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.
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.
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.