound is considered to be the most important unit in structuring music. The timbre or the ‘tone colour’ is the sum total of the simultaneous motions produced by any vibrating medium. This is unique for each instrument. The timbre also defines the fineness of the sound produced by an instrument. Differences in timbre of sound produced by different instruments are responsible for the different musical qualities of the various instruments. Thus musical notes produced by different instruments sound different inspite of the pitch being the same.
When we try to break a stick by bending it we use lever of the first kind in which the fulcrum lies between the load points, in this case, right below the point where the stick breaks. Since the force generated at the break point depends on the length of the lever arm, which in this case is the distance from the break point to the stick end, a longer stick is easier to break as it needs less effort compared to that needed to break a short stick.
Natural rubber is made up of long chains of the molecule isoprene with a few cross links between the chains. These kinked, looping polymers are intertwined in un-stretched rubber. When these polymers are subjected to a stretching force, the tangled polymer opens up because of the flexible bonds between the molecules and becomes elongated. When the applied force is released, the chemical bonds in the chains try to come back to their original state. This accounts for rubber’s elasticity.
When an electrical switch is operated it produces a spark at the contact point. This spark emits electromagnetic radiation. Since radio and TV signals are also electromagnetic in nature, the bursts of electromagnetic radiation produced by a spark is also received by the radio or TV set. This produces the disturbance which is heard as crackling sounds on the radio and snowy lines on the TV picture.
One of the laws of fusion is that the melting point of substances which expand on freezing is lowered by the increase of pressure, while it is raised in the case of those which contract on solidification. Ice belongs to the first category of substances, that is, it expands on freezing. Ice has an open structure which collapses when subjected to pressure, producing water which occupies lesser volume. That is why ice melts when subjected to pressure.