Light – Reflection and RefractionIntroduction
It is seven in the evening, Riya is halfway through her homework, and the power goes off. A moment ago she could see her books, the steel tumbler on the table and the mirror on the wall. Now she cannot see any of them. Then her brother Aman strikes a match and lights a candle, and everything comes back.
What changed? Not the books, not the tumbler. Only the light. A few things give out light of their own. Everything else we see only because light falls on it and some of that light bounces off, or is reflected, into our eyes. Drag each thing into the right bucket.
Even the Moon is in the second bucket: it shines only with reflected sunlight. In daylight the Sun lights up everything around us; each object reflects some of that light, and when the reflected light enters our eyes, we see the object. We can see through a transparent medium like glass or water because light passes through it. Light is behind many everyday wonders: the images in a mirror, the twinkling of stars, the colours of a rainbow, a straw that looks bent in a glass of water. This chapter is about two of them, reflection and refraction.
Light Travels in Straight Lines
Switch on a small torch in a dark room and hold a cricket ball in front of it. The ball's shadow on the wall has sharp edges. What does that sharp shadow tell you about light?
A small source of light casts a sharp shadow of an opaque object because light travels along straight lines. We draw such a straight-line path as a ray of light, an arrow showing the direction in which the light travels.
More to know
If the opaque object in the path of light is very, very small, light does bend round it a little. This is diffraction, and to explain it light is treated as a wave. Early in the 20th century it turned out that light also behaves like a stream of particles. Today's quantum theory treats light as neither just a wave nor just a particle. In this chapter, rays travelling in straight lines are all we need.
We will use rays to understand how light is reflected by spherical mirrors and refracted by glass, water and lenses, and where we use them in real life.