Light:reflection

We see luminous objects because the light they give out comes to our eyes. Everything else we see because of reflection. Light reflects in very different ways from a smooth object compared to a rough one.

Scattering

Rought objects scatter light. The light comes in parallel but bounces off in all directions. This is why we don't see an image in them.

The law of reflection

Smooth surfaces scatter light according to the law of reflection. To do this the surface must be really smooth, like a mirror, a still lake or a plane of glass.

When this happens the light reflects as in this diagram. The angle of incidence = the angle of reflection when we measure it from the normal (which is an imaginary line at 90 degrees to the reflecting surface).

This means light rays that come in parallel leave the surface parallel and you get an image.

The window reflects some of the light from the flowers so you get a dim image compared to a mirror. But why are the flowers on the right here reflected 3 times and those on the left only once?

Light : Basics

Light is a form of energy. What we call light is a small part of the electromagnetic spectrum which includes ultra violet and infrared as well as radio waves and microwaves. All these things are basically the same but we can only see a small section of the whole lot (spectrum). This make sense, when we were evolving the only radio waves on the planet would have come from the stars or lightning. Not helpful to see only those things when a predator is creeping up on you. The difference between all these parts of the electromagnetic spectrum is their wave length and our eyes evolved to see the most useful wavelengths around on this planet.

Remember light travels in straight lines (unless it goes past a star or black hole when they can be bent). In space light travels at 300,000,000 m/s around a million times faster than sound travels through the air. Luminous objects are light sources they give out light. Materials can be described as transparent (let's through all the light), translucent (let's through some light) or opaque (let's through no light). Remember the thickness of the material is important! One sheet of paper is translucent but 100 sheets will be opaque. We also need to know about reflection, refraction and dispersion which are coming in the next post.

Pressure

Pressure in physics tells us about the concentration of a force. Imagine sitting on a chair. Ah! Relax. It's rather different if someone leaves a drawing pin on the seat!  In the first case your weight (gravity force)  is spread out over the whole of the area you press on. When the pin is there you put the same force onto a tiny area. This increases the pressure and the pin sinks into you. 

Many things are designed to have a high or a low pressure when used. A garden fork is made to have a high pressure. The ends of the prongs are sharp. This makes a high pressure so when you press on it it will sink into the earth. The drawing pin is another example the sharp end makes a high pressure so it will sink into display boards. The other end has a bigger area. This gives a low pressure so it will not think into your thumb. Snow shoes are also designed to have a low pressure they spread out your weight so you don't sink too far into the snow.

To calculate the pressure made by something you divide the force by the area it is exerted (made) on. So

Pressure=Force/area

Forces are measured in Newtons and areas are measured in cm2 (centimetres squared) or m2. This means pressure is measured in N\cm2 (Force divided by area) or N\m2 which is also called a Pascal (Pa). Check your bicycle tyres they will probably have this but also lb\in2 or pounds per square inch.