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In this chapter:

  • Everything reflects some light
  • Smooth and shiny surfaces reflect light best
  • Reflections occur when light bounces off a smooth and shiny surface
  • Glass mirrors have a thin layer of silver on the back so that light is reflected
  • There are three types of mirrors: flat; convex and concave
  • Each type of mirror reflects light in a different way
  • One way mirrors are sometimes used in police stations
  • Mirrors are used in different types of technology

Introduction

When a ray of light hits a mirror it is reflected. That means it bounces back and a reflection is made. You might think that mirrors are the only things that reflect light but almost every surface reflects some light otherwise we would not be able to see anything. (Refer Chapter 2) We see objects because light reflects off them and into our eyes. This chapter will describe what happens to a ray of light when it hits a surface such as a mirror and then bounces off it to make a reflection. The different types of mirrors are then discussed.

Reflections from rough or smooth surfaces

Most surfaces are uneven or rough. When light rays strike these rough surfaces they bounce off in all directions. So there is no clear image reflected. This is called diffuse reflection.

When light hits a very smooth surface, however, it is reflected in a regular pattern and we see a very clear image of the object. This is called specular reflection.

See image 1
 
When the surface of a pool of water is very smooth, for example, a clear image is reflected. When the water becomes ruffled by the wind, the image becomes jumbled as the reflected light is scattered in all directions.
 
See image 2
 
The amount of light that is reflected depends on the surface of the object. If the object is shiny and smooth it will be a better reflector of light and a clearer image will be seen. Silver is the best reflector. It reflects about 96 percent of light and absorbs only 4 percent. That is why the backs of mirrors are now covered by a thin layer of silver.

Mirrors

Humans have always had a fascination with looking at themselves in mirrors. A polished rock called obsidian was used for making mirrors in Turkey about 7500 years ago. Later mirrors were made from polished copper or bronze. Then glass mirrors began to be made in about 1300. These mirrors had a thin layer of tin coated on the back to reflect light. In 1835 the German scientist Justus von Liebig found a way of covering the back of glass with a layer of pure silver which is the best reflector of light. This is still the way mirrors are made today.

See image 3
 
There are three types of mirrors:
  • flat or plane;
  • concave (curved inwards like a bowl or 'cave'); and
  • convex (curved outwards like a dome).

Images from each mirror are reflected in different ways.

Reflection from a flat mirror

Reflection always involves two light rays; an incoming or incident ray and an outgoing or reflected ray. When light hits a flat mirror the light rays are reflected in a very regular pattern. Each ray of light that hits a flat mirror leaves the surface at the same angle but in a different direction.

See animation 1
 
A flat mirror reflects a very clear image but the image is reversed from left to right. You can see this when you look at words in a mirror. They seem to be written backwards.
 
See animation 2

Reflection from a concave mirror

A concave mirror is a mirror that curves inwards like a bowl. When light strikes a concave mirror it is reflected in a different way from a flat mirror. When rays of light hit a concave mirror they are reflected so that they come together at one point.

What is seen from a concave mirror depends on the distance the mirror is from the object. If an object is far away from the concave mirror the image is upside down and magnified (larger). If an object is close to the mirror the image is magnified but it is the right way up.

Investigate this by looking at your reflection in the front of a spoon. Your reflection will be upside down unless you hold the spoon very close to your eye.

Concave mirrors are used by dentists as they make the teeth look bigger. They are also used in some telescopes to gather light from far away stars.

Reflection from a convex mirror

A convex mirror is a mirror that curves outwards like a dome. When light rays hit a convex mirror they are reflected so that they spread out in all directions. Light can hit the mirror from many directions and this gives the mirror a wide view.

Reflections from a convex mirror appear the right way up but are smaller than the actual objects. Security mirrors in shops are convex mirrors and some car mirrors are convex. This is because convex mirrors give a wide view but everything looks smaller.

See image 4

One-way mirrors

You may have seen one-way mirrors on detective TV shows. If the lighting is right, one-way mirrors can allow people to see through one side of the mirror. A one-way mirror is made from a darker type of glass than an ordinary mirror and has a thinner coating on the back. This allows half of the light to go through the mirror and the other half to be reflected. If one room is brightly lit and the other is dimly lit then the people in the dimly lit room will be able to see through the mirror but the people on the other side will see only a reflection of themselves.

Using mirrors

Flat and curved mirrors are used in lots of different technology. Flat mirrors are used to make periscopes. Periscopes are used on submarines to see ships floating on the sea above or to see around corners. Two mirrors are set at an angle of 45 degrees at each end of a long box. This allows light to come out the bottom of the box in the same direction as it went in. Making a periscope is easy - you just need two flat mirrors and a long box.

Concave mirrors are used in torches and car headlights to make a strong, straight beam of reflected light. They can also be used in telescopes, such as the Hubble Space Telescope, so that we can see deep into space.

Mirrors reflect light in straight lines but what happens if light hits a new substance? The next chapter will discuss bending light.


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Question 1/5

1. What metal is the best reflector of light?

Silver

Lead

Gold

Aluminium

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