Waves and Electromagnetic Radiation
Mirrors and lenses
Tell concave from convex for both mirrors and lenses, use the laws of reflection to find an angle, and say which of them gathers a beam and which spreads it.
Meena looked into one curved mirror at a science center and her face filled it. Her brother, standing further back, appeared upside down in the same mirror. At the next one she was tiny. A plane mirror had done nothing of the sort — it gave her back at her own size, the right way up, wherever she stood.
Curved in, curved out
A spherical mirror whose reflecting surface curves inwards is concave; one that curves outwards is convex. Lenses are named the same way by their thickness: a convex lens is thicker in the middle than at the edges, a concave lens thicker at the edges than in the middle. Light bounces off a mirror and passes through a lens, so we see things in a mirror but through a lens.
Worked example
Three mirrors sit side by side. In the first, a pen cap looks the same size and the right way up. In the second it is upside down. In the third it is upright but small. Which is which?
The same size and upright is the plane mirror — it does that whatever the distance.
A plane mirror is the baseline the other two are being compared against, so it is the safe one to name first.
Try it together
A ray of light strikes a plane mirror, making an angle of 30° with the normal.
Answer with a bare number each time.
1.The ray makes 30° with the normal. What is the angle of reflection, in degrees?
Have a go
Have a go on your own. A ray falls on a mirror along the normal itself. What is the angle of reflection, in degrees? Answer with a number.
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.