Light, Rays and Reflection പ്രകാശരശ്മി
This chapter is about what happens to light when it goes from one material into another. Before that, we need to be sure about some simple facts: light travels in straight lines, we draw its path as a ray, we see a thing only when light from it enters our eyes, and light bounces back from a mirror in a very exact way. This page builds all of these from zero, with experiments you can do on the screen and at home.
- Nothing from earlier classes is assumed – every idea is explained here.
- Angles are used a little in section 4; page 2 explains angles and the protractor in full.
1. Light travels in straight lines
Look at sunlight coming into a dusty room through a gap in the window, or through the gaps in a coconut-leaf roof. You see straight shafts of light. Light from a source goes out in straight lines in every direction, as long as it stays in the same material – air, water or glass. It does not curve round corners. Two simple experiments show this.
Experiment A · Three cards with holes
Make a small hole in the middle of three cards at the same height. Stand them in a row and look at a candle flame through the holes. You can see the flame only when the three holes are exactly in one straight line. Push the middle card a little up or down and the flame disappears.
Light from the flame can reach the eye only along the straight line through the three holes. It cannot bend round the card to find the hole. The dashed curve shows a bent path that light simply does not take.
Experiment B · Shadows
An opaque object (one that light cannot pass through, page 4) blocks the straight lines of light coming from a lamp. Behind it is a dark area with sharp edges – its shadow. If light could bend round corners, there would be no sharp shadows. Draw two straight lines from the lamp past the top and the bottom of the object and you get exactly the edges of the shadow.
The card is 6 cm tall and the screen is 64 cm from the lamp. Move the card towards the lamp and its shadow grows; move it towards the screen and the shadow shrinks to nearly its own size. The straight lines decide everything.
In Tholpavakoothu, the shadow-puppet play of Palakkad temples, leather puppets are held between a row of oil lamps and a white cloth screen. The puppeteers make a puppet's shadow grow huge by moving it towards the lamps, exactly as in the lab above. At noon in summer, when the Sun is almost overhead, your own shadow shrinks to a small patch round your feet.
2. Rays and beams
We cannot draw every bit of light, so we draw its path. A straight line with an arrow on it, showing the direction in which light travels, is called a ray of light (പ്രകാശരശ്മി). The route it follows is the path of light (പ്രകാശപാത). A bundle of many rays is a beam. There are three kinds of beams:
- Parallel beam – the rays stay the same distance apart, like sunlight reaching the Earth.
- Diverging beam – the rays spread out from a point, like light from a torch bulb or a lamp.
- Converging beam – the rays come together to a point, like sunlight through a magnifying glass.
Set the number of rays to 1: a single ray. A laser torch gives a very narrow beam – almost a single ray. That is why the textbook uses a laser torch for its activities, with a little incense smoke or milk in water so that you can see the path of the ray.
3. How do we see things?
Some things give out their own light: the Sun, a lamp, a candle flame, a burning nilavilakku. Most things – a mango, a book, the Moon – do not. We see them because light from a source falls on them and bounces off them in all directions. Some of that light enters our eyes.
We see an object when light from the object enters our eyes. The eye does not send anything out. In a completely dark room you cannot see a mango, however hard you look, because no light comes from it to your eyes.
The textbook's first question is about a straw in a glass of lime juice that looks broken. The light that comes from the part of the straw inside the juice travels through the juice, the glass and the air before it reaches your eye. If its direction changes on the way, your eye is fooled about where that part of the straw really is. Keep this in mind – it is the key to the whole chapter.
4. Reflection from a plane mirror
When light falls on a shiny surface such as a mirror, it bounces back. This is reflection (പ്രതിപതനം). A flat mirror is a plane mirror (സമതല ദർപ്പണം). To describe reflection exactly, we name a few things:
- the ray arriving at the mirror is the incident ray (പതനരശ്മി);
- the point where it meets the mirror is the point of incidence (പതനബിന്ദു);
- the line drawn at that point at right angles (90°) to the mirror is the normal (ലംബം) – we always draw it dashed;
- the ray going away from the mirror is the reflected ray;
- the angle between the incident ray and the normal is the angle of incidence (പതനകോൺ), i; the angle between the reflected ray and the normal is the angle of reflection.
Both angles are measured from the normal, not from the mirror. The grey angles are the angles with the mirror surface; they are equal too, but they are not the angles we use. At 0° the ray falls along the normal and comes straight back on itself.
(1) The angle of incidence = the angle of reflection, both measured from the normal. (2) The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane (the same flat sheet – here, the screen).
You will meet the same words – incident ray, normal, point of incidence, angle of incidence – again and again in refraction. The second law has a twin in the rules of refraction (page 7).
Two mirrors at right angles
Put two plane mirrors together at 90°, like the corner of a box. A ray that hits one mirror is reflected on to the other and then comes back parallel to the direction it came from, whatever angle it arrives at. Try it.
At each mirror the angle of incidence equals the angle of reflection. The two turns together reverse the ray. Cycle and lorry reflectors (റിഫ്ലക്റ്റർ) send light back to the driver behind in a similar way, but with glass prisms instead of mirrors – you will see how on page 11.
5. Images in a plane mirror
When you look into a mirror, light from your face reflects off the mirror and enters your eyes. Your brain assumes that light always travels in straight lines, so it traces the rays backwards in straight lines. They seem to come from a point behind the mirror. That is where you see the image (പ്രതിബിംബം).
Two people (eye 1 and eye 2) look from different places, yet both see the image at the same point. The solid rays are the real light. The dashed lines are only where the eye thinks the light came from: no light actually goes behind the mirror. The image is as far behind the mirror as the candle is in front of it.
It is the same size as the object, upright, as far behind the mirror as the object is in front, and left–right swapped (your right hand looks like the image's left hand). Light does not really come from it – it is where the reflected rays appear to come from.
On a still morning, a temple pond (kulam) shows a perfect upside-down copy of the temple and the coconut palms: the flat water surface acts like a plane mirror. The famous Aranmula kannadi of Pathanamthitta is a mirror of polished metal, not glass – the reflection happens at its front surface. Later in this chapter you will see that “seeing a thing where it is not” also happens with refraction: a coin in water and the bottom of a pond look higher than they really are.
Worked example 1 · Angles of a reflected ray
Q. A ray of light falls on a plane mirror, making an angle of 30° with the surface of the mirror. Find (a) the angle of incidence, (b) the angle of reflection and (c) the angle between the incident ray and the reflected ray.
The angle of incidence is measured from the normal, which is at 90° to the mirror. So angle with the normal = 90° − angle with the surface.
Worked example 2 · How far is the image?
Q. Anu stands 1.5 m in front of a plane mirror. (a) How far behind the mirror is her image? (b) How far is she from her image? (c) She walks 0.5 m towards the mirror. How far is she from her image now?
Image distance behind the mirror = object distance in front. The distance from Anu to her image is the sum of the two.