Media, Slabs and Prisms മാധ്യമം
The chapter keeps talking about light going from one medium into another: from air into water, from glass into air, from kerosene into water, through the layers of the atmosphere. It uses glass blocks of special shapes – a rectangular slab, a semicircular slab and prisms. And it warns that optical density is not the same as ordinary density. This page explains all these words and shapes before you need them, with labs where the rays are drawn exactly as real light travels.
- Rays, the normal and angles from the normal (pages 1 and 2).
- Speed and the comparison of decimals like 1.33 and 1.44 (page 3).
1. Transparent, translucent and opaque
Materials behave in three different ways when light falls on them:
- Transparent – almost all the light passes straight through, so you can see clearly through it: clear glass, clean water, air, cooking oil.
- Translucent – some light passes through but is scattered in all directions, so you see only a blur: butter paper, frosted bathroom glass, milky water.
- Opaque – no light passes through; there is a shadow behind it: wood, cardboard, a steel plate, a brick wall.
Refraction happens in transparent materials, because light must get through them to change direction. In the textbook's laser activity a drop or two of milk makes the water very slightly translucent – just enough to scatter a little light sideways so that you can see the path of the ray. Incense smoke does the same in the air above.
2. What is a medium?
Any material through which light travels is called a medium (മാധ്യമം) – plural media. Air, water, glass, kerosene, coconut oil and diamond are all media for light. Light can also travel through vacuum (ശൂന്യത) – completely empty space with no material at all, as between the Sun and the Earth. (Sound cannot do that – it needs a medium.)
The surface where one medium meets another is the interface (വിഭജനതലം): the surface of the water in a glass, the face of a glass slab, the boundary between oil and water. In the chapter, light changes direction only at an interface – never in the middle of one uniform medium.
Here the beam meets every wall straight on – along the normal – so it does not bend at all (you will see why on page 5). It still passes through five media and four interfaces.
3. Mass density is not optical density
In your earlier classes, density meant how much mass is packed into each cubic centimetre: density = mass ÷ volume. The textbook calls this mass density (പദാർഥ സാന്ദ്രത). Kerosene has less mass density than water (about 0.8 g/cm³ against 1 g/cm³), which is why kerosene floats on water.
This chapter introduces a different idea: optical density (പ്രകാശികസാന്ദ്രത) – how much a medium slows light down. A medium in which light is slower is optically denser; one in which light is faster is optically rarer (പ്രകാശികസാന്ദ്രത കൂടിയ / കുറഞ്ഞ മാധ്യമം). The surprise: kerosene, which floats on water, is optically denser than water – light is slower in kerosene.
Watch the two interfaces. Going from air into kerosene the ray bends towards the normal; going from kerosene into the water below it bends a little away from the normal. So water is optically rarer than kerosene, even though water is the heavier liquid at the bottom. (The refractive index on the bars is explained on page 6 – a bigger number means light is slower.)
Mass density decides what floats and what sinks. Optical density decides how slow light is and how much it bends. They are different properties: kerosene has lower mass density but higher optical density than water.
Worked example · Which floats?
Q. 50 cm³ of kerosene has a mass of 40 g, and 50 cm³ of water has a mass of 50 g. Find the mass density of each and say which liquid floats on the other.
Mass density = mass ÷ volume. The liquid with the smaller mass density floats on top.
4. Glass slabs and prisms
The textbook uses four shapes of glass. All are transparent blocks with flat (or, for one, curved) polished faces:
▭ Rectangular glass slab
A glass slab (ഗ്ലാസ് സ്ലാബ്) is a rectangular block with two pairs of parallel faces. Light enters one face and leaves the opposite face (Figure 1.9).
◓ Semicircular glass slab
Half a circular disc: one flat face and one curved face. A ray aimed at the centre crosses the curved face along the normal, so only the flat face bends it (Figure 1.16).
△ Triangular prism
A prism (പ്രിസം) has two triangular ends and three rectangular faces. Its faces are not parallel, so the ray leaves in a new direction.
◺ Right-angled prism
A prism whose triangle has angles 90°, 45° and 45°. It is used in cycle reflectors, periscopes and binoculars (pages 11 and 15).
Glass, n = 1.5. Each dashed line is the normal where the ray meets a face. For now just notice what happens – the rectangular slab sends the ray out parallel to where it came in; the triangular prism turns it; the semicircular slab and the right-angled prism can trap the light inside by reflecting it completely. Pages 7–11 explain why.
5. The atmosphere: a medium in layers
The air around the Earth – the atmosphere – is not the same all the way up. Near the ground the air is pressed together by the air above it, so it is denser; higher up it gets thinner and thinner until there is almost nothing (vacuum). The temperature changes with height too, and the air is always moving. So the atmosphere is like a stack of many thin layers, each with a slightly different optical density.
Light from a star passing down through these layers crosses one interface after another and bends a tiny bit at each. This is atmospheric refraction (അന്തരീക്ഷത്തിലെ അപവർത്തനം), the reason stars twinkle and the Sun is seen above the horizon (ചക്രവാളം) a little before it actually rises (page 9). The same layering, of hot and cool air over a sun-baked road, makes the “water on the road” mirage (page 11).
The bending is hugely exaggerated so that you can see it: real air changes the direction of starlight by less than half a degree. At 0 all the layers are the same, so the ray goes straight and the star is seen where it really is.
On a hot April afternoon on the NH 66, the road surface heats the air just above it. Look along the road and you can see the air above it shimmer and wobble: the layers of hot and cooler air keep moving, and light passing through them bends one way and then another. That shimmer is atmospheric refraction you can see with your own eyes.
6. Check yourself
You now know rays and reflection, angles and normals, speed and powers of ten, and the media and shapes the chapter uses. Time for the real thing: why light changes direction when it enters a new medium.